PERIPATUS
BY
ADAM SEDGWICK, M.A., F.R.S.
Fellow of Trinity College, Cambridge.
CHAPTER I
PERIPATUS
INTRODUCTION–EXTERNAL FEATURES–HABITS–BREEDING–ANATOMY–ALIMENTARY CANAL–NERVOUS SYSTEM–THE BODY WALL–THE TRACHEAL SYSTEM–THE MUSCULAR SYSTEM–THE VASCULAR SYSTEM–THE BODY CAVITY–NEPHRIDIA–GENERATIVE ORGANS–DEVELOPMENT–SYNOPSIS OF THE SPECIES–SUMMARY OF DISTRIBUTION.
The genus Peripatus was established in 1826 by Guilding,[[1]] who first obtained specimens of it from St. Vincent in the Antilles. He regarded it as a Mollusc, being no doubt deceived by the slug-like appearance given by the antennae. Specimens were subsequently obtained from other parts of the Neotropical region and from South Africa and Australia, and the animal was variously assigned by the zoologists of the day to the Annelida and Myriapoda. Its true place in the system, as a primitive member of the group Arthropoda, was first established in 1874 by Moseley,[[2]] who discovered the tracheae. The genus has been monographed by Sedgwick,[[3]] who has also written an account of the development of the Cape species.[[4]] A bibliography will be found in Sedgwick's Monograph.
There can be no doubt that Peripatus is an Arthropod, for it possesses the following features, all characteristic of that group, and all of first-class morphological importance: (1) The presence of appendages modified as jaws; (2) the presence of paired lateral ostia perforating the wall of the heart and putting its cavity in communication with the pericardium; (3) the presence of a vascular body cavity and pericardium (haemocoelic body cavity); (4) absence of a perivisceral section of the coelom. Finally, the tracheae, though not characteristic of all the classes of the Arthropoda, are found nowhere outside that group, and constitute a very important additional reason for uniting Peripatus with it.
Peripatus, though indubitably an Arthropod, differs in such important respects from all the old-established Arthropod classes, that a special class, equivalent in rank to the others, and called Prototracheata, has had to be created for its sole occupancy. This unlikeness to other Arthropoda is mainly due to the Annelidan affinities which it presents, but in part to the presence of the following peculiar features: (1) The number and diffusion of the tracheal apertures; (2) the restriction of the jaws to a single pair; (3) the disposition of the generative organs; (4) the texture of the skin; and (5) the simplicity and similarity of all the segments of the body behind the head.
The Annelidan affinities are superficially indicated in so marked a manner by the thinness of the cuticle, the dermo-muscular body wall, the hollow appendages, that, as already stated, many of the earlier zoologists who examined Peripatus placed it amongst the segmented worms; and the discovery that there is some solid morphological basis for this determination constitutes one of the most interesting points of the recent work on the genus. The Annelidan features are: (1) The paired nephridia in every segment of the body behind the first two (Saenger, Balfour[[5]]); (2) the presence of cilia in the generative tracts (Gaffron). It is true that neither of these features are absolutely distinctive of the Annelida, but when taken in conjunction with the Annelidan disposition of the chief systems of organs, viz. the central nervous system, and the main vascular trunk or heart, may be considered as indicating affinities in that direction. Peripatus, therefore, is zoologically of extreme interest from the fact that, though in the main Arthropodan, it possesses features which are possessed by no other Arthropod, and which connect it to the group to which the Arthropoda are in the general plan of their organisation most closely related. It must, therefore, according to our present lights, be regarded as a very primitive form; and this view of it is borne out by its extreme isolation at the present day. Peripatus stands absolutely alone as a kind of half-way animal between the Arthropoda and Annelida. There is no gradation of structure within the genus; the species are very limited in number, and in all of them the peculiar features above mentioned are equally sharply marked.
Peripatus, though a lowly organised animal, and of remarkable sluggishness, with but slight development of the higher organs of sense, with eyes the only function of which is to enable it to avoid the light—though related to those animals most repulsive to the aesthetic sense of man, animals which crawl upon their bellies and spit at, or poison, their prey—is yet, strange to say, an animal of striking beauty. The exquisite sensitiveness and constantly changing form of the antennae, the well-rounded plump body, the eyes set like small diamonds on the side of the head, the delicate feet, and, above all, the rich colouring and velvety texture of the skin, all combine to give these animals an aspect of quite exceptional beauty. Of all the species which I have seen alive, the most beautiful are the dark green individuals of Capensis, and the species which I have called Balfouri. These animals, so far as skin is concerned, are not surpassed in the animal kingdom. I shall never forget my astonishment and delight when on bearing away the bark of a rotten tree-stump in the forest on Table Mountain, I first came upon one of these animals in its natural haunts, or when Mr. Trimen showed me in confinement at the South African Museum a fine fat, full-grown female, accompanied by her large family of thirty or more just-born but pretty young, some of which were luxuriously creeping about on the beautiful skin of their mother's back.
External Features.
The anterior part of the body may be called the head, though it is not sharply marked off from the rest of the body (Fig. 1).
Fig. 1.—Peripatus capensis, drawn from life. Life size. (After Sedgwick.)
Fig. 2.—Ventral view of hind-end of P. Novae-Zealandiae. (After Sedgwick.) g, Generative opening; a, anus.
Fig. 3.—Ventral view of the head of P. capensis. (After Sedgwick.) ant, Antennae; or.p, oral papillae; F.1, first leg; T, tongue.
The head carries three pairs of appendages, a pair of simple eyes, and a ventrally placed mouth. The body is elongated and vermiform; it bears a number of paired appendages, each terminating in a pair of claws, and all exactly alike. The number varies in the different species. The anus is always at the posterior end of the body, and the generative opening is on the ventral surface just in front of the anus; it may be between the legs of the last pair (Fig. 2), or it may be behind them. There is in most species a thin median white line extending the whole length of the dorsal surface of the body, on each side of which the skin pigment is darker than elsewhere. The colour varies considerably in the different species, and even in different individuals of the same species. The ventral surface is nearly always flesh-coloured, while the dorsal surface has a darker colour. In the South African species the colour of the dorsal surface varies from a dark green graduating to a bluish gray, to a brown varying to a red orange. The colour of the Australasian species varies in like manner, while that of the Neotropical species (S. American and W. Indian) is less variable. The skin is thrown into a number of transverse ridges, along which wart-like papillae are placed. The papillae, which are found everywhere, are specially developed on the dorsal surface, less so on the ventral. Each papilla carries at its extremity a well-marked spine.
The appendages of the head are the antennae, the jaws and the oral papillae.
The antennae, which are prolongations of the dorso-lateral parts of the head, are ringed, and taper slightly till near their termination, where they are slightly enlarged. The rings bear a number of spines, and the free end of the antennae is covered by a cap of spiniferous tissue like that of the rings.
Fig. 4.—Inner jaw-claw of P. capensis. (After Balfour.)
Fig. 5.—Outer jaw-claw of P. capensis. (After Balfour.)
The mouth is at the hinder end of a depression called the buccal cavity, and is surrounded by an annular tumid lip, raised into papilliform ridges and bearing a few spines (Fig. 3). Within the buccal cavity are the two jaws. They are short, stump-like, muscular structures, armed at their free extremities by a pair of cutting blades or claws, and are placed one on each side of the mouth. In the median line of the buccal cavity in front is placed a thick muscular protuberance, which may be called the tongue, though attached to the dorsal instead of to the ventral wall of the mouth (Fig. 3). The tongue bears a row of small chitinous teeth. The jaw-claws (Figs. 4 and 5), which resemble in all essential points the claws borne by the feet, and like these are thickenings of the cuticle, are sickle-shaped. They have their convex edge directed forwards and their concave or cutting edge turned backwards. The inner cutting plate (Fig. 4) usually bears a number of cutting teeth. The jaws appear to be used for tearing the food, to which the mouth adheres by means of the tumid suctorial lips. The oral papillae are placed at the sides of the head (Fig. 3). The ducts of the slime-glands open at their free end. They possess two main rings of projecting tissue, and their extremities bear papillae irregularly arranged.
The ambulatory appendages vary in number. There are seventeen pairs in P. capensis and eighteen in P. Balfouri, while in P. Edwardsii the number varies from twenty-nine to thirty-four pairs. They consist of two main divisions, which we may call the leg and the foot (Figs. 6 and 7). The leg (l) has the form of a truncated cone, the broad end of which is attached to the ventro-lateral wall of the body, of which it is a prolongation. It is marked by a number of rings of papillae placed transversely to its long axis, the dorsal of which are pigmented like the dorsal surface of the body, and the ventral like the ventral surface. At the narrow distal end of the leg there are on the ventral surface three spiniferous pads, each of which is continued dorsally into a row of papillae.
Fig. 6.—Ventral view of last leg of a male P. capensis. (After Sedgwick.) f, Foot; l, leg; p, spiniferous pads. The white papilla on the proximal part of this leg is characteristic of the male of this species.
Fig. 7.—Leg of P. capensis seen from the front. (After Sedgwick.) f, Foot; l, leg; p, spiniferous pads.
The foot is attached to the distal end of the leg. It is slightly narrower at its attached extremity than at its free end. It bears two sickle-shaped claws and a few papillae. The part of the foot which carries the claws is especially retractile, and is generally found more or less telescoped into the proximal part. The legs of the fourth and fifth pairs differ from the others in the fact that the proximal pad is broken up into three, a small central and two larger lateral. The enlarged nephridia of these legs open on the small central division.
The males are generally rather smaller than the females. In those species in which the number of legs varies, the male has a smaller number of legs than the female.
Habits.
They live beneath the bark of rotten stumps of trees, in the crevices of rock, and beneath stones. They require a moist atmosphere, and are exceedingly susceptible to drought. They avoid light, and are therefore rarely seen. They move with great deliberation, picking their course by means of their antennae and eyes. It is by the former that they acquire a knowledge of the ground over which they are travelling, and by the latter that they avoid the light. The antennae are extraordinarily sensitive, and so delicate, indeed, that they seem to be able to perceive the nature of objects without actual contact. When irritated they eject with considerable force the contents of their slime reservoirs from the oral papillae. The force is supplied by the sudden contraction of the muscular body wall. They can squirt the slime to the distance of almost a foot. The slime, which appears to be perfectly harmless, is extremely sticky, but it easily comes away from the skin of the animal itself.
I have never seen them use this apparatus for the capture of prey, but Hutton describes the New Zealand species as using it for this purpose. So far as I can judge, it is used as a defensive weapon; but this of course will not exclude its offensive use. They will turn their heads to any part of the body which is being irritated and violently discharge their slime at the offending object. Locomotion is effected entirely by means of the legs, with the body fully extended.
Of their food in the natural state we know little; but it is probably mainly, if not entirely, animal. Hutton describes his specimens as sucking the juices of flies which they had stuck down with their slime, and those which I kept in captivity eagerly devoured the entrails of their fellows, and the developing young from the uterus. They also like raw sheep's liver. They move their mouths in a suctorial manner, tearing the food with their jaws. They have the power of extruding their jaws from the mouth, and of working them alternately backwards or forwards. This is readily observed in individuals immersed in water.
Breeding.
All species are viviparous. It has been lately stated that one of the Australian species is normally oviparous, but this has not been proved. The Australasian species come nearest to laying eggs, inasmuch as the eggs are large, full of yolk, and enclosed in a shell; but development normally takes place in the uterus, though, abnormally, incompletely developed eggs are extruded.
The young of P. capensis are born in April and May. They are almost colourless at birth, excepting the antennae, which are green, and their length is 10 to 15 mm. A large female will produce thirty to forty young in one year. The period of gestation is thirteen months, that is to say, the ova pass into the oviducts about one month before the young of the preceding year are born. They are born one by one, and it takes some time for a female to get rid of her whole stock of embryos; in fact, the embryos in any given female differ slightly in age, those next the oviduct being a little older (a few hours) than those next the vagina. The mother does not appear to pay any special attention to her young, which wander away and get their own food.
There does not appear to be any true copulation. The male deposits small, white, oval spermatophores, which consist of small bundles of spermatozoa cemented together by some glutinous substance, indiscriminately on any part of the body of the female. Such spermatophores are found on the bodies of both males and females from July to January, but they appear to be most numerous in our autumn. It seems probable that the spermatozoa make their way from the adherent spermatophore through the body wall into the body, and so by traversing the tissues reach the ovary. The testes are active from June to the following March. From March to June the vesiculae of the male are empty.
There are no other sexual differences except in some of the South African species, in which the last or penultimate leg of the male bears a small white papilla on its ventral surface (Fig. 6).
Whereas in the Cape species embryos in the same uterus are all practically of the same age (except in the month of April, when two broods overlap in P. capensis), and birth takes place at a fixed season; in the Neotropical species the uterus, which is always pregnant, contains embryos of different ages, and births probably take place all the year round.
In all species of Peripatus the young are fully formed at birth, and differ from the adults only in size and colour.
ANATOMY
The Alimentary Canal (Fig. 8).
Fig. 8.—Peripatus capensis dissected so as to show the alimentary canal, slime glands, and salivary glands. (After Balfour.) The dissection is viewed from the ventral side, and the lips (L) have been cut through in the middle line behind and pulled outwards so as to expose the jaws (j), which have been turned outwards, and the tongue (T) bearing a median row of chitinous teeth, which branches behind into two. The muscular pharynx, extending back into the space between the first and second pairs of legs, is followed by a short tubular oesophagus. The latter opens into the large stomach with plicated walls, extending almost to the hind end of the animal. The stomach at its point of junction with the rectum presents an S-shaped ventro-dorsal curve. A, Anus; at, antenna; F.1, F.2, first and second feet; j, jaws; L, lips; oe, oesophagus; or.p, oral papilla; ph, pharynx; R, rectum; s.d, salivary duct; s.g, salivary gland; sl.d, slime reservoir; sl.g, portion of tubules of slime gland; st, stomach; T, tongue in roof of mouth.
The buccal cavity, as explained above, is a secondary formation around the true mouth, which is at its dorsal posterior end. It contains the tongue and the jaws, which have already been described, and into the hind end of it there opens ventrally by a median opening the salivary glands (s.g). The mouth leads into a muscular pharynx (ph), which is connected by a short oesophagus (oe) with a stomach (st). The stomach forms by far the largest part of the alimentary canal. It is a dilated soft-walled tube, and leads behind into the short narrow rectum (R), which opens at the anus. There are no glands opening into the alimentary canal.
Nervous System.
The central nervous system consists of a pair of supra-oesophageal ganglia united in the middle line, and of a pair of widely divaricated ventral cords, continuous in front with the supra-oesophageal ganglia (Fig. 9).
The ventral cords at first sight appear to be without ganglionic thickenings, but on more careful examination they are found to be enlarged at each pair of legs (Fig. 9). These enlargements may be regarded as imperfect ganglia. There are, therefore, as many pairs of ganglia as there are pairs of legs. There is in addition a ganglionic enlargement at the commencement of the oesophageal commissures, where the nerves to the oral papillae are given off (Fig. 9, or.g).
Fig. 9.—Brain and anterior part of the ventral nerve-cords of Peripatus capensis enlarged and viewed from the ventral surface. (After Balfour.) The paired appendages (d) of the ventral surface of the brain are seen, and the pair of sympathetic nerves (sy) arising from the ventral surface of the hinder part. From the commencement of the oesophageal commissures pass off on each side a pair of nerves to the jaws (Jn). The three anterior commissures between the ventral nerve-cords are placed close together; immediately behind them the nerve-cords are swollen, to form the ganglionic enlargements from which pass off to the oral papillae a pair of large nerves on each side (orn). Behind this the cords present a series of enlargements, one pair for each pair of feet, from which a pair of large nerves pass off on each side to the feet (pn). atn, Antennary nerves; co, commissures between ventral cords; d, ventral appendages of brain; E, eye; en, nerves passing outwards from ventral cord; F.g.1, ganglionic enlargements from which nerves to feet pass off; jn, nerves to jaws; org, ganglionic enlargement from which nerves to oral papillae pass off; orn, nerves to oral papillae; pc, posterior lobe of brain; pn, nerves to feet; sy, sympathetic nerves.
The ventral cords are placed each in the lateral compartments of the body cavity, immediately within the longitudinal layer of muscles. They are connected with each other, rather like the pedal nerves of Chiton and the lower Prosobranchiata, by a number of commissures. These commissures exhibit a fairly regular arrangement from the region included between the first and the last pair of true feet. There are nine or ten of them between each pair of feet. They pass along the ventral wall of the body, perforating the ventral mass of longitudinal muscles. On their way they give off nerves which innervate the skin.
Posteriorly the two nerve-cords nearly meet immediately in front of the generative aperture, and then, bending upwards, fall into each other dorsally to the rectum. They give off a series of nerves from their outer borders, which present throughout the trunk a fairly regular arrangement. From each ganglion two large nerves (pn) are given off, which, diverging somewhat from each other, pass into the feet.
From the oesophageal commissures, close to their junction with the supra-oesophageal ganglia, a nerve arises on each side which passes to the jaws, and a little in front of this, apparently from the supra-oesophageal ganglion itself, a second nerve to the jaws also takes its origin.
The supra-oesophageal ganglia (Fig. 9) are large, somewhat oval masses, broader in front than behind, completely fused in the middle, but free at their extremities. Each of them is prolonged anteriorly into an antennary nerve, and is continuous behind with one of the oesophageal commissures. On the ventral surface of each, rather behind the level of the eye, is placed a hollow protuberance (Fig. 9, d), of which I shall say more in dealing with the development. About one-third of the way back the two large optic nerves take their origin, arising laterally, but rather from the dorsal surface (Fig. 9). Each of them joins a large ganglionic mass placed immediately behind the retina.
The histology of the ventral cords and oesophageal commissures is very simple and uniform. They consist of a cord almost wholly formed of nerve-fibres placed dorsally, and of a ventral layer of ganglion cells.
The Body Wall.
The skin is formed of three layers.
(1) The cuticle.
(2) The epidermis or hypodermis.
(3) The dermis.
The cuticle is a thin layer. The spines, jaws, and claws are special developments of it. Its surface is not, however, smooth, but is everywhere, with the exception of the perioral region, raised into minute secondary papillae, which in most instances bear at their free extremity a somewhat prominent spine. The whole surface of each of the secondary papillae just described is in its turn covered by numerous minute spinous tubercles.
The epidermis, placed immediately within the cuticle, is composed of a single layer of cells, which vary, however, a good deal in size in different regions of the body. The cells excrete the cuticle, and they stand in a very remarkable relation to the secondary papillae of the cuticle just described. Each epidermis cell is in fact placed within one of these secondary papillae, so that the cuticle of each secondary papilla is the product of a single epidermis cell. The pigment which gives the characteristic colour to the skin is deposited in the protoplasm of the outer ends of the cells in the form of small granules.
At the apex of most, if not all, the primary wart-like papillae there are present oval aggregations, or masses of epidermis cells, each such mass being enclosed in a thickish capsule and bearing a long projecting spine. These structures are probably tactile organs. In certain regions of the body they are extremely numerous; more especially is this the case in the antennae, lips, and oral papillae. On the ventral surface of the peripheral rings of the thicker sections of the feet they are also very thickly set and fused together so as to form a kind of pad (Figs. 6 and 7). In the antennae they are thickly set side by side on the rings of skin which give such an Arthropodan appearance to these organs in Peripatus.
The Tracheal System.
The apertures of the tracheal system are placed in the depressions between the papillae or ridges of the skin. Each of them leads into a tube, which may be called the tracheal pit (Fig. 10), the walls of which are formed of epithelial cells bounded towards the lumen of the pit by a very delicate cuticular membrane continuous with the cuticle covering the surface of the body. The pits vary somewhat in depth; the pit figured was about 0.09 mm. It perforates the dermis and terminates in the subjacent muscular layer.
Internally it expands in the transverse plane and from the expanded portion the tracheal tubes arise in diverging bundles. Nuclei similar in character to those in the walls of the tracheal pit are placed between the tracheae, and similar but slightly more elongated nuclei are found along the bundles. The tracheae are minute tubes exhibiting a faint transverse striation which is probably the indication of a spiral fibre. They appear to branch, but only exceptionally. The tracheal apertures are diffused over the surface of the body, but are especially developed in certain regions.
Fig. 10.—Section through a tracheal pit and diverging bundles of tracheal tubes taken transversely to the long axis of the body. (After Balfour.) tr, Tracheae, showing rudimentary spiral fibre; tr.c, cells resembling those lining the tracheal pits, which occur at intervals along the course of the tracheae; tr.o, tracheal stigma; tr.p, tracheal pit.
The Muscular System.
The general muscular system consists of—(1) the general wall of the body; (2) the muscles connected with the mouth, pharynx, and jaws; (3) the muscles of the feet; (4) the muscles of the alimentary tract.
The muscular wall of the body is formed of—(1) an external layer of circular fibres; (2) an internal layer of longitudinal muscles.
The main muscles of the body are unstriated and divided into fibres, each invested by a delicate membrane. The muscles of the jaws alone are transversely striated.
The Vascular System.
The vascular system consists of a dorsal tubular heart with paired ostia leading into it from the pericardium, of the pericardium, and the various other divisions of the perivisceral cavity (Fig. 14, D). As in all Arthropoda, the perivisceral cavity is a haemocoele; i.e. it contains blood and forms part of the vascular system. The heart extends from close to the hind end of the body to the head.
The Body Cavity.
The body cavity is formed of four compartments—one central, two lateral, and a pericardial (Fig. 14, D). The former is by far the largest, and contains the alimentary tract, the generative organs, and the slime glands. It is lined by a delicate endothelial layer, and is not divided into compartments nor traversed by muscular fibres. The lateral divisions are much smaller than the central, and are shut off from it by the inner transverse band of muscles. They are almost entirely filled with the nerve-cord and salivary gland in front and with the nerve-cord alone behind, and their lumen is broken up by muscular bands. They further contain the nephridia. They are prolonged into the feet, as is the embryonic body cavity of most Arthropoda. The pericardium contains a peculiar cellular tissue, probably, as suggested by Moseley, equivalent to the fat-bodies of insects.
Nephridia.
In Peripatus capensis nephridia are present in all the legs. In all of them (except the first three) the following parts may be recognised (Fig. 11):—
(1) A vesicular portion opening to the exterior on the ventral surface of the legs by a narrow passage.
(2) A coiled portion, which is again subdivided into several sections.
(3) A section with closely packed nuclei ending by a somewhat enlarged opening.
(4) The terminal portion, which consists of a thin-walled vesicle.
The last twelve pairs of these organs are all constructed in a very similar manner, while the two pairs situated in the fourth and fifth pairs of legs are considerably larger than those behind, and are in some respects very differently constituted.
It will be convenient to commence with one of the hinder nephridia. Such a nephridium from the ninth pair of legs is represented in Fig. 11. The external opening is placed at the outer end of a transverse groove at the base of one of the legs, while the main portion of the organ lies in the body cavity in the base of the leg, and extends into the trunk to about the level of the outer edge of the nerve-cord of its side. The external opening (o.s) leads into a narrow tube (s.d), which gradually dilates into a large sac (s). The narrow part is lined by small epithelial cells, which are directly continuous with and perfectly similar to those of the epidermis. The sac itself, which forms a kind of bladder or collecting vesicle for the organ, is provided with an extremely thin wall, lined with very large flattened cells. The second section of the nephridium is formed by the coiled tube, the epithelial lining of which varies slightly in the different parts. The third section (s.o.t), constitutes the most distinct portion of the whole organ. Its walls are formed of columnar cells almost filled by oval nuclei, which absorb colouring matters with very great avidity, and thus render this part extremely conspicuous. The nuclei are arranged in several rows. It ends by opening into a vesicle (Fig. 14, D), the wall of which is so delicate that it is destroyed when the nephridium is removed from the body, and consequently is not shown in Fig. 11.
Fig. 11.—Nephridium from the 9th pair of legs of P. capensis. o.s, External opening of segmental organ; p.f, internal opening of nephridium into the body cavity (lateral compartment); s, vesicle of segmental organ; s.c.1, s.c.2, s.c.3, s.c.4, successive regions of coiled portion of nephridium; s.o.t, third portion of nephridium broken off at p.f from the internal vesicle, which is not shown.
The fourth and fifth pairs are very considerably larger than those behind, and are in other respects peculiar. The great mass of each organ is placed behind the leg on which the external opening is placed, immediately outside one of the lateral nerve-cords. The external opening, instead of being placed near the base of the leg, is placed on the ventral side of the third ring (counting from the outer end) of the thicker portion of the leg. It leads into a portion which clearly corresponds with the collecting vesicle of the hinder nephridia. This part is not, however, dilated into a vesicle. The three pairs of nephridia in the three foremost pairs of legs are rudimentary, consisting solely of a vesicle and duct. The salivary glands are the modified nephridia of the segment of the oral papillae.
Generative Organs.
Male.—The male organs (Fig. 12) consist of a pair of testes (te), a pair of vesicles (v), vasa deferentia (v.d), and accessory glandular tubules (f). All the above parts lie in the central compartment of the body cavity. In P. capensis the accessory glandular bodies or crural glands of the last (17th) pair of legs are enlarged and prolonged into an elongated tube placed in the lateral compartment of the body cavity (a.g).
Fig. 12.—Male generative organs of Peripatus capensis, viewed from the dorsal surface. (After Balfour.) a.g, Enlarged crural glands of last pair of legs; F.16, 17, last pairs of legs; f, small accessory glandular tubes; p, common duct into which the vasa deferentia open; te, testis; v, seminal vesicle; v.c, nerve-cord; v.d, vas deferens.
The right vas deferens passes under both nerve-cords to join the left, and form the enlarged tube (p), which, passing beneath the nerve-cord of its side, runs to the external orifice. The enlarged terminal portion possesses thick muscular walls, and possibly constitutes a spermatophore maker, as has been shown to be the case in P. N. Zealandiae, by Moseley. In some specimens a different arrangement obtains, in that the left vas deferens passes under both nerve-cords to join the right.
Female.—The ovaries consist of a pair of tubes closely applied together, and continued posteriorly into the oviducts. The oviducts, after a short course, become dilated into the uteruses, which join behind and open to the exterior by a median opening. The ovaries always contain spermatozoa, some of which project through the ovarian wall into the body cavity. Spermatozoa are not found in the uterus and oviducts, and it appears probable that they reach the ovary directly by boring through the skin and traversing the body cavity.[[6]] In the neotropical species there is a globular receptaculum seminis opening by two short ducts close together into the oviduct, and there is a small receptaculum ovorum with extremely thin walls opening into the oviduct by a short duct just in front of the receptaculum seminis. The epithelium of the latter structure is clothed with actively moving cilia. In the New Zealand species there is a receptaculum seminis with two ducts, but the receptacula ovorum has not been seen.
There appear to be present in most, if not all, the legs some accessory glandular structures opening just externally to the nephridia. They are called the crural glands.
DEVELOPMENT.
As stated at the outset, Peripatus is found in three of the great regions, viz. in Africa, in Australasia, and in South America and the West Indies. It is a curious and remarkable fact that although the species found in these various localities are really closely similar, the principal differences relating to the structure of the female generative organs and to the number of the legs, they do differ in the most striking manner in the structure of the ovum and in the early development. In all the Australasian species the egg is large and heavily charged with food-yolk, and is surrounded by a tough membrane. In the Cape species the eggs are smaller, though still of considerable size; the yolk is much less developed, and the egg membrane is thinner though dense. In the neotropical species the egg is minute and almost entirely devoid of yolk. The unsegmented uterine ovum of P. Novae-Zealandiae measures 1.5 mm. in length by .8 mm. in breadth; that of P. capensis is .56 mm. in length; and that of P. Trinidadensis .04 mm. in diameter. In correspondence with these differences in the ovum there are differences in the early development, though the later stages are closely similar.
Fig. 13.—A series of embryos of P. capensis. The hind end of embryos B, C, D is uppermost in the figures, the primitive streak is the white patch behind the blastopore. (After Sedgwick.) A, Gastrula stage, ventral view, showing blastopore. B, Older gastrula stage, ventral view, showing elongated blastopore and primitive streak. C, Ventral view of embryo with three pairs of mesoblastic somites, dumb-bell-shaped blastopore and primitive streak. D, Ventral view of embryo, in which the blastopore has completely closed in its middle portion, and given rise to two openings, the embryonic mouth and anus. The anterior pair of somites have moved to the front end of the body, and the primitive groove has appeared on the primitive streak. E, Side view of embryo, in which the hind end of the body has begun to elongate in a spiral manner, and in which the appendages have begun. At, antenna; d, dorsal projection; p.s, preoral somite. F, Ventral view of head of embryo intermediate between E and G. The cerebral grooves are wide and shallow. The lips have appeared, and have extended behind the openings of the salivary glands, but have not yet joined in the middle line. At, antennae; c.g, cerebral groove; j, jaws; j.s, swelling at base of jaws; L, lips; M, mouth; or.p, oral papillae; o.s, opening of salivary gland. G, Side view of older embryo with the full number of appendages, to show the position in which the embryos lie in the uterus.
But unfortunately the development has only been fully worked out in one species, and to that species—P. capensis—the following description refers. The ova are apparently fertilised in the ovary, and they pass into the oviducts in April and May. In May the brood of the preceding year are born, and the new ova, which have meanwhile undergone cleavage, pass into the uterus. There are ten to twenty ova in each uterus. The segmentation is peculiar, and leads to the formation of a solid gastrula, consisting of a cortex of ectoderm nuclei surrounding a central endodermal mass, which consists of a much-vacuolated tissue with some irregularly-shaped nuclei. The endoderm mass is exposed at one point—the blastopore (gastrula mouth). The central vacuoles of the endoderm now unite and form the enteron of the embryo, and at the same time the embryo elongates into a markedly oval form, and an opacity—the primitive streak—appears at the hind end of the blastopore (Fig. 13, B). This elongation of the embryo is accompanied by an elongation of the blastopore, which soon becomes dumb-bell shaped (Fig. 13, C). At the same time the mesoblastic somites (embryonic segments of mesoderm) have made their appearance in pairs at the hind end, and gradually travel forward on each side of the blastopore to the front end, where the somites of the anterior pair soon meet in front of the blastopore (Fig. 13, D). Meanwhile the narrow middle part of the blastopore has closed by a fusion of its lips, so that the blastopore is represented by two openings, the future mouth and anus. A primitive groove makes its appearance behind the blastopore (Fig. 13, D). At this stage the hind end of the body becomes curved ventrally into a spiral (Fig. 13, E), and at the same time the appendages appear as hollow processes of the body wall, a mesoblastic somite being prolonged into each of them. The first to appear are the antennae, into which the praeoral somites are prolonged. The remainder appear from before backwards in regular order, viz. jaw, oral papillae, legs 1-17. The full number of somites and their appendages is not, however, completed until a later stage. The nervous system is formed as an annular thickening of ectoderm passing in front of the mouth and behind the anus, and lying on each side of the blastopore along the lines of the somites. The praeoral part of this thickening, which gives rise to the cerebral ganglia, becomes pitted inwards on each side (Fig. 13, F, c.g). These pits are eventually closed, and form the hollow ventral appendages of the supra-pharyngeal ganglia of the adult (Fig. 9, d). The lips are formed as folds of the side wall of the body, extending from the praeoral lobes to just behind the jaw (Fig. 13, F, L). They enclose the jaws (j) mouth (M), and opening of the salivary glands (o.s), and so give rise to the buccal cavity. The embryo has now lost its spiral curvature, and becomes completely doubled upon itself, the hind end being in contact with the mouth (Fig. 13, G). It remains in this position until birth. The just-born young are from 10-15 mm. in length and have green antennae, but the rest of the body is either quite white or of a reddish colour. This red colour differs from the colour of the adult in being soluble in spirit.
The mesoblastic somites are paired sacs formed from the anterior lateral portions of the primitive streak (Fig. 13, C). As they are formed they become placed in pairs on each side of the blastopore. The somites of the first pair eventually obtain a position entirely in front of the blastopore (Fig. 13, D). They form the somites of the praeoral lobes. The full complement of somites is acquired at about the stage of Fig. 13, E.
Fig. 14.—A series of diagrams of transverse sections through Peripatus embryos to show the relations of the coelom at successive stages. (After Sedgwick.) A, Early stage: 1, gut; 2, mesoblastic somite; no trace of the vascular space; endoderm and ectoderm in contact. B, Endoderm has separated from the dorsal and ventral ectoderm. The somite is represented as having divided on the left side into a dorsal and ventral portion: 1, gut; 2, somite; 3, haemocoele. C, The haemocoele (3) has become divided up into a number of spaces, the arrangement of which is unimportant. The dorsal part of each somite has travelled dorsalwards, and now constitutes a small space (triangular in section) just dorsal to the gut. The ventral portion (2′) has assumed a tubular character, and has acquired an external opening. The internal vesicle is already indicated, and is shown in the diagram by the thinner black line: 1, gut; 2′, nephridial part of coelom; 3, haemocoele; 3′, part of haemocoele which will form the heart—the part of the haemocoele on each side of this will form the pericardium; 4, nerve-cord. D represents the conditions at the time of birth; numbers as in C, except 5, slime glands. The coelom is represented as surrounded by a thick black line, except in the part which forms the internal vesicle of the nephridium.
The relations of the somites is shown in Fig. 14, A, which represents a transverse section taken between the mouth and anus of an embryo of the stage of Fig. 13, D. The history of these somites is an exceedingly interesting one, and may be described shortly as follows:—They divide into two parts—a ventral part, which extends into the appendage, and a dorsal part (Fig. 14, B). The ventral part acquires an opening to the exterior just outside the nerve-cord, and becomes entirely transformed into a nephridium (Fig. 14, D, 2′). The dorsal part shifts dorsalwards and diminishes relatively in size (Fig. 14, C). Its fate differs in the different parts of the body. In the anterior somites it dwindles and disappears, but in the posterior part it unites with the dorsal divisions of contiguous somites of the same side, and forms a tube—the generative tube (Fig. 14, D, 2). The last section of this tube retains its connexion with the ventral portion of the somite, and so acquires an external opening, which is at first lateral, but soon shifts to the middle line, and fuses with its fellow, to form the single generative opening. The praeoral somite develops the rudiment of a nephridium, but eventually entirely disappears. The jaw somite also disappears; the oral papilla somite forms ventrally the salivary glands, which are thus serially homologous with nephridia. The perivisceral cavity of Peripatus is, as in all Arthropoda, a haemocoele. Its various divisions develop as a series of spaces between the ectoderm and endoderm, and later in the mesoderm. The mesoderm seems to be formed entirely from the proliferation of the cells of the mesoblastic somites. It thus appears that in Peripatus the coelom does not develop a perivisceral portion, but gives rise only to the renal and reproductive organs.
Synopsis of the Species of Peripatus.
Peripatus, Guilding.
Soft-bodied vermiform animals, with one pair of ringed antennae, one pair of jaws, one pair of oral papillae, and a varying number of claw-bearing ambulatory legs. Dorsal surface arched and more darkly pigmented than the flat ventral surface. Skin transversely ridged and beset by wart-like spiniferous papillae. Mouth anterior, ventral; anus posterior, terminal. Generative opening single, median, ventral, and posterior. One pair of simple eyes. Brain large, with two ventral hollow appendages; ventral cords widely divaricated, without distinct ganglia. Alimentary canal simple, uncoiled. Segmentally arranged, paired nephridia are present. Body cavity is continuous with the vascular system, and does not communicate with the paired nephridia. Heart tubular, with paired ostia. Respiration by means of tracheae. Dioecious; males smaller and generally less numerous than females. Generative glands tubular, continuous with the ducts. Viviparous. Young born fully developed. They shun the light, and live in damp places beneath stones, leaves, and bark of rotten stumps. They eject when irritated a viscid fluid through openings at the apex of the oral papillae.
Distribution: South Africa, New Zealand, and Australia, South America and the West Indies [and in Sumatra?].
South African Species.
With three spinous pads on the legs and two primary papillae on the anterior side of the foot, and one accessory tooth on the outer blade of the jaw; with a white papilla on the ventral surface of the last fully developed leg of the male. Genital opening subterminal, behind the last pair of fully-developed legs. The terminal unpaired portion of vas deferens short. Ova of considerable size, but with only a small quantity of food-yolk. (Colour highly variable, number of legs constant in same species (?).)
P. capensis (Grube).—South African Peripatus, with seventeen pairs of claw-bearing ambulatory legs. Locality, Table Mountain.
P. Balfouri (Sedgwick).—South African Peripatus, with eighteen pairs of claw-bearing ambulatory legs, of which the last pair is rudimentary. With white papillae on the dorsal surface. Locality, Table Mountain.
P. brevis (De Blainville).—South African Peripatus, with fourteen pairs of ambulatory legs. Locality, Table Mountain. (I have not seen this species. Presumably it has the South African characters.)
P. Moseleyi (Wood Mason).—South African Peripatus, with twenty-one and twenty-two pairs of claw-bearing ambulatory legs. Locality, near Williamstown, Cape Colony; and Natal.[[7]]
Doubtful Species.
(1) South African Peripatus, with twenty pairs of claw-bearing ambulatory legs (Sedgwick). Locality, Table Mountain. (Also Peters, locality not stated.)
(2) South African Peripatus, with nineteen pairs of ambulatory legs (Trimen). Locality, Plettenberg Bay, Cape Colony. (Also Peters, locality not stated.)
Australasian Species.
With fifteen pairs of claw-bearing ambulatory legs, with three spinous pads on the legs, and a primary papilla projecting from the median dorsal portion of the feet. Genital opening between the legs of the last pair. Receptacula seminis present. Unpaired portion of vas deferens long and complicated. Ova large and heavily charged with yolk. (Colour variable, number of legs constant in same species (?).)
P. Novae Zealandiae (Hutton).—Australasian Peripatus, without an accessory tooth on the outer blade of the jaw, and without a white papilla on the base of the last leg of the male. New Zealand.
P. Leuckarti (Saenger).—Australasian Peripatus, with an accessory tooth on the outer blade of the jaw, and a white papilla on the base of the last leg of the male. Queensland.
Neotropical Species.
With four spinous pads on the legs, and the generative aperture between the legs of the penultimate pair. Dorsal white line absent. Primary papillae divided into two portions. Inner blade of jaw with gap between the first minor tooth and the rest. Oviducts provided with receptacula ovorum and seminis. Unpaired part of vas deferens very long and complicated. Ova minute, without food-yolk. (Colour fairly constant, number of legs variable in same species (?).)
P. Edwardsii.[[8]]—Neotropical Peripatus from Caracas, with a variable number of ambulatory legs (twenty-nine to thirty-four). Males with twenty-nine or thirty legs, and tubercles on a varying number of the posterior legs. The basal part or the primary papilla is cylindrical.
P. Trinidadensis (n. sp.).—Neotropical Peripatus from Trinidad, with twenty-eight to thirty-one pairs of ambulatory legs, and a large number of teeth on the inner blade of the jaw. The basal portion of the primary papillae is conical.
P. torquatus (Kennel).— Neotropical Peripatus from Trinidad, with forty-one to forty-two pairs of ambulatory legs. With a transversely placed bright yellow band on the dorsal surface behind the head.
Doubtful Species.
The above are probably distinct species. Of the remainder we do not know enough to say whether they are distinct species or not. The following is a list of these doubtful species, with localities and principal characters:—
P. juliformis (Guilding).—Neotropical Peripatus from St. Vincent, with thirty-three pairs of ambulatory legs.
P. Chiliensis (Gay).—Neotropical Peripatus from Chili, with nineteen pairs of ambulatory legs.
P. demeraranus (Sclater).—Neotropical Peripatus from Maccasseema, Demerara, with twenty-seven to thirty-one pairs of ambulatory legs and conical primary papillae.
Peripatus from Cayenne (Audouin and Milne-Edwards).—With thirty pairs of legs. Named P. Edwardsii by Blanchard.
Peripatus from Valentia Lake, Columbia (Wiegmann).—With thirty pairs of legs.
Peripatus from St. Thomas (Moritz).—No description.
Peripatus from Colonia Towar, Venezuela (Grube).—With twenty-nine to thirty-one pairs of ambulatory legs. Named P. Edwardsii by Grube.
Peripatus from Santo Domingo, Nicaragua (Belt).—With thirty-one pairs of ambulatory legs.
Peripatus from Dominica (Angas).—Neotropical Peripatus, with twenty-six to thirty (Pollard) pairs of ambulatory legs.
Peripatus from Jamaica (Gosse).—With thirty-one and thirty-seven pairs of ambulatory legs.
Peripatus from Santaram.—Neotropical Peripatus, with thirty-one pairs of ambulatory legs.
Peripatus from Cuba.—No details.
Peripatus from Hoorubea Creek, Demerara (Quelch).—With thirty pairs of legs.
Peripatus from Marajo (Branner).—No details.
Peripatus from Utuado, Porto Rico (Peters).—With twenty-seven, thirty, thirty-one, and thirty-two pairs of legs.
Peripatus from Surinam (Peters).—No details.
Peripatus from Puerto Cabello, Venezuela (Peters).—With thirty and thirty-two pairs of legs.
Peripatus from Laguayra, Venezuela (Peters).—No details.
Peripatus Quitensis (Schmarda).—From Quito, with thirty-six pairs of legs.
Peripatus from Sumatra (?).
P. Sumatranus (Horst).—Peripatus from Sumatra, with twenty-four pairs of ambulatory legs, and four spinous pads on the legs. The primary papillae of the neotropical character with conical bases. Generative opening between the legs of the penultimate pair. Feet with only two papillae.[[9]]
Summary of Distribution
Distribution of the South African Species—
Slopes of Table Mountain, neighbourhood of Williamstown, Plettenberg Bay—Cape Colony—Natal.
Distribution of the Australasian Species—
Queensland—Australia.
North and South Islands—New Zealand.
Oriental Region (?)—
Sumatra.
Distribution of the Neotropical Species—
Nicaragua.
Valencia Lake, Caracas, Puerto Cabello, Laguayra, Colonia Towar—Venezuela.
Quito—Ecuador.
Maccasseema, Hoorubea Creek—Demerara.
Surinam (Peters).
Cayenne.
Santarem, Marajo, at the mouth of the Amazon—Brazil.
Chili.
And in the following West Indian Islands—Cuba, Dominica, Porto Rico (Peters), Jamaica, St. Thomas, St. Vincent, Trinidad.