THE ENCYCLOPÆDIA BRITANNICA

A DICTIONARY OF ARTS, SCIENCES, LITERATURE AND GENERAL INFORMATION

ELEVENTH EDITION

VOLUME XI SLICE V
Gassendi, Pierre to Geocentric

Articles in This Slice

GASSENDI[1] [Gassend], PIERRE (1592-1655), French philosopher, scientist and mathematician, was born of poor parents at Champtercier, near Digne, in Provence, on the 22nd of January 1592. At a very early age he gave indications of remarkable mental powers and was sent to the college at Digne. He showed particular aptitude for languages and mathematics, and it is said that at the age of sixteen he was invited to lecture on rhetoric at the college. Soon afterwards he entered the university of Aix, to study philosophy under P. Fesaye. In 1612 he was called to the college of Digne to lecture on theology. Four years later he received the degree of doctor of theology at Avignon, and in 1617 he took holy orders. In the same year he was called to the chair of philosophy at Aix, and seems gradually to have withdrawn from theology. He lectured principally on the Aristotelian philosophy, conforming as far as possible to the orthodox methods. At the same time, however, he followed with interest the discoveries of Galileo and Kepler, and became more and more dissatisfied with the Peripatetic system. It was the period of revolt against the Aristotelianism of the schools, and Gassendi shared to the full the empirical tendencies of the age. He, too, began to draw up objections to the Aristotelian philosophy, but did not at first venture to publish them. In 1624, however, after he had left Aix for a canonry at Grenoble, he printed the first part of his Exercitationes paradoxicae adversus Aristoteleos. A fragment of the second book was published later at La Haye (1659), but the remaining five were never composed, Gassendi apparently thinking that after the Discussiones Peripateticae of Francesco Patrizzi little field was left for his labours.

After 1628 Gassendi travelled in Flanders and Holland. During this time he wrote, at the instance of Mersenne, his examination of the mystical philosophy of Robert Fludd (Epistolica dissertatio in qua praecipua principia philosophiae Ro. Fluddi deteguntur, 1631), an essay on parhelia (Epistola de parheliis), and some valuable observations on the transit of Mercury which had been foretold by Kepler. He returned to France in 1631, and two years later became provost of the cathedral church at Digne. Some years were then spent in travelling through Provence with the duke of Angoulême, governor of the department. The only literary work of this period is the Life of Peiresc, which has been frequently reprinted, and was translated into English. In 1642 he was engaged by Mersenne in controversy with Descartes. His objections to the fundamental propositions of Descartes were published in 1642; they appear as the fifth in the series contained in the works of Descartes. In these objections Gassendi’s tendency towards the empirical school of speculation appears more pronounced than in any of his other writings. In 1645 he accepted the chair of mathematics in the Collège Royal at Paris, and lectured for many years with great success. In addition to controversial writings on physical questions, there appeared during this period the first of the works by which he is known in the history of philosophy. In 1647 he published the treatise De vita, moribus, et doctrina Epicuri libri octo. The work was well received, and two years later appeared his commentary on the tenth book of Diogenes Laërtius, De vita, moribus, et placitis Epicuri, seu Animadversiones in X. librum Diog. Laër. (Lyons, 1649; last edition, 1675). In the same year the more important Syntagma philosophiae Epicuri (Lyons, 1649; Amsterdam, 1684) was published.

In 1648 ill-health compelled him to give up his lectures at the Collège Royal. He travelled in the south of France, spending nearly two years at Toulon, the climate of which suited him. In 1653 he returned to Paris and resumed his literary work, publishing in that year lives of Copernicus and Tycho Brahe. The disease from which he suffered, lung complaint, had, however, established a firm hold on him. His strength gradually failed, and he died at Paris on the 24th of October 1655. A bronze statue of him was erected by subscription at Digne in 1852.

His collected works, of which the most important is the Syntagma philosophicum (Opera, i. and ii.), were published in 1658 by Montmort (6 vols., Lyons). Another edition, also in 6 folio volumes, was published by N. Averanius in 1727. The first two are occupied entirely with his Syntagma philosophicum; the third contains his critical writings on Epicurus, Aristotle, Descartes, Fludd and Lord Herbert, with some occasional pieces on certain problems of physics; the fourth, his Institutio astronomica, and his Commentarii de rebus celestibus; the fifth, his commentary on the tenth book of Diogenes Laërtius, the biographies of Epicurus, N.C.F. de Peiresc, Tycho Brahe, Copernicus, Georg von Peuerbach, and Regiomontanus, with some tracts on the value of ancient money, on the Roman calendar, and on the theory of music, to all which is appended a large and prolix piece entitled Notitia ecclesiae Diniensis; the sixth volume contains his correspondence. The Lives, especially those of Copernicus, Tycho and Peiresc, have been justly admired. That of Peiresc has been repeatedly printed; it has also been translated into English. Gassendi was one of the first after the revival of letters who treated the literature of philosophy in a lively way. His writings of this kind, though too laudatory and somewhat diffuse, have great merit; they abound in those anecdotal details, natural yet not obvious reflections, and vivacious turns of thought, which made Gibbon style him, with some extravagance certainly, though it was true enough up to Gassendi’s time—“le meilleur philosophe des littérateurs, et le meilleur littérateur des philosophes.”

Gassendi holds an honourable place in the history of physical science. He certainly added little to the stock of human knowledge, but the clearness of his exposition and the manner in which he, like Bacon, urged the importance of experimental research, were of inestimable service to the cause of science. To what extent any place can be assigned him in the history of philosophy is more doubtful. The Exercitationes on the whole seem to have excited more attention than they deserved. They contain little or nothing beyond what had been already advanced against Aristotle. The first book expounds clearly, and with much vigour, the evil effects of the blind acceptance of the Aristotelian dicta on physical and philosophical study; but, as is the case with so many of the anti-Aristotelian works of this period, the objections show the usual ignorance of Aristotle’s own writings. The second book, which contains the review of Aristotle’s dialectic or logic, is throughout Ramist in tone and method. The objections to Descartes—one of which at least, through Descartes’s statement of it in the appendix of objections in the Meditationes has become famous—have no speculative value, and in general are the outcome of the crudest empiricism. His labours on Epicurus have a certain historical value, but the want of consistency inherent in the philosophical system raised on Epicureanism is such as to deprive it of genuine worth. Along with strong expressions of empiricism we find him holding doctrines absolutely irreconcilable with empiricism in any form. For while he maintains constantly his favourite maxim “that there is nothing in the intellect which has not been in the senses” (nihil in intellectu quod non prius fuerit in sensu), while he contends that the imaginative faculty (phantasia) is the counterpart of sense—that, as it has to do with material images, it is itself, like sense, material, and essentially the same both in men and brutes; he at the same time admits that the intellect, which he affirms to be immaterial and immortal—the most characteristic distinction of humanity—attains notions and truths of which no effort of sensation or imagination can give us the slightest apprehension (Op. ii. 383). He instances the capacity of forming “general notions”; the very conception of universality itself (ib. 384), to which he says brutes, who partake as truly as men in the faculty called phantasia, never attain; the notion of God, whom he says we may imagine to be corporeal, but understand to be incorporeal; and lastly, the reflex action by which the mind makes its own phenomena and operations the objects of attention.

The Syntagma philosophicum, in fact, is one of those eclectic systems which unite, or rather place in juxtaposition, irreconcilable dogmas from various schools of thought. It is divided, according to the usual fashion of the Epicureans, into logic (which, with Gassendi as with Epicurus, is truly canonic), physics and ethics. The logic, which contains at least one praiseworthy portion, a sketch of the history of the science, is divided into theory of right apprehension (bene imaginari), theory of right judgment (bene proponere), theory of right inference (bene colligere), theory of right method (bene ordinare). The first part contains the specially empirical positions which Gassendi afterwards neglects or leaves out of account. The senses, the sole source of knowledge, are supposed to yield us immediately cognition of individual things; phantasy (which Gassendi takes to be material in nature) reproduces these ideas; understanding compares these ideas, which are particular, and frames general ideas. Nevertheless, he at the same time admits that the senses yield knowledge—not of things—but of qualities only, and holds that we arrive at the idea of thing or substance by induction. He holds that the true method of research is the analytic, rising from lower to higher notions; yet he sees clearly, and admits, that inductive reasoning, as conceived by Bacon, rests on a general proposition not itself proved by induction. He ought to hold, and in disputing with Descartes he did apparently hold, that the evidence of the senses is the only convincing evidence; yet he maintains, and from his special mathematical training it was natural he should maintain, that the evidence of reason is absolutely satisfactory. The whole doctrine of judgment, syllogism and method is a mixture of Aristotelian and Ramist notions.

In the second part of the Syntagma, the physics, there is more that deserves attention; but here, too, appears in the most glaring manner the inner contradiction between Gassendi’s fundamental principles. While approving of the Epicurean physics, he rejects altogether the Epicurean negation of God and particular providence. He states the various proofs for the existence of an immaterial, infinite, supreme Being, asserts that this Being is the author of the visible universe, and strongly defends the doctrine of the foreknowledge and particular providence of God. At the same time he holds, in opposition to Epicureanism, the doctrine of an immaterial rational soul, endowed with immortality and capable of free determination. It is altogether impossible to assent to the supposition of Lange (Gesch. des Materialismus, 3rd ed., i. 233), that all this portion of Gassendi’s system contains nothing of his own opinions, but is introduced solely from motives of self-defence. The positive exposition of atomism has much that is attractive, but the hypothesis of the calor vitalis (vital heat), a species of anima mundi (world-soul) which is introduced as physical explanation of physical phenomena, does not seem to throw much light on the special problems which it is invoked to solve. Nor is his theory of the weight essential to atoms as being due to an inner force impelling them to motion in any way reconcilable with his general doctrine of mechanical causes.

In the third part, the ethics, over and above the discussion on freedom, which on the whole is indefinite, there is little beyond a milder statement of the Epicurean moral code. The final end of life is happiness, and happiness is harmony of soul and body (tranquillitas animi et indolentia corporis). Probably, Gassendi thinks, perfect happiness is not attainable in this life, but it may be in the life to come.

The Syntagma is thus an essentially unsystematic work, and clearly exhibits the main characteristics of Gassendi’s genius. He was critical rather than constructive, widely read and trained thoroughly both in languages and in science, but deficient in speculative power and original force. Even in the department of natural science he shows the same inability steadfastly to retain principles and to work from them; he wavers between the systems of Brahe and Copernicus. That his revival of Epicureanism had an important influence on the general thinking of the 17th century may be admitted; that it has any real importance in the history of philosophy cannot be granted.

Authorities.—Gassendi’s life is given by Sorbière in the first collected edition of the works, by Bugerel, Vie de Gassendi (1737; 2nd ed., 1770), and by Damiron, Mémoire sur Gassendi (1839). An abridgment of his philosophy was given by his friend, the celebrated traveller, Bernier (Abrégé de la philosophie de Gassendi, 8 vols., 1678; 2nd ed., 7 vols., 1684). The most complete surveys of his work are those of G.S. Brett (Philosophy of Gassendi, London, 1908), Buhle (Geschichte der neuern Philosophie, iii. 1, 87-222), Damiron (Mémoires pour servir à l’histoire de philosophie au XVIIe siècle), and P.F. Thomas (La Philosophie de Gassendi, Paris, 1889). See also Ritter, Geschichte der Philosophie, x. 543-571; Feuerbach, Gesch. d. neu. Phil. von Bacon bis Spinoza, 127-150; F.X. Kiefl, P. Gassendis Erkenntnistheorie und seine Stellung zum Materialismus (1893) and “Gassendi’s Skepticismus” in Philos. Jahrb. vi. (1893); C. Güttler, “Gassend oder Gassendi?” in Archiv f. Gesch. d. Philos. x. (1897), pp. 238-242.

(R. Ad.; X.)

[1] It was formerly thought that Gassendi was really the genitive of the Latin form Gassendus. C. Güttler, however, holds that it is a modernized form of the O. Fr. Gassendy (see paper quoted in bibliography).

GASTEIN, in the duchy of Salzburg, Austria, a side valley of the Pongau or Upper Salzach, about 25 m. long and 1¼ m. broad, renowned for its mineral springs. It has an elevation of between 3000 and 3500 ft. Behind it, to the S., tower the mountains Mallnitz or Nassfeld-Tauern (7907 ft.) and Ankogel (10,673 ft.), and from the right and left of these mountains two smaller ranges run northwards forming its two side walls. The river Ache traverses the valley, and near Wildbad-Gastein forms two magnificent waterfalls, the upper, the Kesselfall (196 ft.), and the lower, the Bärenfall (296 ft.). Near these falls is the Schleierfall (250 ft.), formed by the stream which drains the Bockhart-see. The valley is also traversed by the so-called Tauern railway (opened up to Wildbad-Gastein in September 1905), which goes to Mallnitz, piercing the Tauern range by a tunnel 9260 yds. in length. The principal villages of the valley are Hof-Gastein, Wildbad-Gastein and Böckstein.

Hof-Gastein, pop. (1900) 840, the capital of the valley, is also a watering-place, the thermal waters being conveyed here from Wildbad-Gastein by a conduit 5 m. long, constructed in 1828 by the emperor Francis I. of Austria. Hof-Gastein was, after Salzburg, the richest place in the duchy, owing to its gold and silver mines, which were already worked during the Roman period. During the 16th century these mines were yielding annually 1180 ℔ of gold and 9500 ℔ of silver, but since the 17th century they have been much neglected and many of them are now covered by glaciers.

Wildbad-Gastein, commonly called Bad-Gastein, one of the most celebrated watering-places in Europe, is picturesquely situated in the narrow valley of the Gasteiner Ache, at an altitude of 3480 ft. The thermal springs, which issue from the granite mountains, have a temperature of 77°-120° F., and yield about 880,000 gallons of water daily. The water contains only 0.35 to 1000 of mineral ingredients and is used for bathing purposes. The springs are resorted to in cases of nervous affections, senile and general debility, skin diseases, gout and rheumatism. Wildbad-Gastein is annually visited by over 8500 guests. The springs were known as early as the 7th century, but first came into fame by a successful visit paid to them by Duke Frederick of Austria in 1436. Gastein was a favourite resort of William I. of Prussia and of the Austrian imperial family, and it was here that, on the 14th of August 1865, was signed the agreement known as the Gastein Convention, which by dividing the administration of the conquered provinces of Schleswig and Holstein between Austria and Prussia postponed for a while the outbreak of war between the two powers. It was also here (August-September 1879) that Prince Bismarck negotiated with Count Julius Andrássy the Austro-German treaty, which resulted in the formation of the Triple Alliance.

See Pröll, Gastein, Its Springs and Climate (Vienna, 5th ed., 1893).

GASTRIC ULCER (ulcer of the stomach), a disease of much gravity, commonest in females, and especially in anaemic domestic servants. It is connected in many instances with impairment of the circulation in the stomach and the formation of a clot in a small blood-vessel (thrombosis). It may be due to an impoverished state of the blood (anaemia), but it may also arise from disease of the blood-vessels, the result of long-continued indigestion and gastric catarrh.

When clotting takes place in a blood-vessel the nutrition of that limited area of the stomach is cut off, and the patch undergoes digestion by the unresisted action of the gastric juices, an ulcer being formed. The ulcer is usually of the size of a silver threepence or sixpence, round or oval, and, eating deeply, is apt to make a hole right through the coats of the stomach. Its usual site is upon the posterior wall of the upper curvature, near to the pyloric orifice. It may undergo a healing process at any stage, in which case it may leave but little trace of its existence; while, on the other hand, it may in the course of cicatrizing produce such an amount of contraction as to lead to stricture of the pylorus, or to a peculiar hour-glass deformity of the stomach. Perforation is in most cases quickly fatal, unless previously the stomach has become adherent to some neighbouring organ, by which the dangerous effects of this occurrence may be averted, or unless the condition has been promptly recognized and an operation has been quickly done. Usually there is but one ulcer, but sometimes there are several ulcers.

The symptoms of ulcer of the stomach are often indefinite and obscure, and in some cases the diagnosis has been first made on the occurrence of a fatal perforation. First among the symptoms is pain, which is present at all times, but is markedly increased after food. The pain is situated either at the lower end of the breast-bone or about the middle of the back. Sometimes it is felt in the sides. It is often extremely severe, and is usually accompanied with localized tenderness and also with a sense of oppression, and by an inability to wear tight clothing. The pain is due to the movements of the stomach set up by the presence of the food, as well as to the irritation of the inflamed nerve filaments in the floor of the ulcer. Vomiting is a usual symptom. It occurs either soon after the food is swallowed or at a later period, and generally relieves the pain and discomfort. Vomiting of blood (haematemesis) is a frequent and important symptom. The blood may show itself in the form of a brown or coffee-like mixture, or as pure blood of dark colour and containing clots. It comes from some vessel or vessels which the ulcerative process has ruptured. Blood is also found mixed with the discharges from the bowels, rendering them dark or tarry-looking. The general condition of the patient with gastric ulcer is, as a rule, that of extreme ill-health, with pallor, emaciation and debility. The tongue is red, and there is usually constipation. In most of the cases the disease is chronic, lasting for months or years; and in those cases where the ulcers are large or multiple, incomplete healing may take place, relapses occurring from time to time. But the ulcers may give rise to no marked symptoms, and there have been instances where fatal perforation suddenly took place, and where post-mortem examination revealed the existence of long-standing ulcers which had given rise to no suggestive symptoms. While gastric ulcer is to be regarded as dangerous, its termination, in the great majority of cases, is in recovery. It frequently, however, leaves the stomach in a delicate condition, necessitating the utmost care as regards diet. Occasionally the disease proves fatal by sudden haemorrhage, but a fatal result is more frequently due to perforation and the escape of the contents of the stomach into the peritoneal cavity, in which case death usually occurs in from twelve to forty-eight hours, either from shock or from peritonitis. Should the stomach become adherent to another organ, and fatal perforation be thus prevented, chronic “indigestion” may persist, owing to interference with the natural movements of the stomach. Stricture of the pylorus and consequent dilatation of the stomach may be caused by the cicatrization of an ulcer.

The patient should at once be sent to bed and kept there, and allowed for a while nothing stronger than milk and water or milk and lime water. But if bleeding has recently taken place no food whatever should be allowed by the stomach, and the feeding should be by nutrient enemata. As the symptoms quiet down, eggs may be given beaten up with milk, and later, bread and milk and home-made broths and soups. Thus the diet advances to chicken and vegetables rubbed through a sieve, to custard pudding and bread and butter. As regards medicines, iron is the most useful, but no pills of any sort should be given. Under the influence of rest and diet most gastric ulcers get well. The presence of healthy-looking scars upon the surface of the stomach, which are constantly found in operating upon the interior of the abdomen, or as revealed in post-mortem examinations, are evidence of the truth of this statement. It is unlikely that under the treatment just described perforation of the stomach will take place, and if the surgeon is called in to assist he will probably advise that operation is inadvisable. Moreover, he knows that if he should open the abdomen to search for an ulcer of the stomach he might fail to find it; more than that, his search might also be in vain if he opened the stomach itself and examined the interior. Serious haemorrhages, however, may make it necessary that a prompt and thorough search should be made in order that the surgeon may endeavour to locate the ulcer, and, having found it, secure the damaged vessel and save the patient from death by bleeding.

Perforation of a gastric ulcer having taken place, the septic germs, which were harmless whilst in the stomach, escape with the rest of the contents of the stomach into the general peritoneal cavity. The immediate effects of this leakage are sudden and severe pain in the upper part of the abdomen and a great shock to the system (collapse). The muscles of the abdominal wall become hard and resisting, and as peritonitis appears and the intestines are distended with gas, the abdomen is distended and becomes greatly increased in size and ceases to move, the respiratory movements being short and quick. At first, most likely, the temperature drops below normal, and the pulse quickens. Later, the temperature rises. If nothing is done, death from the septic poisoning of peritonitis is almost certain.

The treatment of ruptured gastric ulcer demands immediate operation. An incision should be made in the upper part of the middle line of the abdomen, and the perforation should be looked for. There is not, as a rule, much difficulty in finding it, as there are generally deposits of lymph near the spot, and other signs of local inflammation; moreover, the contents of the stomach may be seen escaping from the opening. The ulcer is to be closed by running a “purse-string” suture in the healthy tissue around it, and the place is then buried in the stomach by picking up small folds of the stomach-wall above and below it and fixing them together by suturing. This being done, the surface of the stomach, and the neighbouring viscera which have been soiled by the leakage, are wiped clean and the abdominal wound is closed, provision being made for efficient drainage. A large proportion of cases of perforated gastric ulcer thus treated recover.

(E. O.*)

GASTRITIS (Gr. γαστήρ, stomach), an inflammatory affection of the stomach, of which the condition of catarrh, or irritation of its mucous membrane, is the most frequent and most readily recognized. This may exist in an acute or a chronic form, and depends upon some condition, either local or general, which produces a congested state of the circulation in the walls of the stomach (see [Digestive Organs]: Pathology).

Acute Gastritis may arise from various causes. The most intense forms of inflammation of the stomach are the toxic conditions which follow the swallowing of corrosive poisons, such as strong mineral acids of alkalis which may extensively destroy the mucous membrane. Other non-corrosive poisons cause acute degeneration of the stomach wall (see [Poisons]). Acute inflammatory conditions may be secondary to zymotic diseases such as diphtheria, pyaemia, typhus fever and others. Gastritis is also caused by the ingestion of food which has begun to decompose, or may result from eating unsuitable articles which themselves remain undigested and so excite acute catarrhal conditions. These give rise to the symptoms well known as characterizing an acute “bilious attack,” consisting in loss of appetite, sickness or nausea, and headache, frontal or occipital, often accompanied with giddiness. The tongue is furred, the breath foetid, and there is pain or discomfort in the region of the stomach, with sour eructations, and frequently vomiting, first of food and then of bilious matter. An attack of this kind tends to subside in a few days, especially if the exciting cause be removed. Sometimes, however, the symptoms recur with such frequency as to lead to the more serious chronic form of the disease.

The treatment bears reference, in the first place, to any known source of irritation, which, if it exist, may be expelled by an emetic or purgative (except in cases due to poisoning). This, however, is seldom necessary, since vomiting is usually present. For the relief of sickness and pain the sucking of ice and counter-irritation over the region of the stomach are of service. Further, remedies which exercise a soothing effect upon an irritable mucous membrane, such as bismuth or weak alkaline fluids, and along with these the use of a light milk diet, are usually sufficient to remove the symptoms.

Chronic Gastric Catarrh may result from the acute or may arise independently. It is not infrequently connected with antecedent disease in other organs, such as the lungs, heart, liver or kidneys, and it is especially common in persons addicted to alcoholic excess. In this form the texture of the stomach is more altered than in the acute form, except in the toxic and febrile forms above referred to. It is permanently in a state of congestion, and its mucous membrane and muscular coat undergo thickening and other changes, which markedly affect the function of digestion. The symptoms are those of dyspepsia in an aggravated form (see [Dyspepsia]), of which discomfort and pain after food, with distension and frequently vomiting, are the chief; and the treatment must be conducted in reference to the causes giving rise to it. The careful regulation of the diet, alike as to the amount, the quality, and the intervals between meals, demands special attention. Feeding on artificially soured milk may in many cases be useful. Lavage or washing out of the stomach with weak alkaline solutions has been used with marked success in the treatment of chronic gastritis. Of medicinal agents, bismuth, arsenic, nux vomica, and the mineral acids are all of acknowledged efficacy, as are also preparations of pepsin.

GASTROPODA, the second of the five classes of animals constituting the phylum Mollusca. For a discussion of the relationship of the Gastropoda to the remaining classes of the phylum, see [Mollusca].

The Gastropoda are mainly characterized by a loss of symmetry, produced by torsion of the visceral sac. This torsion may be resolved into two successive movements. The first is a ventral flexure in the antero-posterior or sagittal plane; the result of this is to approximate the two ends of the alimentary canal. In development, the openings of the mantle-cavity and the anus are always originally posterior; later they are brought forward ventrally. During this first movement flexure is also produced by the coiling of the visceral sac and shell; primitively the latter was bowl-shaped; but the ventral flexure, which brings together the two extremities of the digestive tube, gives the visceral sac the outline of a more or less acute cone. The shell necessarily takes this form also, and then becomes coiled in a dorsal or anterior plane—that is to say, it becomes exogastric. This condition may be seen in embryonic Patellidae, Fissurellidae and Trochidae (fig. 1, A), and agrees with the method of coiling of a mollusc without lateral torsion, such as Nautilus. But ultimately the coil becomes ventral or endogastric, in consequence of the second torsion movement then apparent.

From Lankester’s Treatise on Zoology.
Fig. 1.—Three stages in the development of Trochus, during theprocess of torsion. (After Robert.)
A, Nearly symmetrical larva (veliger). B, A stage 1½ hours later than A. C, A stage 3½ hours later than B. f, Foot.	op, Operculum. pac, Pallial cavity. ve, Velum.

The shell is represented as fixed, while the head and foot rotate from left to right. In reality the head and foot are fixed and the shell rotates from right to left.

The second movement is a lateral torsion of the visceral mass, the foot remaining a fixed point; this torsion occurs in a plane approximately at right angles to that of the first movement, and carries the pallial aperture and the anus from behind forwards. If, at this moment, the animal were placed with mouth and ventral surface turned towards the observer, this torsion carries the circumanal complex in a clockwise direction (along the right side in dextral forms) through 180° as compared with its primitive condition. The (primitively) right-hand organs of the complex thus become left-hand, and vice versa. The visceral commissure, while still surrounding the digestive tract, becomes looped; its right half, with its proper ganglion, passes to the left side over the dorsal face of the alimentary canal (whence the name supra-intestinal), while the left half passes below towards the right side, thus originating the name infra-intestinal given to this half and to its ganglion. Next, the shell, the coil of which was at first exogastric, being also included in this rotation through 180°, exhibits an endogastric coiling (fig. 1, B, C). This, however, is not generally retained in one plane, and the spire projects, little by little, on the side which was originally left, but finally becomes right (in dextral forms, with a clockwise direction, if viewed from the side of the spire; but counter-clockwise in sinistral forms). Finally, the original symmetry of the circumanal complex vanishes; the anus leaves the centre of the pallial cavity and passes towards the right side (left side in sinistral forms); the organs of this side become atrophied and disappear. The essential feature of the asymmetry of Gastropoda is the atrophy or disappearance of the primitively left half of the circumanal complex (the right half in sinistral forms), including the gill, the auricle, the osphradium, the hypobranchial gland and the kidney.

In dextral Gastropods the only structure found on the topographically right side of the rectum is the genital duct. But this is not part of the primitive complex. It is absent in the most primitive and symmetrical forms, such as Haliotis and Pleurotomaria. Originally the gonads opened into the kidneys. In the most primitive existing Gastropods the gonad opens into the right kidney (Patellidae, Trochidae, Fissurellidae). The gonaduct, therefore, is derived from the topographically right kidney. The transformation has been actually shown to take place in the development of Paludina. In a dextral Gastropod the shell is coiled in a right-handed spiral from apex to mouth, and the spiral also projects to the right of the median plane of the animal.

When the shell is sinistral the asymmetry of the organs is usually reversed, and there is a complete situs inversus viscerum, the direction of the spiral of the shell corresponding to the position of the organs of the body. Triforis, Physa, Clausilia are examples of sinistral Gastropods, but reversal also occurs as an individual variation among forms normally dextral. But there are forms in which the involution is “hyperstrophic,” that is to say, the turns of the spire projecting but slightly, the spire, after flattening out gradually, finally becomes re-entrant and transformed into a false umbilicus; at the same time that part which corresponds to the umbilicus of forms with a normal coil projects and constitutes a false spire; the coil thus appears to be sinistral, although the asymmetry remains dextral, and the coil of the operculum (always the opposite to that of the shell) sinistral (e.g. Lanistes among Streptoneura, Limacinidae among Opisthobranchia). The same, mutatis mutandis, may occur in sinistral shells.

The problem of the causes of the torsion of the Gastropod body has been much discussed. E.R. Lankester in the ninth edition of this work attributed it to the pressure of the shell and visceral hump towards the right side. He referred also to the nautiloid shell of the larva falling to one side. But these are two distinct processes. In the larva a nautiloid shell is developed which is coiled exogastrically, that is, dorsally, and the pallial cavity is posterior or ventral (fig. 2, C): the larva therefore resembles Nautilus in the relations of body and shell. The shell then rotates towards the left side through 180°, so that it becomes ventral or endogastric (fig. 2, D). The pallial cavity, with its organs, is by this torsion moved up the right side of the larva to the dorsal surface, and thus the left organs become right and vice versa. In the subsequent growth of the shell the spire comes to project on the right side, which was originally the left. Neither the rotation of the shell as a whole nor its helicoid spiral coiling is the immediate cause of the torsion of the body in the individual, for the direction of the torsion is indicated in the segmentation of the ovum, in which there is a complete reversal of the cleavage planes in sinistral as compared with dextral forms. The facts, however, strongly suggest that the original cause of the torsion was the weight of the exogastric shell and visceral hump, which in an animal creeping on its ventral surface necessarily fell over to one side. It is not certain that the projection of the spire to the originally left side of the shell has anything to do with the falling over of the shell to that side. The facts do not support such a suggestion. In the larva there is no projection at the time the torsion takes place. In some forms the coiling disappears in the adult, leaving the shell simply conical as in Patellidae, Fissurellidae, &c., and in some cases the shell is coiled in one plane, e.g. Planorbis. In all these cases the torsion and asymmetry of the body are unaffected.

Fig 3.—Sketch of a model designed so as to show the effect oftorsion or rotation of the visceral hump in Streptoneurous Gastropoda.
A, Unrotated ancestral condition. B, Quarter-rotation. C, Complete semi-rotation (the limit). an, Anus. ln, rn, Primarily left nephridium and primarily right nephridium. lvg, Primarily left (subsequently the sub-intestinal) visceral ganglion. rvg, Primarily right (subsequently the sub-intestinal) visceral ganglion.	cerg, Cerebral ganglion. plg, Pleural ganglion. pedg, Pedal ganglion. abg, Abdominal ganglion. bucc, Buccal mass. W, Wooden arc representing the base-line of the wall of the visceral hump. x, x′, Pins fastening the elastic cord (representing the visceral nerve loop) to W.

The characteristic torsion attains its maximum effect among the majority of the Streptoneura. It is followed in some specialized Heteropoda and in the Euthyneura by a torsion in the opposite direction, or detorsion, which brings the anus farther back and untwists the visceral commissure (see Euthyneura, below). This conclusion has shown that the Euthyneura do not represent an archaic form of Gastropoda, but are themselves derived from streptoneurous forms. The difference between the two sub-classes has been shown to be slight; certain of the more archaic Tectibranchia (Actaeon) and Pulmonata (Chilina) still have the visceral commissure long and not untwisted. The fact that all the Euthyneura are hermaphrodite is not a fundamental difference; several Streptoneura are so, likewise Valvata, Oncidiopsis, Marsenina, Odostomia, Bathysciadium, Entoconcha.

Classification.—The class Gastropoda is subdivided as follows:

Sub-Class I.—Streptoneura

In this division the torsion of the visceral mass and visceral commissure is at its maximum, the latter being twisted into a figure of eight. The right half of the commissure with its ganglion is supra-intestinal, the left half with its ganglion infra-intestinal. In some cases each pleural ganglion is connected with the opposite branch of the visceral commissure by anastomosis with the pallial nerve, a condition which is called dialyneury; or there may be a direct connective from the pleural ganglion to the visceral ganglion of the opposite side, which is called zygoneury. The head bears only one pair of tentacles. The radular teeth are of several different kinds in each transverse row. The heart is usually posterior to the branchia (proso-branchiate). The sexes are usually separate.

The old division into Zygobranchia and Azygobranchia must be abandoned, for the Azygobranchiate Rhipidoglossa have much greater affinity to the Zygobranchiate Haliotidae and Fissurellidae than to the Azygobranchia in general. This is shown by the labial commissure and pedal cords of the nervous system, by the opening of the gonad into the right kidney, and by other points. Further, the Pleurotomariidae have been discovered to possess two branchiae. The sub-class is now divided into two orders: the Aspidobranchia in which the branchia or ctenidium is bipectinate and attached only at its base, and the Pectinibranchia in which the ctenidium is monopectinate and attached to the mantle throughout its length.

Fig. 4.—The Common Limpet (Patella vulgata) in its shell, seen fromthe pedal surface. (Lankester.)
x, y, The median antero-posterior axis. a, Cephalic tentacle. b, Plantar surface of the foot. c, Free edge of the shell. d, The branchial efferent vessel carrying aerated blood to theauricle, and here interrupting the circlet of gill lamellae.	e, Margin of the mantle-skirt. f, Gill lamellae (not ctenidia, but special pallial growths, comparablewith those of Pleurophyllidia). g, The branchial efferent vessel. h, Factor of the branchial advehent vessel. i, Interspaces between the muscular bundles of the root ofthe foot, causing the separate areae seen in fig. 5, c.

Order I. Aspidobranchia.—These are the most primitive Gastropods, retaining to a great degree the original symmetry of the organs of the pallial complex, having two kidneys, in some cases two branchiae, and two auricles. The gonad has no accessory organs and except in Neritidae no duct, but discharges into the right kidney.

Forms adapted to terrestrial life and to aerial respiration occur in various divisions of Gastropods, and do not constitute a single homogeneous group. Thus the Helicinidae, which are terrestrial, are now placed among the Aspidobranchia. In these there are neither branchia nor osphradium, and the pallial chamber which retains its large opening serves as a lung. Degeneration of the shell occurs in some members of the order. It is largely covered by the mantle in some Fissurellidae, is entirely internal in Pupilia and absent in Titiscaniidae.

The common limpet is a specially interesting and abundant example of the more primitive Aspidobranchia. The foot of the limpet is a nearly circular disk of muscular tissue; in front, projecting from and raised above it, are the head and neck (figs. 4, 13). The visceral hump forms a low conical dome above the sub-circular foot, and standing out all round the base of this dome so as completely to overlap the head and foot, is the circular mantle-skirt. The depth of free mantle-skirt is greatest in front, where the head and neck are covered in by it. Upon the surface of the visceral dome, and extending to the edge of the free mantle-skirt, is the conical shell. When the shell is taken away (best effected by immersion in hot water) the surface of the visceral dome is found to be covered by a black-coloured epithelium, which may be removed, enabling the observer to note the position of some organs lying below the transparent integument (fig. 5). The muscular columns (c) attaching the foot to the shell form a ring incomplete in front, external to which is the free mantle-skirt. The limits of the large area formed by the flap over the head and neck (ecr) can be traced, and we note the anal papilla showing through and opening on the right shoulder, so to speak, of the animal into the large anterior region of the sub-pallial space. Close to this the small renal organ (i, mediad) and the larger renal organ (k, to the right and posteriorly) are seen, also the pericardium (l) and a coil of the intestine (int) embedded in the compact liver.

Fig. 6.—Anterior portion of the sameLimpet, with the overhanging cephalichood removed. (Lankester.)
a, Cephalic tentacle. b, Foot. c, Muscular substance forming the root of the foot. d, The capito-pedal organs of Lankester (= rudimentary ctenidia). e, Mantle-skirt. f, Papilla of the larger nephridium. g, Anus.	h, Papilla of the smaller nephridium. i, Smaller nephridium. k, Larger nephridium. l, Pericardium. m, Cut edge of the mantle-skirt. n, Liver. p, Snout.

On cutting away the anterior part of the mantle-skirt so as to expose the sub-pallial chamber in the region of the neck, we find the right and left renal papillae (discovered by Lankester in 1867) on either side of the anal papilla (fig. 6), but no gills. If a similar examination be made of the allied genus Fissurella (fig. 17, d), we find right and left of the two renal apertures a right and left gill-plume or ctenidium, which here as in Haliotis and Pleurotomaria retain their original paired condition. In Patella no such plumes exist, but right and left of the neck are seen a pair of minute oblong yellow bodies (fig. 6, d), which were originally described by Lankester as orifices possibly connected with the evacuation of the generative products. On account of their position they were termed by him the “capito-pedal orifices,” being placed near the junction of head and foot. J.W. Spengel has, however, in a most ingenious way shown that these bodies are the representatives of the typical pair of ctenidia, here reduced to a mere rudiment. Near to each rudimentary ctenidium Spengel has discovered an olfactory patch or osphradium (consisting of modified epithelium) and an olfactory nerve-ganglion (fig. 8). It will be remembered that, according to Spengel, the osphradium of mollusca is definitely and intimately related to the gill-plume or ctenidium, being always placed near the base of that organ; further, Spengel has shown that the nerve-supply of this olfactory organ is always derived from the visceral loop. Accordingly, the nerve-supply affords a means of testing the conclusion that we have in Lankester’s capito-pedal bodies the rudimentary ctenidia. The accompanying diagrams (figs. 9, 10) of the nervous systems of Patella and of Haliotis, as determined by Spengel, show the identity in the origin of the nerves passing from the visceral loop to Spengel’s olfactory ganglion of the Limpet, and that of the nerves which pass from the visceral loop of Haliotis to the olfactory patch or osphradium, which lies in immediate relation on the right and on the left side to the right and left gill-plumes (ctenidia) respectively. The same diagrams serve to demonstrate the streptoneurous condition of the visceral loop in Aspidobranchia.

Thus, then, we find that the limpet possesses a symmetrically disposed pair of ctenidia in a rudimentary condition, and justifies its position among Aspidobranchia. At the same time it possesses a totally distinct series of functional gills, which are not derived from the modification of the typical molluscan ctenidium. These gills are in the form of delicate lamellae (fig. 4, f), which form a series extending completely round the inner face of the depending mantle-skirt. This circlet of gill-lamellae led Cuvier to class the limpets as Cyclobranchiata, and, by erroneous identification of them with the series of metamerically repeated ctenidia of Chiton, to associate the latter mollusc with the former. The gill-lamellae of Patella are processes of the mantle comparable with the plait-like folds often observed on the roof of the branchial chamber in other Gastropoda (e.g. Buccinum and Haliotis). They are termed pallial gills. The only other molluscs in which they are exactly represented are the curious Opisthobranchs Phyllidia and Pleurophyllidia (fig. 55). In these, as in Patella, the typical ctenidia are aborted, and the branchial function is assumed by close-set lamelliform processes arranged in a series beneath the mantle-skirt on either side of the foot. In fig. 4, d, the large branchial vein of Patella bringing blood from the gill-series to the heart is seen; where it crosses the series of lamellae there is a short interval devoid of lamellae.

The heart in Patella consists of a single auricle (not two as in Haliotis and Fissurella) and a ventricle; the former receives the blood from the branchial vein, the latter distributes it through a large aorta which soon leads into irregular blood-lacunae.

The existence of two renal organs in Patella, and their relation to the pericardium (a portion of the coelom), is important. Each renal organ is a sac lined with glandular epithelium (ciliated cell, with concretions) communicating with the exterior by its papilla, and by a narrow passage with the pericardium. The connexion with the pericardium of the smaller of the two renal organs was demonstrated by Lankester in 1867, at a time when the fact that the renal organ of the Mollusca, as a rule, opens into the pericardium, and is therefore a typical nephridium, was not known. Subsequent investigations carried on under the direction of the same naturalist have shown that the larger as well as the smaller renal sac is in communication with the pericardium. The walls of the renal sacs are deeply plaited and thrown into ridges. Below the surface these walls are excavated with blood-vessels, so that the sac is practically a series of blood-vessels covered with renal epithelium, and forming a meshwork within a space communicating with the exterior. The larger renal sac (remarkably enough, that which is aborted in other Anisopleura) extends between the liver and the integument of the visceral dome very widely. It also bends round the liver as shown in fig. 12, and forms a large sac on half of the upper surface of the muscular mass of the foot. Here it lies close upon the genital body (ovary or testis), and in such intimate relationship with it that, when ripe, the gonad bursts into the renal sac, and its products are carried to the exterior by the papilla on the right side of the anus (Robin, Dall). This fact led Cuvier erroneously to the belief that a duct existed leading from the gonad to this papilla. The position of the gonad, best seen in the diagrammatic section (fig. 13), is, as in other Aspidobranchia, devoid of a special duct communicating with the exterior. This condition, probably an archaic one, distinguishes the Aspidobranchia from other Gastropoda.

Fig. 10.—Nervous system of Haliotis; the visceral loop is lightlyshaded; the buccal ganglia are omitted. (After Spengel.)
ce, Cerebral ganglion. pl.pe, The fused pleural and pedal ganglia. pe, The right pedal nerve. ce.pl, The cerebro-pleural connective.	ce.pe, The cerebro-pedal connective. s, s′, Right and left mantle nerves. ab, Abdominal ganglion or site of same. o, o, Right and left olfactory ganglia and osphardia receiving nerve from visceral loop.

Fig. 11.—Nervous system ofFissurella. (From Gegenbaur, after Jhering.) pl, Pallial nerve. p, Pedal nerve. A, Abdominal ganglia in the streptoneurous visceral commissure, with supra- and sub-intestineganglion on each side. B, Buccal ganglia. C, C, Cerebral ganglia. es, Cerebral commissure. o, Otocysts attached to the cerebro-pedal connectives.

Fig. 12.—Diagram of the tworenal organs (nephridia), to show their relation to the rectum andto the pericardium. (Lankester.) f, Papilla of the larger nephridium. g, Anal papilla with rectum leading from it. h, Papilla of the smaller nephridium, which is only represented by dotted outlines. l, Pericardium indicated by a dotted outline—at its rightside are seen the two reno-pericardial pores. ff, The sub-anal tract of the large nephridium given off near itspapilla and seen through the unshaded smaller nephridium. ks.a, Anterior superior lobe of the large nephridium. ks.l, Left lobe of same. ks.p, Posterior lobe of same. ks.i, Inferior sub-visceral lobe of same.

Fig. 13.—Diagram of a vertical antero-postero median sectionof a Limpet. Letters as in figs. 6, 7, with following additions.(Lankester.)
q, Intestine in transverse section. r, Lingual sac (radular sac). rd, Radula. s, Lamellated stomach. t, Salivary gland. u, Duct of same. v, Buccal cavity	w, Gonad. br.a, Branchial advehent vessel (artery). br.v, Branchial efferent vessel (vein). bv, Blood-vessel. odm, Muscles and cartilage of the odontophore. cor, Heart within the pericardium.

Fig. 14.—Vertical section in a plane running right and left throughthe anterior part of the visceral hump of Patella to show the two renalorgans and their openings into the pericardium. (J.T. Cunningham.)
a, Large or external or right renal organ. ab, Narrow process of the same running below the intestine and leading by k into the pericardium. b, Small or median renal organ. c, Pericardium. d, Rectum. e, Liver.	f, Manyplies. g, Epithelium of the dorsal surface. h, Renal epithelium lining the renal sacs. i, Aperture connecting the small sac with the pericardium. k, Aperture connecting the large sac with the pericardium.

The digestive tract of Patella offers some interesting features. The odontophore is powerfully developed; the radular sac is extraordinarily long, lying coiled in a space between the mass of the liver and the muscular foot. The radula has 160 rows of teeth with twelve teeth in each row. Two pairs of salivary ducts, each leading from a salivary gland, open into the buccal chamber. The oesophagus leads into a remarkable stomach, plaited like the manyplies of a sheep, and after this the intestine takes a very large number of turns embedded in the yellow liver, until at last it passes between the two renal sacs to the anal papilla. A curious ridge (spiral? valve) which secretes a slimy cord is found upon the inner wall of the intestine. The general structure of the Molluscan intestine has not been sufficiently investigated to render any comparison of this structure of Patella with that of other Mollusca possible. The eyes of the limpet deserve mention as examples of the most primitive kind of eye in the Molluscan series. They are found one on each cephalic tentacle, and are simply minute open pits or depressions of the epidermis, the epidermic cells lining them being pigmented and connected with nerves (compare fig. 14, art. [Cephalopoda]). The limpet breeds upon the southern English coast in the early part of April, but its development has not been followed. It has simply been traced as far as the formation of a diblastula which acquires a ciliated band, and becomes a nearly spherical trochosphere. It is probable that the limpet takes several years to attain full growth, and during that period it frequents the same spot, which becomes gradually sunk below the surrounding surface, especially if the rock be carbonate of lime. At low tide the limpet (being a strictly intertidal organism) is exposed to the air, and (according to trustworthy observers) quits its attachment and walks away in search of food (minute encrusting algae), and then once more returns to the identical spot, not an inch in diameter, which belongs, as it were, to it. Several million limpets—twelve million in Berwickshire alone—are annually used on the east coast of Britain as bait.

Sub-order 1. Docoglossa.—Nervous system without dialyneury. Eyes are open invaginations without crystalline lens. Two osphradia present but no hypobranchial glands nor operculum. Teeth of radula beam-like, and at most three marginal teeth on each side. Heart has only a single auricle, neither heart nor pericardium traversed by rectum. Shell conical without spire.

Fam. 1.—Acmaeidae. A single bipectinate ctenidium on left side. Acmaea, without pallial branchiae, British. Scurria, with pallial branchiae in a circle beneath the mantle.

Fam. 2.—Tryblidiidae. Muscle scar divided into numerous impressions. Tryblidium, Silurian.

Fam. 3.—Patellidae. No ctenidia but pallial branchiae in a circle between mantle and foot. Patella, pallial branchiae forming a complete circle, no epipodial tentacles, British. Ancistromesus, radula with median central tooth. Nacella, epipodial tentacles present. Helcion, circlet of branchiae interrupted anteriorly, British.

Fam. 4.—Lepetidae. Neither ctenidia nor pallial branchiae. Lepeta, without eyes. Pilidium. Propilidium.

Fam. 5.—Bathysciadidae. Hermaphrodite; head with appendage on right side; radula without central tooth. Bathysciadium, abyssal.

Sub-order 2. Rhipidoglossa.—Aspidobranchia with a palliovisceral anastomosis (dialyneurous); eye-vesicle closed, with crystalline lens; ctenidia, osphradia and hypobranchial glands paired or single. Radula with very numerous marginal teeth arranged like the rays of a fan. Heart with two auricles; ventricle traversed by the rectum, except in the Helicinidae. An epipodial ridge on each side of the foot and cephalic expansions between the tentacles often present.

Fam. 1.—Pleurotomariidae. Shell spiral; mantle and shell with an anterior fissure; two ctenidia; a horny operculum. Pleurotomaria, epipodium without tentacles. Genus includes several hundred extinct species ranging from the Silurian to the Tertiary. Five living species from the Antilles, Japan and the Moluccas. Moluccan species is 19 cm. in height.

Fam. 2.—Bellerophontidae. 300 species, all fossil, from Cambrian to Trias.

Fam. 3.—Euomphalidae. Also extinct, from Cambrian to Cretaceous.

Fam. 4.—Haliotidae. Spire of shell much reduced; two bipectinate ctenidia, the right being the smaller; no operculum. Haliotis.

Fam. 5.—Velainiellidae, an extinct family from the Eocene.

Fam. 6.—Fissurellidae. Shell conical; slit or hole in anterior part of mantle; two symmetrical ctenidia; no operculum. Emarginula, mantle and shell with a slit, British. Scutum, mantle split anteriorly and reflected over shell, which has no slit. Puncturella, mantle and shell with a foramen in front of the apex, British. Fissurella, mantle and shell perforated at apex, British.

Fam. 7.—Cocculinidae. Shell conical, symmetrical, without slit or perforation. Cocculina, abyssal.

Fam. 8.—Trochidae. Shell spirally coiled; a single ctenidium; eyes perforated; a horny operculum; lobes between the tentacles. Trochus, shell umbilicated, spire pointed and prominent, British. Monodonta, no jaws, spire not prominent, no umbilicus, columella toothed. Gibbula, with jaws, three pairs of epipodial cirri without pigment spots at their bases, British. Margarita, five to seven pairs of epipodial cirri with a pigment spot at base of each.

Fig. 16.—Scutum,seen from the pedal surface. (Lankester.) o, Mouth. T, Cephalic tentacle. br, One of the two symmetrical gills placed on the neck.

Fig. 17.—Dorsal aspect of a specimen of Fissurella fromwhich the shell has been removed, whilst the anterior area of the mantle-skirt hasbeen longitudinally slit and its sides reflected. (Lankester.) a, Cephalic tentacle. b, Foot. d, Left (archaic right) gill-plume. e, Reflected mantle-flap. fi, The fissure or hole in the mantle-flap traversed by the longitudinal incision. f, Right (archaic left) nephridium’s aperture. g, Anus. h, Left (archaic right) aperture of nephridium. p, Snout.

Fam. 9.—Stomatellidae. Spire of shell much reduced; a single ctenidium. Stomatella, foot truncated posteriorly, an operculum present, no epipodial tentacles. Gena, foot elongated posteriorly, no operculum.

Fam. 10.—Delphinulidae. Shell spirally coiled; operculum horny; intertentacular lobes absent. Delphinula.

Fam. 11.—Liotiidae, shell globular, margin of aperture thickened. Liotia.

Fam. 12.—Cyclostrematidae. Shell flattened, umbilicated; foot anteriorly truncated with angles produced into lobes. Cyclostrema. Teinostoma.

Fam. 13.—Trochonematidae. All extinct, Cambrian to Cretaceous.

Fam. 14.—Turbinidae. Shell spirally coiled; epipodial tentacles present; operculum thick and calcareous. Turbo. Astralium. Molleria. Cyclonema.

Fam. 15.—Phasianellidae. Shell not nacreous, without umbilicus, with prominent spire and polished surface. Phasianella.

Fam. 16.—Umboniidae. Shell flattened, not umbilicated, generally smooth; operculum horny. Umbonium. Isanda.

Fam. 17.—Neritopsidae. Shell semi-globular, with short spire; operculum calcareous, not spiral. Neritopsis. Naticopsis, extinct.

Fam. 18.—Macluritidae. Extinct, Cambrian and Silurian.

Fam. 19.—Neritidae. Shell with very low spire, without umbilicus, internal partitions frequently absorbed; a single ctenidium; a cephalic penis present. Nerita, marine. Neritina, freshwater, British. Septaria, shell boat-shaped.

Fam. 20.—Titiscaniidae. Without shell and operculum, but with pallial cavity and ctenidium. Titiscania, Pacific.

Fam. 21.—Helicinidae. No ctenidium, but a pulmonary cavity; heart with a single auricle, not traversed by the rectum. Helicina. Eutrochatella. Stoastoma. Bourceria.

Fam. 22.—Hydrocenidae. No ctenidium, but a pulmonary cavity; operculum with an apophysis. Hydrocena, Dalmatia.

Fam. 23.—Proserpinidae. No operculum. Proserpina, Central America.

Order 2. Pectinibranchia.—In this order there is no longer any trace of bilateral symmetry in the circulatory, respiratory and excretory organs, the topographically right half of the pallial complex having completely disappeared, except the right kidney, which is represented by the genital duct. There is usually a penis in the male. The ctenidium is monopectinate and attached to the mantle along its whole length, except in Adeorbis and Valvata; in the latter alone it is bipectinate. There is a single well-developed, often pectinated osphradium. The eye is always a closed vesicle, and the internal cornea is extensive. In the radula there is a single central tooth or none.

Fig. 18.—Animal and shell of Pyrula laevigata. (From Owen.)
a, Siphon. b, Head-tentacles. C, Head, the letter placed near the right eye.	d, The foot, expanded as in crawling. h, The mantle-skirt reflected over the sides of the shell.

The former classification into Holochlamyda, Pneumochlamyda and Siphonochlamyda has been abandoned, as it was founded on adaptive characters not always indicative of true affinities. The order is now divided into two sub-orders: the Taenioglossa, in which there are three teeth on each side of the median tooth of the radula, and the Stenoglossa, in which there is only one tooth on each side of the median tooth. In the latter a pallial siphon, a well-developed proboscis and an unpaired oesophageal gland are always present, in the former they are usually absent. The siphon is an incompletely tubular outgrowth of the mantle margin on the left side, contained in a corresponding outgrowth of the edge of the shell-mouth, and serving to conduct water to the respiratory cavity.

The condition usually spoken of as a “proboscis” appears to be derived from the condition of a simple rostrum (having the mouth at its extremity) by the process of incomplete introversion of that simple rostrum. There is no reason in the actual significance of the word why the term “proboscis” should be applied to an alternately introversible and eversible tube connected with an animal’s body, and yet such is a very customary use of the term. The introversible tube may be completely closed, as in the “proboscis” of Nemertine worms, or it may have a passage in it leading into a non-eversible oesophagus, as in the present case, and in the case of the eversible pharynx of the predatory Chaetopod worms. The diagrams here introduced (fig. 19) are intended to show certain important distinctions which obtain amongst the various “introverts,” or intro- and e-versible tubes so frequently met with in animal bodies. Supposing the tube to be completely introverted and to commence its eversion, we then find that eversion may take place, either by a forward movement of the side of the tube near its attached base, as in the proboscis of the Nemertine worms, the pharynx of Chaetopods and the eye-tentacle of Gastropods, or by a forward movement of the inverted apex of the tube, as in the proboscis of the Rhabdocoel Planarians, and in that of Gastropods here under consideration. The former case we call “pleurecbolic” (fig. 19, A, B, C, H, I, K), the latter “acrecbolic” tubes or introverts (fig. 19, D, E, F, G). It is clear that, if we start from the condition of full eversion of the tube and watch the process of introversion, we shall find that the pleurecbolic variety is introverted by the apex of the tube sinking inwards; it may be called acrembolic, whilst conversely the acrecbolic tubes are pleurembolic. Further, it is obvious enough that the process either of introversion or of eversion of the tube may be arrested at any point, by the development of fibres connecting the wall of the introverted tube with the wall of the body, or with an axial structure such as the oesophagus; on the other hand, the range of movement of the tubular introvert may be unlimited or complete. The acrembolic proboscis or frontal introvert of the Nemertine worms has a complete range. So has the acrembolic pharynx of Chaetopods, if we consider the organ as terminating at that point where the jaws are placed and the oesophagus commences. So too the acrembolic eye-tentacle of the snail has a complete range of movement, and also the pleurembolic proboscis of the Rhabdocoel prostoma. The introverted rostrum of the Pectinibranch Gastropods presents in contrast to these a limited range of movement. The “introvert” in these Gastropods is not the pharynx as in the Chaetopod worms, but a prae-oral structure, its apical limit being formed by the true lips and jaws, whilst the apical limit of the Chaetopod’s introvert is formed by the jaws placed at the junction of pharynx and oesophagus, so that the Chaetopod’s introvert is part of the stomodaeum or fore-gut, whilst that of the Gastropod is external to the alimentary canal altogether, being in front of the mouth, not behind it, as is the Chaetopod’s. Further, the Gastropod’s introvert is pleurembolic (and therefore acrecbolic), and is limited both in eversion and in introversion; it cannot be completely everted owing to the muscular bands (fig. 19, G), nor can it be fully introverted owing to the bands (fig. 19, F) which tie the axial pharynx to the adjacent wall of the apical part of the introvert. As in all such intro- and e-versible organs, eversion of the Gastropod proboscis is effected by pressure communicated by the muscular body-wall to the liquid contents (blood) of the body-space, accompanied by the relaxation of the muscles which directly pull upon either the sides or the apex of the tubular organ. The inversion of the proboscis is effected directly by the contraction of these muscles. In various members of the Pectinibranchia the mouth-bearing cylinder is introversible (i.e. is a proboscis)—with rare exceptions these forms have a siphonate mantle-skirt. On the other hand, many which have a siphonate mantle-skirt are not provided with an introversible mouth-bearing cylinder, but have a simple non-introversible rostrum, as it has been termed, which is also the condition presented by the mouth-bearing region in nearly all other Gastropoda. One of the best examples of the introversible mouth-cylinder or proboscis which can be found is that of the common whelk (Buccinum undatum) and its immediate allies. In fig. 23 the proboscis is seen in an everted state; it is only so carried when feeding, being withdrawn when the animal is at rest. Probably its use is to enable the animal to introduce its rasping and licking apparatus into very narrow apertures for the purposes of feeding, e.g. into a small hole bored in the shell of another mollusc.

A, Simple introvert completely introverted.

B, The same, partially everted by eversion of the sides, as in the Nemertine proboscis and Gastropod eye-tentacle = pleurecbolic.

C, The same, fully everted.

D, E, A similar simple introvert in course of eversion by the forward movement, not of its sides, but of its apex, as in the proboscidean Rhabdocoels = acrecbolic.

F, Acrecbolic (= pleurembolic) introvert, formed by the snout of the proboscidiferous Gastropod. al, alimentary canal; d, the true mouth. The introvert is not a simple one with complete range both in eversion and introversion, but is arrested in introversion by the fibrous bands at c, and similarly in eversion by the fibrous bands at b.

G, The acrecbolic snout of a proboscidiferous Gastropod, arrested short of complete eversion by the fibrous band b.

H, The acrembolic (= pleurecbolic) pharynx of a Chaetopod fully introverted. al, alimentary canal; at d, the jaws; at a, the mouth; therefore a to d is stomodaeum, whereas in the Gastropod (F) a to d is inverted body-surface.

I, Partial eversion of H.

K, Complete eversion of H.

Fig. 20.—Male of Littorina littoralis,Lin., removed from its shell; the mantle-skirt cut along its right line ofattachment and thrown over to the left side of the animal so as to exposethe organs on its inner face. a, Anus. i, Intestine. r, Nephridium (kidney). r′, Aperture of the nephridium. c, Heart. br, Ctenidium (gill-plume). pbr, Parabranchia (= the osphradium or olfactory patch). x, Glandular lamellae of the inner face of the mantle-skirt. y, Adrectal (purpuriparous) gland. t, Testis. vd, Vas deferens. p, Penis. mc, Columella muscle (muscular process grasping the shell). v, Stomach. h, Liver. N.B.—Note the simple snout or rostrum not introverted as a “proboscis.”

Fig. 21.—Nervous system of Paludinaas a type of the streptoneurous condition. (From Gegenbaur, after Jhering.) B, Buccal (suboesophageal) ganglion. C, Cerebral ganglion. Co, Pleural ganglion. P, Pedal ganglion with otocyst attached. p, Pedal nerve. A, Abdominal ganglion at the extremity of the twisted visceral “loop.” sp, Supra-intestinal visceral ganglion on the course of the right visceral cord. sb, Sub-intestinal ganglion on the course of the left visceral cord.

The very large assemblage of forms coming under this order comprises the most highly developed predaceous sea-snails, numerous vegetarian species, a considerable number of freshwater and some terrestrial forms. The partial dissection of a male specimen of the common periwinkle, Littorina littoralis, drawn in fig. 20, will serve to exhibit the disposition of viscera which prevails in the group. The branchial chamber formed by the mantle-skirt overhanging the head has been exposed by cutting along a line extending backward from the letters vd to the base of the columella muscle mc, and the whole roof of the chamber thus detached from the right side of the animal’s neck has been thrown over to the left, showing the organs which lie upon the roof. No opening into the body-cavity has been made; the organs which lie in the coiled visceral hump show through its transparent walls. The head is seen in front resting on the foot and carrying a median non-retractile snout or rostrum, and a pair of cephalic tentacles at the base of each of which is an eye. In many Gastropoda the eyes are not thus sessile but raised upon special eye-tentacles (figs. 25, 56). To the right of the head is seen the muscular penis p, close to the termination of the vas deferens (spermatic duct) vd. The testis t occupies a median position in the coiled visceral mass. Behind the penis on the same side is the hook-like columella muscle, a development of the retractor muscle of the foot, which clings to the spiral column or columella of the shell (see fig. 33). This columella muscle is the same thing as the muscles adhering to the shell in Patella, and the posterior adductor of Lamellibranchs.

The surface of the neck is covered by integument forming the floor of the branchial cavity. It has not been cut into. Of the organs lying on the reflected mantle-skirt, that which in the natural state lay nearest to the vas deferens on the right side of the median line of the roof of the branchial chamber is the rectum i′, ending in the anus a. It can be traced back to the intestine i near the surface of the visceral hump, and it is found that the apex of the coil formed by the hump is occupied by the liver h and the stomach v. Pharynx and oesophagus are concealed in the head. The enlarged glandular structure of the walls of the rectum is frequent in the Pectinibranchia, as is also though not universal the gland marked y, next to the rectum. It is the adrectal gland, and in the genera Murex and Purpura secretes a colourless liquid which turns purple upon exposure to the atmosphere, and was used by the ancients as a dye. Near this and less advanced into the branchial chamber is the single renal organ or nephridium r with its opening to the exterior r′. Internally this glandular sac presents a second slit or aperture which leads into the pericardium (as is now found to be the case in all Mollusca). The heart c lying in the pericardium is seen in close proximity to the renal organ, and consists of a single auricle receiving blood from the gill, and of a single ventricle which pumps it through the body by an anterior and posterior aorta. The surface x of the mantle between the rectum and the gill-plume is thrown into folds which in many sea-snails (whelks or Buccinidae, &c.) are very strongly developed. The whole of this surface appears to be active in the secretion of a mucous-like substance. The single gill-plume br lies to the left of the median line in natural position. It corresponds to the right of the two primitive ctenidia in the untwisted archaic condition of the molluscan body, and does not project freely into the branchial cavity, but its axis is attached (by concrescence) to the mantle-skirt (roof of the branchial chamber). It is rare for the gill-plume of a Pectinibranch Gastropod to stand out freely as a plume, but occasionally this more archaic condition is exhibited as in Valvata (fig. 30). Next beyond (to the left of) the gill-plume we find the so-called parabranchia, which is here simple, but sometimes lamellated as in Purpura (fig. 22). This organ has, without reason, been supposed to represent the second ctenidium of the typical mollusc, which it cannot do on account of its position. It should be to the right of the anus were this the case. Spengel showed that the parabranchia of Gastropods is the typical olfactory organ or osphradium in a highly developed condition. The minute structure of the epithelium which clothes it, as well as the origin of the nerve which is distributed to the parabranchia, proves it to be the same organ which is found universally in molluscs at the base of each gill-plume, and tests the indrawn current of water by the sense of smell. The nerve to this organ is given off from the superior (original right, see fig. 3) visceral ganglion.

The figures which are given here of various Pectinibranchia are in most cases sufficiently explained by the references attached to them. As an excellent general type of the nervous system, attention may be directed to that of Paludina drawn in fig. 21. On the whole the ganglia are strongly individualized in the Pectinibranchia, nerve-cell tissue being concentrated in the ganglia and absent from the cords. At the same time, the junction of the visceral loop above the intestine prevents in all Streptoneura the shortening of the visceral loop, and it is rare to find a fusion of the visceral ganglia with either pleural, pedal or cerebral—a fusion which can and does take place where the visceral loop is not above but below the intestine, e.g. in the Euthyneura (fig. 48), Cephalopoda and Lamellibranchia. As contrasted with the Aspidobranchia, we find that in the Pectinibranchia the pedal nerves are distinctly nerves given off from the pedal ganglia, rather than cord-like nerve-tracts containing both nerve-cells or ganglionic elements and nerve-fibres. Yet in some Pectinibranchia (Paludina) a ladder-like arrangement of the two pedal nerves and their lateral branches has been detected. The histology of the nervous system of Mollusca has yet to be seriously inquired into.

The alimentary canal of the Pectinibranchia presents little diversity of character, except in so far as the buccal region is concerned. Salivary glands are present, and in some carnivorous forms (Dolium) these secrete free sulphuric acid (as much as 2% is present in the secretion), which assists the animal in boring holes by means of its rasping tongue through the shells of other molluscs upon which it preys. A crop-like dilatation of the gut and a recurved intestine, embedded in the compact yellowish-brown liver, the ducts of which open into it, form the rest of the digestive tract and occupy a large bulk of the visceral hump. The buccal region presents a pair of shelly jaws placed laterally upon the lips, and a wide range of variation in the form of the denticles of the lingual ribbon or radula.

Well-developed glandular invaginations occur in different positions on the foot in Pectinibranchia. The most important of these opens by the ventral pedal pore, situated in the median line in the anterior half of the foot. This organ is probably homologous with the byssogenous gland of Lamellibranchs. The aperture, which was formerly supposed to be an aquiferous pore, leads into an extensive and often ramified cavity surrounded by glandular tubules. The gland has been found in both sub-orders of the Pectinibranchia, in Cyclostoma and Cypraea among the Taenioglossa, in Hemifusus, Cassis, Nassa, Murex, Fasciolariidae, Turbinellidae, Olividae, Marginellidae and Conidae among the Stenoglossa. It was discovered by J.T. Cunningham that in Buccinum the egg-capsules are formed by this pedal gland and not by any accessory organ of the generative system. Such horny egg-capsules doubtless have the same origin in all other species in which they occur, e.g. Fusus, Pyrula, Purpura, Murex, Nassa, Trophon, Voluta, &c. The float of the pelagic Janthina, to which the egg-capsules are attached, probably is also formed by the secretion of the pedal gland.

Fig. 23.—A, Triton variegatum, to show the proboscis or buccalintrovert (e) in a state of eversion.
a, Siphonal notch of the shell occupied by the siphonal fold of the mantle-skirt (Siphonochlamyda). b, Edge of the mantle-skirt resting on the shell. c, Cephalic eye. d, Cephalic tentacle. e, Everted buccal introvert (proboscis).	f, Foot. g, Operculum. h, Penis. i, Under surface of the mantle-skirt forming the roof of the sub-pallial chamber.
B, Sole of the foot of Pyrula tuba, to show a, the pore usually saidto be “aquiferous” but probably the orifice of a gland; b, medianline of foot.

Other glands opening on or near the foot are: (1) The suprapedal gland opening in the middle line between the snout and the anterior border of the foot. It is most commonly found in sessile forms and in terrestrial genera such as Cyclostoma; (2) the anterior pedal gland opening into the anterior groove of the foot, generally present in aquatic species; (3) dorsal posterior mucous glands in certain Cyclostomatidae.

The foot of the Pectinibranchia, unlike the simple muscular disk of the Isopleura and Aspidobranchia, is very often divided into lobes, a fore, middle and hind lobe (pro-, meso- and meta-podium, see figs. 24 and 25). Very usually, but not universally, the metapodium carries an operculum. The division of the foot into lobes is a simple case of that much greater elaboration or breaking up into processes and regions which it undergoes in the class Cephalopoda. Even among some Gastropoda (viz. the Opisthobranchia) we find the lobation of the foot still further carried out by the development of lateral lobes, the parapodia, whilst there are many Pectinibranchia, on the other hand, in which the foot has a simple oblong form without any trace of lobes.

The development of the Pectinibranchia has been followed in several examples, e.g. Paludina, Purpura, Nassa, Vermetus, Neritina. As in other Molluscan groups, we find a wide variation in the early process of the formation of the first embryonic cells, and their arrangement as a diblastula, dependent on the greater or less amount of food-yolk which is present in the egg-cell when it commences its embryonic changes. In fig. 26 the early stages of Paludina vivipara are represented. There is but very little food-material in the egg of this Pectinibranch, and consequently the diblastula forms by invagination; the blastopore or orifice of invagination coincides with the anus, and never closes entirely. A well-marked trochosphere is formed by the development of an equatorial ciliated band; and subsequently, by the disproportionate growth of the lower hemisphere, the trochosphere becomes a veliger. The primitive shell-sac or shell-gland is well marked at this stage, and the pharynx is seen as a new ingrowth (the stomodaeum), about to fuse with and open into the primitively invaginated arch-enteron (fig. 26, F).

Fig. 24.—Animal and shell of Phorus exutus.
a, Snout (not introversible). b, Cephalic tentacles. c, Right eye.	d, Pro- and meso-podium; to the right of this is seen the metapodiumbearing the sculptured operculum.

Fig. 25.—Animal and shell of Rostellaria rectirostris. (FromOwen.)
a, Snout or rostrum. b, Cephalic tentacle. c, Eye. d, Propodium and mesopodium.	e, Metapodium. f, Operculum. h′, Prolonged siphonal notch of the shell occupied by the siphon,or trough-like process of the mantle-skirt.

In other Pectinibranchia (and such variations are representative for all Mollusca, and not characteristic only of Pectinibranchia) we find that there is a very unequal division of the egg-cell at the commencement of embryonic development, as in Nassa. Consequently there is, strictly speaking, no invagination (emboly), but an overgrowth (epiboly) of the smaller cells to enclose the larger. The general features of this process and of the relation of the blastopore to mouth and anus have been explained in treating of the development of Mollusca generally. In such cases the blastopore may entirely close, and both mouth and anus develop as new ingrowths (stomodaeum and proctodaeum), whilst, according to the observations of N. Bobretzky, the closed blastopore may coincide in position with the mouth in some instances (Nassa, &c.), instead of with the anus. But in these epibolic forms, just as in the embolic Paludina, the embryo proceeds to develop its ciliated band and shell-gland, passing through the earlier condition of a trochosphere to that of the veliger. In the veliger stage many Pectinibranchia (Purpura, Nassa, &c.) exhibit, in the dorsal region behind the head, a contractile area of the body-wall. This acts as a larval heart, but ceases to pulsate after a time. Similar rhythmically contractile areas are found on the foot of the embryo Pulmonate Limax and on the yolk-sac (distended foot-surface) of the Cephalopod Loligo. The preconchylian invagination or shell-gland is formed in the embryo behind the velum, on the surface opposite the blastopore. It is surrounded by a ridge of cells which gradually extends over the visceral sac and secretes the shell. In forms which are naked in the adult state, the shell falls off soon after the reduction of the velum, but in Cenia, Runcina and Vaginula the shell-gland and shell are not developed, and the young animal when hatched has already the naked form of the adult.

Fig. 26.—Development of the River-Snail, Paludina vivipara.(After Lankester, 17.)
dc, Directive corpuscle (outcast cell). ae, Arch-enteron or cavity lined by the enteric cell-layer or endoderm. bl, Blastopore. vr, Velum or circlet of ciliated cells. dv, Velar area or cephalic dome. sm, Site of the as yet unformed mouth.	f, Foot. mes, Rudiments of the skeleto-trophic tissues. pi, The pedicle of invagination, the future rectum. shgl, The primitive shell-sac or shell-gland. m, Mouth. an, Anus.

A, Diblastula phase (optical section).

B, The diblastula has become a trochosphere by the development of the ciliated ring vr (optical section).

C, Side view of the trochosphere with commencing formation of the foot.

D, Further advanced trochosphere (optical section).

E, The trochosphere passing to the veliger stage, dorsal view showing the formation of the primitive shell-sac.

F, Side view of the same, showing foot, shell-sac (shgl), velum (vr), mouth and anus.

N.B.—In this development the blastopore is not elongated; it persists as the anus. The mouth and stomodaeum form independently of the blastopore.

One further feature of the development of the Pectinibranchia deserves special mention. Many Gastropoda deposit their eggs, after fertilization, enclosed in capsules; others, as Paludina, are viviparous; others, again, as the Zygobranchia, agree with the Lamellibranch Conchifera (the bivalves) in having simple exits for the ova without glandular walls, and therefore discharge their eggs unenclosed in capsules freely into the sea-water; such unencapsuled eggs are merely enclosed each in its own delicate chorion. When egg-capsules are formed they are often of large size, have tough walls, and in each capsule are several eggs floating in a viscid fluid. In some cases all the eggs in a capsule develop; in other cases one egg only in a capsule (Neritina), or a small proportion (Purpura, Buccinum), advance in development; the rest are arrested either after the first process of cell-division (cleavage) or before that process. The arrested embryos or eggs are then swallowed and digested by those in the same capsule which have advanced in development. This is clearly the same process in essence as that of the formation of a vitellogenous gland from part of the primitive ovary, or of the feeding of an ovarian egg by the absorption of neighbouring potential eggs; but here the period at which the sacrifice of one egg to another takes place is somewhat late. What it is that determines the arrest of some eggs and the progressive development of others in the same capsule is at present unknown.

Fig. 27.—Oxygyrus Keraudrenii.(From Owen.)
a, Mouth and odontophore. b, Cephalic tentacles. c, Eye. d, Propodium (B) and mesopodium. e, Metapodium. f, Operculum. h, Mantle-chamber. i, Ctenidium (gill-plume). k, Retractor muscle of foot. l, Optic tentacle. m, Stomach.	n, Dorsal surface overhung by the mantle-skirt; the letter is close to the salivary gland. o, Rectum and anus. p, Liver. q, Renal organ (nephridium). s, Ventricle. u, The otocyst attached to the cerebral ganglion. w, Testis. x, Auricle of the heart. y, Vesicle on genital duct. z, Penis.

In the tribe of Pectinibranchia called Heteropoda the foot takes the form of a swimming organ. The nervous system and sense organs are highly developed. The odontophore also is remarkably developed, its lateral teeth being mobile, and it serves as an efficient organ for attacking the other pelagic forms on which the Heteropoda prey. The sexes are distinct, as in all Streptoneura; and genital ducts and accessory glands and pouches are present, as in all Pectinibranchia. The Heteropoda exhibit a series of modifications in the form and proportions of the visceral mass and foot, leading from a condition readily comparable with that of a typical Pectinibranch such as Rostellaria, with the three regions of the foot strongly marked and a coiled visceral hump of the usual proportions, up to a condition in which the whole body is of a tapering cylindrical shape, the foot a plate-like vertical fin, and the visceral hump almost completely atrophied. Three steps of this modification may be distinguished as three families:—Atlantidae, Carinariidae and Pterotrachaeidae. They are true Pectinibranchia which have taken to a pelagic life, and the peculiarities of structure which they exhibit are strictly adaptations consequent upon their changed mode of life. Such adaptations are the transparency and colourlessness of the tissues, and the modifications of the foot, which still shows in Atlanta the form common in Pectinibranchia (compare fig. 27 and fig. 24). The cylindrical body of Pterotrachaea is paralleled by the slug-like forms of Euthyneura. J.W. Spengel has shown that the visceral loop of the Heteropoda is streptoneurous. Special to the Heteropoda is the high elaboration of the lingual ribbon, and, as an agreement with some of the opisthobranchiate Euthyneura, but as a difference from the Pectinibranchia, we find the otocysts closely attached to the cerebral ganglia. This is, however, less of a difference than it was at one time supposed to be, for it has been shown by H. Lacaze-Duthiers, and also by F. Leydig, that the otocysts of Pectinibranchia even when lying close upon the pedal ganglion (as in fig. 21) yet receive their special nerve (which can sometimes be readily isolated) from the cerebral ganglion (see fig. 11). Accordingly the difference is one of position of the otocyst and not of its nerve-supply. The Heteropoda are further remarkable for the high development of their cephalic eyes, and for the typical character of their osphradium (Spengel’s olfactory organ). This is a groove, the edges of which are raised and ciliated, lying near the branchial plume in the genera which possess that organ, whilst in Firoloida, which has no branchial plume, the osphradium occupies a corresponding position. Beneath the ciliated groove is placed an elongated ganglion (olfactory ganglion) connected by a nerve to the supra-intestinal (therefore the primitively dextral) ganglion of the long visceral nerve-loop, the strands of which cross one another—this being characteristic of Streptoneura (Spengel).

Fig. 28.—Carinaria mediterranea. (From Owen.) A, The animal. B, The shell removed. C, D, Two views of the shell of Cardiopoda.
a, Mouth and odontophore. b, Cephalic tentacles. c, Eye. d, The fin-like mesopodium. d’, Its sucker. e, Metapodium. f, Salivary glands. h, Border of the mantle-flap. i, Ctenidium (gill-plume). m, Stomach.	n, Intestine. o, Anus. p, Liver. t, Aorta, springing from the ventricle. u, Cerebral ganglion. v, Pleural and pedal ganglion. w, Testis. x, Visceral ganglion. y, Vesicula seminalis. z, Penis.

The Heteropoda belong to the “pelagic fauna” occurring near the surface in the Mediterranean and great oceans in company with the Pteropoda, the Siphonophorous Hydrozoa, Salpae, Leptocephali, and other specially-modified transparent swimming representatives of various groups of the animal kingdom. In development they pass through the typical trochosphere and veliger stages provided with boat-like shell.

Sub-order 1.—Taenioglossa. Radula with a median tooth and three teeth on each side of it. Formula 3 : 1 : 3.

Tribe 1.—Platypoda. Normal Taenioglossa of creeping habit. The foot is flattened ventrally, at all events in its anterior part (Strombidae). Otocysts situated close to the pedal nerve-centres. Accessory organs are rarely found on the genital ducts, but occur in Paludina, Cyclostoma, Naticidae, Calyptraeidae, &c. Mandibles usually present. This is the largest group of Mollusca, including nearly sixty families, some of which are insufficiently known from the anatomical point of view.

Fam. 1.—Paludinidae. Pedal centres in the form of ganglionated cords; kidney provided with a ureter; viviparous; fluviatile. Paludina. Neothauma, from Lake Tanganyika. Tylopoma, extinct, Tertiary.

Fig. 29.—Pterotrachea mutica seen from the right side.(After Keferstein.)
a, Pouch for reception of the snout when retracted. c, Pericardium. ph, Pharynx. oc, Cephalic eye. g, Cerebral ganglion. g’, Pleuro-pedal ganglion. pr, Foot (mesopodium).	v, Stomach. i, Intestine. n, So-called nucleus. br, Branchial plume (ctenidium). w, Osphradium. mt, Foot (metapodium). z, Caudal appendage.

Fam. 2.—Cyclophoridae. No ctenidium, pallial cavity transformed into a lung; aperture of shell circular; terrestrial. Pomatias, shell turriculated. Diplommatina. Hybocystis. Cyclophorus, shell umbilicated, with a short spire and horny operculum. Cyclosurus, shell uncoiled. Dermatocera, foot with a horn-shaped protuberance at its posterior end. Spiraculum.

Fam. 3.—Ampullariidae. To the left of the ctenidium a pulmonary sac, separated from it by an incomplete septum, amphibious. Ampullaria, shell dextral, coiled. Lanistes, shell sinistral, spire short or obsolete. Meladomus.

Fam. 4.—Littorinidae. Oesophageal pouches present; pedal nerve-centres concentrated; a pedal penis near the right tentacle. Littorina, shell not umbilicated, littoral habit. Lacuna, foot with two posterior appendages, marine, entirely aquatic. Cremnoconchus, entirely aerial, Indian. Risella. Tectarius.

Fam. 5.—Fossaridae. Head with two lobes in some Rhipidoglossa. Fossaria.

Fam. 6.—Purpurinidae, extinct.

Fam. 7.—Planaxidae. Shell with pointed spire; a short pallial siphon. Planaxis.

Fam. 8.—Cyclostomatidae. Pallial cavity transformed into a lung; pedal centres concentrated; a deep pedal groove. Cyclostoma, shell turbinated, operculum calcareous, British. Omphalotropis.

Fam. 9.—Aciculidae. Pallial cavity transformed into a lung; operculum horny; shell narrow and elongated. Acicula.

Fam. 10.—Valvatidae. Ctenidium bipectinate, free; hermaphrodite; fluviatile. Valvata, British.

Fam. 11.—Rissoidae. Epipodial filaments present; one or two pallial tentacles. Rissoa. Rissoina. Stiva.

Fam. 12.—Litiopidae. An epipodium bearing three pairs of tentacles and an operculigerous lobe with two appendages; inhabitants of the Sargasso weed. Litiopa.

Fam. 13.—Adeorbiidae. Mantle with two posterior appendages; ctenidium large and capable of protrusion from pallial cavity. Adeorbis, British.

Fam. 14.—Jeffreysiidae. Head with two long labial palps; shell ovoid; operculum horny, semicircular, carinated. Jeffreysia.

Fam. 15.—Homalogyridae. Shell flattened; no cephalic tentacles. Homalogyra, British. Ammoniceras.

Fam. 16.—Skeneidae. Shell depressed, with rounded aperture; cephalic tentacles long. Skenea, British.

Fam. 17.—Choristidae. Shell spiral; four cephalic tentacles; eyes absent; two pedal appendages. Choristes.

Fam. 18.—Assimineidae. Eyes at free extremities of tentacles. Assiminea, estuarine, British.

Fam. 19.—Truncatellidae. Snout very long, bilobed; foot short. Truncatella.

Fam. 20.—Hydrobiidae. Shell with prominent spire; penis distant from right tentacle, generally appendiculated; brackish water or fluviatile. Hydrobia, British. Baikalia, from Lake Baikal. Pomatiopsis. Bithynella. Lithoglyphus. Spekia, viviparous, from Lake Tanganyika. Tanganyicia. Limnotrochus, from Lake Tanganyika. Chytra. Littorinida. Bithynia, British, fluviatile. Stenothyra.

Fam. 21.—Melaniidae. Spire of shell somewhat elongated; mantle-border fringed; viviparous; fluviatile. Melania. Faunus. Paludomus. Melanopsis. Nassopsis. Bythoceras, from Lake Tanganyika.

Fam. 22.—Typhobiidae. Foot wide; shell turriculated, with carinated whorls, the carinae tuberculated or spiny. Typhobia. Bathanalia, from Lake Tanganyika.

Fam. 23.—Pleuroceridae. Like Melaniidae, but mantle-border not fringed and reproduction oviparous. Pleurocera. Anculotus.

Fam. 24.—Pseudomelaniidae. All extinct.

Fam. 25.—Subulitidae. All extinct.

Fam. 26.—Nerineidae. All extinct.

Fam. 27.—Cerithiidae. Shell with numerous tuberculated whorls; aperture canaliculated anteriorly; short pallial siphon. Cerithium. Bittium. Potamides. Triforis. Laeocochlis. Cerithiopsis.

Fam. 28.—Modulidae. Shell with short spire; no siphon. Modulus.

Fam. 29.—Vermetidae. Animal fixed by the shell, the last whorls of which are not in contact with each other; foot small; two anterior pedal tentacles. Vermetus. Siliquaria.

Fam. 30.—Caecidae. Shell almost completely uncoiled, in one plane, with internal septa. Caecum, British.

Fam. 31.—Turritellidae. Shell very long; head large; foot broad. Turritella, British. Mesalia. Mathilda.

Fam. 32.—Struthiolariidae. Shell conical; aperture slightly canaliculated; siphon slightly developed. Struthiolaria.

Fam. 33.—Chenopodidae. Shell elongated; aperture expanded; siphon very short. Chenopus, British. Alaria, Spinigera, Diartema, extinct.

Fam. 34.—Strombidae. Foot narrow, compressed, without sole. Strombus. Pteroceras. Rostellaria. Terebellum.

Fam. 35.—Xenophoridae. Foot transversely divided into two parts. Xenophorus. Eotrochus, Silurian.

Fam. 36.—Capulidae. Shell conical, not coiled, but slightly incurved posteriorly; a tongue-shaped projection between snout and foot. Capulus. Thyca, parasitic on asterids. Platyceras, extinct.

Fam. 37.—Hipponycidae. Shell conical; foot secreting a ventral calcareous plate; animal fixed. Hipponyx. Mitrularia.

Fam. 38.—Calyptraeidae. Shell with short spire; lateral cervical lobes present; accessory genital glands. Calyptraea, British. Crepidula. Crucibulum.

Fam. 39.—Naricidae. Foot divided into two, posterior half bearing the operculum; a wide epipodial velum; shell turbinated. Narica.

Fam. 40.—Naticidae. Foot large, with aquiferous system; propodium reflected over head; eyes degenerate; burrowing habit. Natica, British. Amaura. Sigaretus.

Fam. 41.—Lamellariidae. Shell thin, more or less covered by the mantle; no operculum. Lamellaria. Velutina. Marsenina, Oncidiopsis, hermaphrodite.

Fam. 42.—Trichotropidae. Shell with short spire, carinate and pointed. Trichotropis.

Fam. 43.—Seguenziidae. Shell trochiform, with canaliculated aperture and twisted columella. Seguenzia, abyssal.

Fam. 44.—Janthinidae. Shell thin; operculum absent; tentacles bifid; foot secretes a float; pelagic. Janthina. Recluzia.

Fam. 45.—Cypraeidae. Shell inrolled, solid, polished, aperture very narrow in adult; short siphon; anus posterior; osphradium with three lobes; mantle reflected over shell. Cypraea. Pustularia. Ovula. Pedicularia, attached to corals. Erato.

Fam. 46.—Tritonidae. Shell turriculated and siphonated, thick, each whorl with varices; foot broad and truncated anteriorly; pallial siphon well developed; proboscis present. Triton. Persona. Ranella.

Fam. 47.—Columbellinidae. All extinct.

Fam. 48.—Cassididae. Shell ventricose, with elongated aperture, and short spire; proboscis and siphon long; operculum with marginal nucleus. Cassis. Cassidaria. Oniscia.

Fam. 49—Oocorythidae. Shell globular and ventricose; aperture oval and canaliculated; operculum spiral. Oocorys, abyssal.

Fam. 50.—Doliidae. Shell ventricose, with short spire, and wide aperture; no varices and no operculum; foot very broad, with projecting anterior angles; siphon long. Dolium. Pyrula.

Fam. 51.—Solariidae. Solarium. Torinia. Fluxina.

Fam. 52.—Scalariidae. Shell turriculated, with elongated spire; proboscis short; siphon rudimentary. Scalaria. Eglisia. Crossea. Aclis.

The three following families have neither radula nor jaws, and are therefore called Aglossa. They have a well-developed proboscis which is used as a suctorial organ; some are abyssal, but the majority are either commensals or parasites of Echinoderms.

Fam. 53.—Pyramidellidae. Summit of spire heterostrophic; a projection, the mentum, between head and foot; operculum present. Pyramidella. Turbonilla. Odostomia, British. Myxa.

Fam. 54.—Eulimidae. Visceral mass still coiled spirally; shell thin and shining. Eulima, foot well developed, with an operculum, animal usually free, but some live in the digestive cavity of Holothurians. Mucronalia, foot reduced, but still operculate, eyes present, animal fixed by its very long proboscis which is deeply buried in the tissues of an Echinoderm, no pseudopallium. Stylifer, the operculum is lost, animal fixed by a large proboscis which forms a pseudopallium covering the whole shell except the extremity of the spire, parasitic on all groups of Echinoderms. Entosiphon, visceral mass still coiled; shell much reduced, proboscis very long forming a pseudopallium which covers the whole body and projects beyond in the form of a siphon, foot and nervous system present, eyes, branchia and anus absent, parasite in the Holothurian Deima blakei in the Indian Ocean.

Fam. 55.—Entoconchidae. No shell; visceral mass not coiled; no sensory organs, nervous system, branchia or anus; body reduced to a more or less tubular sac; hermaphrodite and viviparous; parasitic in Holothurians; larvae are veligers, with shell and operculum. Entocolax, mouth at free extremity, animal fixed by aboral orifice of pseudopallium, Pacific. Entoconcha, body elongated and tubular, animal fixed by the oral extremity, protandric hermaphrodite, parasitic in testes of Holothurians causing their abortion. Enteroxenos, no pseudopallium and no intestine, hermaphrodite, larvae with operculum.

Tribe 2.—Heteropoda. Pelagic Taenioglossa with foot large and laterally compressed to form a fin.

Fam. 1. Atlantidae. Visceral sac and shell coiled in one plane; foot divided transversely into two parts, posterior part bearing an operculum, anterior part forming a fin provided with a sucker. Atlanta. Oxygyrus.

Fam. 2.—Carinariidae. Visceral sac and shell small in proportion to the rest of the body, which cannot be withdrawn into the shell; foot elongated, fin-shaped, with sucker, but without operculum. Carinaria. Cardiopoda.

Fam. 3.—Pterotrachaeidae. Visceral sac very much reduced; without shell or mantle; anus posterior; foot provided with sucker in male only. Pterotrachaea. Firoloida. Pterosoma.

Sub-order 2.—Stenoglossa. Radula narrow with one lateral tooth on each side, and one median tooth or none.

Tribe 1.—Rachiglossa. Radula with a median tooth and a single tooth on each side of it. Formula 1 : 1 : 1. Rudimentary jaws present.

Fam. 1.—Turbinellidae. Shell solid, piriform, with thick folded columella; lateral teeth of radula bicuspidate. Turbinella. Cynodonta. Fulgur. Hemifusus. Tudicla. Strepsidura.

Fam. 2.—Fasciolariidae. Shell elongated, with long siphon; lateral teeth of radula multicuspidate. Fasciolaria. Fusus. Clavella. Latirus.

Fam. 3.—Mitridae. Shell fusiform and solid, aperture elongated, columella folded; no operculum; eyes on sides of tentacles. Mitra. Turricula. Cylindromitra. Imbricaria.

Fam. 4.—Buccinidae. Foot large and broad; eyes at base of tentacles; operculum horny. Buccinum. Chrysodomus. Liomesus. Cominella. Tritonidea. Pisania. Euthria. Phos. Dipsacus.

Fam. 5.—Nassidae. Foot broad, with two slender posterior appendages; operculum unguiculate. Nassa, marine, British. Canidia, fluviatile. Bullia.

Fam. 6.—Muricidae. Shell with moderately long spire and canal, ornamented with ribs, often spiny; foot truncated anteriorly. Murex, British. Trophon, British. Typhis. Urosalpinx. Lachesis.

Fam. 7.—Purpuridae. Shell thick, with short spire, last whorl large and canal short; aperture wide; operculum horny. Purpura, British. Rapana. Monoceros. Sistrum. Concholepas.

Fam. 8.—Haliidae. Shell ventricose, thin and smooth, with wide aperture; foot large and thick, without operculum. Halia.

Fam. 9.—Cancellariidae. Shell ovoid, with short spire and folded columella; foot small, no operculum; siphon short. Cancellaria.

Fam. 10.—Columbellidae. Spire of shell prominent, aperture narrow, canal very short, columella crenelated; foot large. Columbella.

Fam. 11.—Coralliophilidae. Shell irregular; radula absent; foot and siphon short; sedentary animals, living in corals. Coralliophila. Rhizochilus. Leptoconchus. Magilus. Rapa.

Fam. 12.—Volutidae. Head much flattened and wide, with eyes on sides; foot broad; siphon with internal appendages. Valuta. Guivillea. Cymba.

Fam. 13.—Olividae. Foot with anterior transverse groove; a posterior pallial tentacle; generally burrowing. Olivia. Olivella. Ancillaria. Agaronia.

Fam. 14.—Marginellidae. Foot very large; mantle reflected over shell. Marginella. Pseudomarginella.

Fam. 15.—Harpidae. Foot very large; without operculum; shell with short spire and longitudinal ribs; siphon long. Harpa.

Tribe 2.—Toxiglossa. No jaws. No median tooth in radula. Formula: 1 : 0 : 1. Poison-gland present whose duct traverses the nerve-collar.

Fam. 1.—Pleurotomatidae. Shell fusiform, with elongated spire; margin of shell and mantle notched. Pleurotoma. Clavatula. Mangilia. Bela. Pusionella. Pontiothauma.

Fam. 2.—Terebridae. Shell turriculated, with numerous whorls; aperture and operculum oval; eyes at summits of tentacles; siphon long. Terebra.

Fam. 3.—Conidae. Shell conical, with very short spire, and narrow aperture with parallel borders; operculum unguiform Conus.

Sub-Class II.—Euthyneura

The most important general character of the Euthyneura is the absence of torsion in the visceral commissure, and the more posterior position of the anus and pallial organs. Comparative anatomy and embryology prove that this condition is due, not as formerly supposed to a difference in the relations of the visceral commissure which prevented it from being included in the torsion of the visceral hump, but to an actual detorsion which has taken place in evolution and is repeated to a great extent in individual development. In several of the more primitive forms the same torsion occurs as in Streptoneura, viz. in Actaeon and Limacina among Opisthobranchia, and Chilina among Pulmonata. Actaeon is proso-branchiate, the visceral commissure is twisted in Actaeon and Chilina, and even slightly still in Bulla and Scaphander; in Actaeon and Limacina the osphradium is to the left, innervated by the supra-intestinal ganglion. But in the other members of the sub-class the detorsion of the visceral mass has carried back the anus and circumanal complex from the anterior dorsal region to the right side, as in Bulla and Aplysia, or even to the posterior end of the body, as in Philine, Oncidium, Doris, &c. Different degrees of the same process of detorsion are, as we have seen, exhibited by the Heteropoda among the Streptoneura, and both in them and in the Euthyneura the detorsion is associated with degeneration of the shell. Where the modification is carried to its extreme degree, not only the shell but the pallial cavity, ctenidium and visceral hump disappear, and the body acquires a simple elongated form and a secondary external symmetry, as in Pterotrachaea and in Doris, Eolis, and other Nudibranchia. These facts afford strong support to the hypothesis that the weight of the shell is the original cause of the torsion of the dorsal visceral mass in Gastropods. But this hypothesis leaves the elevation of the visceral mass and the exogastric coiling of the shell in the ancestral form unexplained. In those Euthyneura in which the shell is entirely absent in the adult, it is, except in the three genera Cenia, Runcina and Vaginula, developed in the larva and then falls off. In other cases (Tectibranchs) the reduced shell is enclosed by upgrowths of the edge of the mantle and becomes internal, as in many Cephalopods. A few Euthyneura in which the shell is not much reduced retain an operculum in the adult state, e.g. Actaeon, Limacina, and the marine Pulmonate, Amphibola. The detorted visceral commissure shows a tendency to the concentration of all its elements round the oesophagus, so that except in the Bullomorpha and in Aplysia the whole nervous system is aggregated in the cephalic region, either dorsally or ventrally. The radula has a number of uniform teeth on each side of the median tooth in each transverse row. The head in most cases bears two pairs of tentacles. All the Euthyneura are hermaphrodite.

In the most primitive condition the genital duct is single throughout its length and has a single external aperture; it is therefore said to be monaulic. The hermaphrodite aperture is on the right side near the opening of the pallial cavity, and a ciliated groove conducts the spermatozoa to the penis, which is situated more anteriorly. This is the condition in the Bullomorpha, the Aplysiomorpha, and in one Pulmonate, Pythia. In some cases while the original aperture remains undivided, the seminal groove is closed and so converted into a canal. This is the modification found in Cavolinia longirostris among the Bullomorpha, and in all the Auriculidae except Pythia. A further degree of modification occurs when the male duct takes its origin from the hermaphrodite duct above the external opening, so that there are two distinct apertures, one male and one female, the latter being the original opening. The genital duct is now said to be diaulic, as in Valvata, Oncidiopsis, Actaeon, and Lobiger among the Bullomorpha, in the Pleurobranchidae, in the Nudibranchia, except the Doridomorpha and most of the Elysiomorpha, and in the Pulmonata. Originally in this condition the female aperture is at some distance from the male, as in the Basommatophora and in other cases; but in some forms the female aperture itself has shifted and come to be contiguous with the male opening and penis as in the Stylommatophora. In all these cases the female duct bears a bursa copulatrix or receptaculum seminis. In some forms this receptacle acquires a separate external opening remaining connected with the oviduct internally. There are thus two female openings, one for copulation, the other for oviposition, as well as a male opening. The genital duct is now trifurcated or triaulic, a condition which is confined to certain Nudibranchs, viz. the Doridomorpha and most of the Elysiomorpha.

The Pteropoda, formerly regarded as a distinct class of the Mollusca, were interpreted by E.R. Lankester as a branch of the Cephalopoda, chiefly on account of the protrusible sucker-bearing processes at the anterior end of Pneumonoderma. These he considered to be homologous with the arms of Cephalopods. He fully recognized, however, the similarity of Pteropods to Gastropods in their general asymmetry and in the torsion of the visceral mass in Limacinidae. It is now understood that they are Euthyneurous Gastropods adapted to natatory locomotion and pelagic life. The sucker-bearing processes of Pneumonoderma are outgrowths of the proboscis. The fins of Pteropods are now interpreted as the expanded lateral margins of the foot, termed parapodia, not homologous with the siphon of Cephalopods which is formed from epipodia. The Thecosomatous Pteropoda are allied to Bulla, the Gymnosomatous forms to Aplysia. The Euthyneura comprises two orders, Opisthobranchia and Pulmonata.

A, Veliger-larva of an Opisthobranch (Polycera). f, Foot; op, operculum; mn, anal papilla; ry, dry, two portions of unabsorbed nutritive yolk on either side of the intestine. The right otocyst is seen at the root of the foot.

B, Trochosphere of an Opisthobranch (Pleurobranchidium) showing—shgr, the shell-gland or primitive shell-sac; v, the cilia of the velum; ph, the commencing stomodaeum or oral invagination; ot, the left otocyst; pg, red-coloured pigment spot.

C, Diblastula of an Opisthobranch (Polycera) with elongated blastopore oi.

(All from Lankester.)

Order 1.—Opisthobranchia. Marine Euthyneura, the more archaic forms of which have a relatively large foot and a small visceral hump, from the base of which projects on the right side a short mantle-skirt. The anus is placed in such forms far back beyond the mantle-skirt. In front of the anus, and only partially covered by the mantle-skirt, is the ctenidium with its free end turned backwards. The heart lies in front of, instead of to the side of, the attachment of the ctenidium—hence Opisthobranchia as opposed to “Prosobranchia,” which correspond to the Streptoneura. A shell is possessed in the adult state by but few Opisthobranchia, but all pass through a veliger larval stage with a nautiloid shell (fig. 36). Many Opisthobranchia have by a process of atrophy lost the typical ctenidium and the mantle-skirt, and have developed other organs in their place. As in some Pectinibranchia, the free margin of the mantle-skirt is frequently reflected over the shell when a shell exists; and, as in some Pectinibranchia, broad lateral outgrowths of the foot (parapodia) are often developed which may be thrown over the shell or naked dorsal surface of the body.

The variety of special developments of structure accompanying the atrophy of typical organs in the Opisthobranchia and general degeneration of organization is very great. The members of the order present the same wide range of superficial appearance as do the Pectinibranchiate Streptoneura, forms carrying well-developed spiral shells and large mantle-skirts being included in the group, together with flattened or cylindrical slug-like forms. But in respect of the substitution of other parts for the mantle-skirt and for the gill which the more degenerate Opisthobranchia exhibit, this order stands alone. Some Opisthobranchia are striking examples of degeneration (some Nudibranchia), having none of those regions or processes of the body developed which distinguish the archaic Mollusca from such flat-worms as the Dendrocoel Planarians. Indeed, were it not for their retention of the characteristic odontophore we should have little or no indication that such forms as Phyllirhoë and Limapontia really belong to the Mollusca at all. The interesting little Rhodope veranyii, which has no odontophore, has been associated by systematists both with these simplified Opisthobranchs and with Rhabdocoel Planarians.

Fig. 38.—Three views of Aplysia sp., in various conditions ofexpansion and retraction. (After Cuvier.)
t, Anterior cephalic tentacles. t², Posterior cephalic tentacles. e, Eyes. f, Metapodium. ep, Epipodium.	g, Gill-plume (ctenidium). m, Mantle-flap reflected over the thin oval shell. os, s, Orifice formed by the unclosed border of the reflectedmantle-skirt, allowing the shell to show. pe, The spermatic groove.

In many respects the sea-hare (Aplysia), of which several species are known (some occurring on the English coast), serves as a convenient example of the fullest development of the organization characteristic of Opisthobranchia. The woodcut (fig. 38) gives a faithful representation of the great mobility of the various parts of the body. The head is well marked and joined to the body by a somewhat constricted neck. It carries two pairs of cephalic tentacles and a pair of sessile eyes. The visceral hump is low and not drawn out into a spire. The foot is long, carrying the oblong visceral mass upon it, and projecting (as metapodium) a little beyond it (f). Laterally the foot gives rise to a pair of mobile fleshy lobes, the parapodia (ep), which can be thrown up so as to cover in the dorsal surface of the animal. Such parapodia are common, though by no means universal, among Opisthobranchia. The torsion of the visceral hump is not carried out very fully, the consequence being that the anus has a posterior position a little to the right of the median line above the metapodium, whilst the branchial chamber formed by the overhanging mantle-skirt faces the right side of the body instead of lying well to the front as in Streptoneura and as in Pulmonate Euthyneura. The gill-plume, which in Aplysia is the typical Molluscan ctenidium, is seen in fig. 39 projecting from the branchial sub-pallial space. The relation of the delicate shell to the mantle is peculiar, since it occupies an oval area upon the visceral hump, the extent of which is indicated in fig. 38, C, but may be better understood by a glance at the figures of the allied genus Umbrella (fig. 40), in which the margin of the mantle-skirt coincides, just as it does in the limpet, with the margin of the shell. But in Aplysia the mantle is reflected over the edge of the shell, and grows over its upper surface so as to completely enclose it, excepting at the small central area s where the naked shell is exposed. This enclosure of the shell is a permanent development of the arrangement seen in many Streptoneura (e.g. Pyrula, Ovula, see figs. 18 and 32), where the border of the mantle can be, and usually is, drawn over the shell, though it is withdrawn (as it cannot be in Aplysia) when they are irritated. From the fact that Aplysia commences its life as a free-swimming veliger with a nautiloid shell not enclosed in any way by the border of the mantle, it is clear that the enclosure of the shell in the adult is a secondary process. Accordingly, the shell of Aplysia must not be confounded with a primitive shell in its shell-sac, such as we find realized in the shells of Chiton and in the plugs which form in the remarkable transitory “shell-sac” or “shell-gland” of Molluscan embryos (see figs. 26, 60). Aplysia, like other Mollusca, develops a primitive shell-sac in its trochosphere stage of development, which disappears and is succeeded by a nautiloid shell (fig. 36). This forms the nucleus of the adult shell, and, as the animal grows, becomes enclosed by a reflection of the mantle-skirt. When the shell of an Aplysia enclosed in its mantle is pushed well to the left, the sub-pallial space is fully exposed as in fig. 39, and the various apertures of the body are seen. Posteriorly we have the anus, in front of this the lobate gill-plume, between the two (hence corresponding in position to that of the Pectinibranchia) we have the aperture of the renal organ. In front, near the anterior attachment of the gill-plume, is the osphradium (olfactory organ) discovered by J.W. Spengel, yellowish in colour, in the typical position, and overlying an olfactory ganglion with typical nerve-connexion (see fig. 43). To the right of Spengel’s osphradium is the opening of a peculiar gland which has, when dissected out, the form of a bunch of grapes; its secretion is said to be poisonous. On the under side of the free edge of the mantle are situated the numerous small cutaneous glands which, in the large Aplysia camelus (not in other species), form the purple secretion which was known to the ancients. In front of the osphradium is the single genital pore, the aperture of the common or hermaphrodite duct. From this point there passes forward to the right side of the head a groove—the spermatic groove—down which the spermatic fluid passes. In other Euthyneura this groove may close up and form a canal. At its termination by the side of the head is the muscular introverted penis. In the hinder part of the foot (not shown in any of the diagrams) is the opening of a large mucus-forming gland very often found in the Molluscan foot.

With regard to internal organization we may commence with the disposition of the renal organ (nephridium), the external opening of which has already been noted. The position of this opening and other features of the renal organ were determined by J.T. Cunningham.

There is considerable uncertainty with respect to the names of the species of Aplysia. There are two forms which are very common in the Gulf of Naples. One is quite black in colour, and measures when outstretched 8 or 9 in. in length. The other is light brown and somewhat smaller, its length usually not exceeding 7 in. The first is flaccid and sluggish in its movements, and has not much power of contraction; its epipodial lobes are enormously developed and extend far forward along the body; it gives out when handled an abundance of purple liquid, which is derived from cutaneous glands situated on the under side of the free edge of the mantle. According to F. Blochmann it is identical with A. camelus of Cuvier. The other species is A. depilans; it is firm to the touch, and contracts forcibly when irritated; the secretion of the mantle-glands is not abundant, and is milky white in appearance. The kidney has similar relations in both species, and is identical with the organ spoken of by many authors as the triangular gland. Its superficial extent is seen when the folds covering the shell are cut away and the shell removed; the external surface forms a triangle with its base bordering the pericardium, and its apex directed posteriorly and reaching the the left-hand posterior corner of the shell-chamber. The dorsal surface of the kidney extends to the left beyond the shell-chamber beneath the skin in the space between the shell-chamber and the left parapodium.

When the animal is turned on its left-hand side and the mantle-chamber widely opened, the gill being turned over to the left, a part of the kidney is seen beneath the skin between the attachment of the gill and the right parapodium (fig. 39). On examination this is found to be the under surface of the posterior limb of the gland, the upper surface of which has just been described as lying beneath the shell. In the posterior third of this portion, close to that edge which is adjacent to the base of the gill, is the external opening (fig. 39, o).

When the pericardium is cut open from above in an animal otherwise entire, the anterior face of the kidney is seen forming the posterior wall of the pericardial chamber; on the deep edge of this face, a little to the left of the attachment of the auricle to the floor of the pericardium, is seen a depression; this depression contains the opening from the pericardium into the kidney.

To complete the account of the relations of the organ: the right anterior corner can be seen superficially in the wall of the mantle-chamber above the gill. Thus the base of the gill passes in a slanting direction across the right-hand side of the kidney, the posterior end being dorsal to the apex of the gland, and the anterior end ventral to the right-hand corner.

As so great a part of the whole surface of the kidney lies adjacent to external surfaces of the body, the remaining part which faces the internal organs is small; it consists of the left part of the under surface; it is level with the floor of the pericardium, and lies over the globular mass formed by the liver and convoluted intestine.

Thus the renal organ of Aplysia is shown to conform to the Molluscan type. The heart lying within the adjacent pericardium has the usual form, a single auricle and ventricle. The vascular system is not extensive, the arteries soon ending in the well-marked spongy tissue which builds up the muscular foot, parapodia, and dorsal body-wall.

The alimentary canal commences with the usual buccal mass; the lips are cartilaginous, but not armed with horny jaws, though these are common in other Opisthobranchs; the lingual ribbon is multidenticulate, and a pair of salivary glands pour in their secretion. The oesophagus expands into a curious gizzard, which is armed internally with large horny processes, some broad and thick, others spinous, fitted to act as crushing instruments. From this we pass to a stomach and a coil of intestine embedded in the lobes of a voluminous liver; a caecum of large size is given off near the commencement of the intestine. The liver opens by two ducts into the digestive tract.

The generative organs lie close to the coil of intestine and liver, a little to the left side. When dissected out they appear as represented in fig. 41. The essential reproductive organ or gonad consists of both ovarian and testicular cells (see fig. 42). It is an ovo-testis. From it passes a common or hermaphrodite duct, which very soon becomes entwined in the spire of a gland—the albuminiparous gland. The latter opens into the common duct at the point k, and here also is a small diverticulum of the duct f. Passing on, we find not far from the genital pore a glandular spherical body (the spermatheca c) opening by means of a longish duct into the common duct, and then we reach the pore (fig. 39, k). Here the female apparatus terminates. But when the male secretion of the ovo-testis is active, the seminal fluid passes from the genital pore along the spermatic groove (fig. 39) to the penis, and is by the aid of that eversible muscular organ introduced into the genital pore of a second Aplysia, whence it passes into the spermatheca, there to await the activity of the female element of the ovo-testis of this second Aplysia. After an interval of some days—possibly weeks—the ova of the second Aplysia commence to descend the hermaphrodite duct; they become enclosed in a viscid secretion at the point where the albuminiparous gland opens into the duct intertwined with it; and on reaching the point where the spermathecal duct debouches they are impregnated by the spermatozoa which escape now from the spermatheca and meet the ova.

The development of Aplysia from the egg presents many points of interest from the point of view of comparative embryology, but in relation to the morphology of the Opisthobranchia it is sufficient to point to the occurrence of a trochosphere and a veliger stage (fig. 36), and of a shell-gland or primitive shell-sac (fig. 36, shgr), which is succeeded by a nautiloid shell.

In the nervous system of Aplysia the great ganglion-pairs are well developed and distinct. The euthyneurous visceral loop is long, and presents only one ganglion (in Aplysia camelus, but two distinct ganglia joined to one another in Aplysia hybrida of the English coast), placed at its extreme limit, representing both the right and left visceral ganglia and the third or abdominal ganglion, which are so often separately present. The diagram (fig. 43) shows the nerve connecting this abdomino-visceral ganglion with the olfactory ganglion of Spengel. It is also seen to be connected with a more remote ganglion—the genital. Such special irregularities in the development of ganglia upon the visceral loop, and on one or more of the main nerves connected with it, are very frequent. Our figure of the nervous system of Aplysia does not give the small pair of buccal ganglia which are, as in all glossophorous Molluscs, present upon the nerves passing from the cerebral region to the odontophore.

For a comparison of various Opisthobranchs, Aplysia will be found to present a convenient starting-point. It is one of the more typical Opisthobranchs, that is to say, it belongs to the section Tectibranchia, but other members of the suborder, namely, Bulla and Actaeon (figs. 44 and 45), are less abnormal than Aplysia in regard to their shells and the form of the visceral hump. They have naked spirally twisted shells which may be concealed from view in the living animal by the expansion and reflection of the parapodia, but are not enclosed by the mantle, whilst Actaeon is remarkable for possessing an operculum like that of so many Streptoneura.

The great development of the parapodia seen in Aplysia is usual in Tectibranchiate Opisthobranchs. The whole surface of the body becomes greatly modified in those Nudibranchiate forms which have lost, not only the shell, but also the ctenidium. Many of these have peculiar processes developed on the dorsal surface (fig. 46, A, B), or retain purely negative characters (fig. 46, D). The chief modification of internal organization presented by these forms, as compared with Aplysia, is found in the condition of the alimentary canal. The liver is no longer a compact organ opening by a pair of ducts into the median digestive tract, but we find very numerous hepatic diverticula on a shortened axial tract (fig. 47). These diverticula extend usually one into each of the dorsal papillae or “cerata” when these are present. They are not merely digestive glands, but are sufficiently wide to act as receptacles of food, and in them the digestion of food proceeds just as in the axial portion of the canal. A precisely similar modification of the liver or great digestive gland is found in the scorpions, where the axial portion of the digestive canal is short and straight, and the lateral ducts sufficiently wide to admit food into the ramifications of the gland there to be digested; whilst in the spiders the gland is reduced to a series of simple caeca.

The typical character is retained by the heart, pericardium, and the communicating nephridium or renal organ in all Opisthobranchs. An interesting example of this is furnished by the fish-like transparent Phyllirhoë (fig. 37), in which it is possible most satisfactorily to study in the living animal, by means of the microscope, the course of the blood-stream, and also the reno-pericardial communication. In many of the Nudibranchiate Opisthobranchs the nervous system presents a concentration of the ganglia (fig. 48), contrasting greatly with what we have seen in Aplysia. Not only are the pleural ganglia fused to the cerebral, but also the visceral to these (see in further illustration the condition attained by the Pulmonate Limnaeus, fig. 59), and the visceral loop is astonishingly short and insignificant (fig. 48, e′). That the parts are rightly thus identified is probable from J.W. Spengel’s observation of the osphradium and its nerve-supply in these forms; the nerve to that organ, which is placed somewhat anteriorly—on the dorsal surface—being given off from the hinder part (visceral) of the right compound ganglion—the fellow to that marked A in fig. 48. The Eolid-like Nudibranchs, amongst other specialities of structure, possess (in some cases at any rate) apertures at the apices of the “cerata” or dorsal papillae, which lead from the exterior into the hepatic caeca. Some amongst them (Tergipes, Eolis) are also remarkable for possessing peculiarly modified cells placed in sacs (cnidosacs) at the apices of these same papillae, which resemble the “thread-cells” of the Coelentera. According to T.S. Wright and J.H. Grosvenor these nematocysts are derived from the hydroids on which the animals feed.

The development of many Opisthobranchia has been examined—e.g. Aplysia, Pleurobranchidium, Elysia, Polycera, Doris, Tergipes. All pass through trochosphere and veliger stages, and in all a nautiloid or boat-like shell is developed, preceded by a well-marked “shell-gland” (see fig. 36). The transition from the free-swimming veliger larva with its nautiloid shell (fig. 36) to the adult form has not been properly observed, and many interesting points as to the true nature of folds (whether parapodia or mantle or velum) have yet to be cleared up by a knowledge of such development in forms like Tethys, Doris, Phyllidia, &c. As in other Molluscan groups, we find even in closely-allied genera (for instance, in Aplysia and Pleurobranchidium, and other genera), the greatest differences as to the amount of food-material by which the egg-shell is encumbered. Some form their diblastula by emboly, others by epiboly; and in the later history of the further development of the enclosed cells (arch-enteron) very marked variations occur in closely-allied forms, due to the influence of a greater or less abundance of food-material mixed with the protoplasm of the egg.

Sub-order 1.—Tectibranchia. Opisthobranchs provided in the adult state with a shell and a mantle, except Runcina, Pleurobranchaea, Cymbuliidae, and some Aplysiomorpha. There is a ctenidium, except in some Thecosomata and Gymnosomata, and an osphradium.

Tribe 1.—Bullomorpha. The shell is usually well developed, except in Runcina and Cymbuliidae, and may be external or internal. No operculum, except in Actaeonidae and Limacinidae. The pallial cavity is always well developed, and contains the ctenidium, at least in part; ctenidium, except in Lophocercidae, of folded type. With the exception of the Aplustridae, Lophocercidae and Thecosomata, the head is devoid of tentacles, and its dorsal surface forms a digging disk or shield. The edges of the foot form parapodia, often transformed into fins. Posteriorly the mantle forms a large pallial lobe under the pallial aperture. Stomach generally provided with chitinous or calcified masticatory plates. Visceral commissure fairly long, except in Runcina, Lobiger and Thecosomata. Hermaphrodite genital aperture, connected with the penis by a ciliated groove, except in Actaeon, Lobiger and Cavolinia longirostris, in which the spermiduct is a closed tube. Animals either swim or burrow.

Fig. 46.
A, Eolis papillosa (Lin.), dorsal view.
a, b, Posterior and anterior cephalic tentacles.	c, The dorsal “cerata.”
B, Tethys leporina, dorsal view.
a, The cephalic hood. b, Cephalic tentacles. c, Neck. d, Genital pore.	e, Anus. f, Large cerata. g, Smaller cerata. h, Margin of the foot.
C, Doris (Actinocyclus) tuberculatus (Cuv.), seen from the pedalsurface.
m, Mouth. b, Margin of the head.	f, Sole of the foot. sp, The mantle-like epipodium.
D, E, Dorsal and lateral view of Elysia (Actaeon) viridis.ep, epipodial outgrowths. (After Keferstein.)

Fig. 47.—Enteric Canal of Eolis papillosa. (FromGegenbaur, after Alder and Hancock.) ph, Pharynx. m, Midgut, with its hepatic appendages h, all of which are not figured. e, Hind gut. an, Anus.

Fig. 48.—Central Nervous System ofFiona (one of the Nudibranchia), showing a tendencyto fusion of the great ganglia. (From Gegenbaur,after Bergh.) A, Cerebral, pleural and visceral ganglia united. B, Pedal ganglion. C, Buccal ganglion. D, Oesophageal ganglion connected with, the Buccal. a, Nerve to superior cephalic tentacle. b, Nerves to inferior cephalic tentacles. c, Nerve to generative organs. d, Pedal nerve. e, Pedal commissure. e′, Visceral loop or commissure (?).

Fam. 1.—Actaeonidae. Cephalic shield bifid posteriorly; margins of foot slightly developed; genital duct diaulic; visceral commissure streptoneurous; shell thick, with prominent spire and elongated aperture; a horny operculum. Actaeon, British. Solidula. Tornatellaea, extinct. Adelactaeon. Bullina. Bullinula.

Fam. 2.—Ringiculidae. Cephalic disk enlarged anteriorly, forming an open tube posteriorly; shell external, thick, with prominent spire; no operculum. Ringicula. Pugnus.

Fam. 3.—Tornatinidae. Margins of foot not prominent; no radula; shell external, with inconspicuous spire. Tornatina, British. Retusa. Volvula.

Fam. 4.—Scaphandridae. Cephalic shield short, truncated posteriorly; eyes deeply embedded; three calcareous stomachal plates; shell external, with reduced spire. Scaphander, British. Atys. Smaragdinella. Cylichna, British. Amphisphyra, British.

Fam. 5.—Bullidae. Margins of foot well developed; eyes superficial; three chitinous stomachal plates; shell external, with reduced spire. Bulla, British. Haminea, British.

Fam. 6.—Aceratidae. Cephalic shield continuous with neck; twelve to fourteen stomachal plates; a posterior pallial filament passing through a notch in shell. Acera, British. Cylindrobulla. Volutella.

Fam. 7.—Aplustridae. Foot very broad; cephalic shield with four tentacles; shell external, thin, without prominent spire. Aplustrum. Hydatina. Micromelo.

Fam. 8.—Philinidae. Cephalic shield broad, thick and simple; shell wholly internal, thin, spire much reduced, aperture very large. Philine, British. Cryptophthalmus. Chelinodura. Phanerophthalmus. Colpodaspis, British. Colobocephalus.

Fam. 9.—Doridiidae. Cephalic shield ending posteriorly in a median point; shell internal, largely membranous; no radula or stomachal plates. Doridium. Navarchus.

Fam. 10.—Gastropteridae. Cephalic shield pointed behind; shell internal, chiefly membranous, with calcified nucleus, nautiloid; parapodia forming fins. Gastropteron.

Fam. 11.—Runcinidae. Cephalic shield continuous with dorsal integument; no shell; ctenidium projecting from mantle cavity. Runcina.

Fam. 12.—Lophocercidae. Shell external, globular or ovoid; foot elongated, parapodia separate from ventral surface; genital duct diaulic. Lobiger. Lophocercus.

The next three families form the group formerly known as Thecosomatous Pteropods. They are all pelagic, the foot being entirely transformed into a pair of anterior fins; eyes are absent, and the nerve centres are concentrated on the ventral side of the oesophagus.

Fam. 13.—Limacinidae. Dextral animals, with shell coiled pseudo-sinistrally; operculum with sinistral spiral; pallial cavity dorsal. Limacina, British. Peraclis, ctenidium present.

Fam. 14.—Cymbuliidae. Adult without shell; a sub-epithelial pseudoconch formed by connective tissue; pallial cavity ventral. Cymbulia. Cymbuliopsis. Gleba. Desmopterus.

Fam. 15.—Cavoliniidae. Shell not coiled, symmetrical; pallial cavity ventral. Cavolinia. Clio. Cuvierina.

Tribe 2.—Aplysiomorpha. Shell more or less internal, much reduced or absent. Head bears two pairs of tentacles. Parapodia separate from ventral surface, and generally transformed into swimming lobes. Visceral commissure much shortened, except in Aplysia. Genital duct monaulic; hermaphrodite duct connected with penis by a ciliated groove. Animals either swim or crawl.