Encyclopaedia Britannica, 11th Edition, "Celtes, Konrad" to "Ceramics" / Volume 5, Slice 6

by Various

Fig. 1.—Lateral view of the female Pearly Nautilus, contracted by spiritand lying in its shell, the right half of which is cut away (from Gegenbaur,after Owen).
a, Visceral hump. b, Portion of the free edge of themantle-skirt reflected on to theshell,—the edge of the mantle-skirtcan be traced downwardsand forwards around the baseof the mid-foot or siphon i. l, l, Superficial origin of the retractormuscle of the mid-foot (siphon),more or less firmly attached tothe shell, of which a small piece(s) is seen between the lettersl, l. s, (farther back) points to the siphuncularpedicle, which is brokenoff short and not continued, asin the perfect state, through thewhole length of the siphuncle ofthe shell, also marked s and s’.	o, points to the right eye. t, is placed near the extremities ofthe contracted tentacles of theouter or annular lobe of thefore-foot—the jointed tentaclesare seen protruding a little fromtheir long cylindrical sheaths. v, The dorsal “hood” formed byan enlargement in this regionof the annular lobe of the fore-foot(m in figs. 2, 3). V, A swelling of the mantle-skirt,indicating the position on itsinner face of the nidamentalgland (see fig. 4, g.n.).

Visceral Hump and Shell.—The visceral hump of Nautilus (if we exclude from consideration the fine siphuncular pedicle which it trails, as it were, behind it) is very little, if at all, affected by the coiled form of the shell which it carries, since the animal always slips forward in the shell as it grows, and inhabits a chamber which is practically cylindrical (fig. 1). Were the deserted chambers thrown off instead of being accumulated behind the inhabited chamber as a coiled series of air-chambers, we should have a more correct indication in the shell of the extent and form of the animal’s body. Amongst Gastropods it is not very unusual to find the animal slipping forward in its shell as growth advances and leaving an unoccupied chamber in the apex of the shell. This may indeed become shut off from the occupied cavity by a transverse septum, and a series of such septa may be formed, but in no Gastropod are these apical chambers known to contain a gas during the life of the animal in whose shell they occur. A further peculiarity of the nautilus shell and of that of the allied extinct Ammonites, Scaphites, Orthoceras, &c., and of the living Spirula, is that the series of deserted air-chambers is traversed by a cord-like pedicle extending from the centro-dorsal area of the visceral hump to the smallest and first-formed chamber of the series. No structure comparable to this siphuncular pedicle is known in any other Mollusca. The siphuncle does not communicate with the coelomic cavity; it is a simple vascular process of the mantle, whose cavity consists of a venous sinus, and whose wall contains a ramification of the pallial artery. There appears to be no doubt that the deserted chambers of the nautilus shell contain in the healthy living animal a gas which serves to lessen the specific gravity of the whole organism. This gas is said to be of the same composition as the atmosphere, with a larger proportion of nitrogen. With regard to its origin we have only conjectures. Each septum shutting off an air-containing chamber is formed during a period of quiescence, probably after the reproductive act, when the visceral mass of the nautilus may be slightly shrunk, and gas is secreted from the dorsal integument so as to fill up the space previously occupied by the animal. A certain stage is reached in the growth of the animal when no new chambers are formed. The whole process of the loosening of the animal in its chamber and of its slipping forward when a new septum is formed, as well as the mode in which the air-chambers may be used as a hydrostatic apparatus, and the relation to this use, if any, of the siphuncular pedicle, is involved in obscurity, and is the subject of much ingenious speculation. In connexion with the secretion of gas by the animal, besides the parallel cases ranging from the protozoon Arcella to the physoclistic fishes, from the hydroid Siphonophora to the insect-larva Corethra, we have the identical phenomenon observed in the closely allied Sepia when recently hatched. Here, in the pores of the internal rudimentary shell, gas is observable, which has necessarily been liberated by the tissues which secrete the shell, and not derived from any external source (Huxley).

The coiled shell of Nautilus, and of the majority of extinct Tetrabranchiata, is peculiar in its relation to the body of the animal, inasmuch as the curvature of the coil proceeding from the centro-dorsal area is towards the head or forwards, instead of away from the head and backwards as in other discoid coiled shells such as Planorbis; the coil is in fact absolutely reversed in the two cases. Such a shell is said to be exogastric. But in some extinct forms, e.g. Phragmoceras, Cyrtoceras, Ptenoceras, the shell is coiled towards the ventral side, when it is termed endogastric. Amongst the extinct allies of the nautilus (Tetrabranchiata) we find shells of a variety of shapes, open coils such as Scaphites, leading on to perfectly cylindrical shells with chamber succeeding chamber in a straight line (Orthoceras), whence again we may pass to the corkscrew spires formed by the shell of Turrilites. In some extinct genera, e.g. Gomphoceras, among the Nautiloidea the aperture of the shell is contracted and the edge of the aperture is lobed. In these cases the animal was probably able only to protrude its appendages and not its whole head. The ventral part of the aperture corresponding to the funnel is separated from the dorsal part by a constriction. Hence it is possible to distinguish the ventral and dorsal sides of the shell and to decide whether it was exogastric or endogastric. The direction of the coil of the shell cannot be determined by the position of the siphuncle, which traverses the septa centrally, ventrally or dorsally. Contracted shell apertures occur also in Ammonitoidea, the condition reaching an extreme in Morphoceras, where the original aperture is subdivided by the ingrowth of the sides, so that only five small separate apertures remain. Of these the central probably corresponded to the mouth, two lateral to the eyes, and the remaining two to the pedal appendages.

Head, Foot, Mantle-skirt and Sub-pallial Chamber.—In the pearly nautilus the ovoid visceral hump is completely encircled by the free flap of integument known as mantle-skirt (figs. 2, 3, d, e). In the antero-dorsal region this flap is enlarged so as to be reflected a little over the coil of the shell which rests on it. In the postero-ventral region the flap is deepest, forming an extensive sub-pallial chamber, at the entrance of which e is placed in fig. 3. A view of the interior of the sub-pallial chamber, as seen when the mantle-skirt is retroverted and the observer faces in the direction indicated by the reference line passing from e in fig. 3, is given in fig. 4. With this should be compared the similar view of the sub-pallial chamber of the Dibranchiate Sepia. It should be noted as a difference between Nautilus and the Dibranchiates that in the former the nidamental gland (in the female) lies on that surface of the pallial chamber formed by the dependent mantle-flap (fig. 4, g.n.; fig. 1, V), whilst in the latter it lies on the surface formed by the body-wall; in fact in the former the base of the fold forming the mantle-skirt comprises in its area a part of what is unreflected visceral hump in the latter.

Fig. 2.—Spirit specimen of female Pearly Nautilus, removed fromits shell, and seen from the antero-dorsal aspect (drawn from natureby A.G. Bourne).
m, The dorsal “hood” formedby the enlargement of theouter or annular lobe of thefore-foot, and correspondingto the sheaths of two tentacles(g, g in fig. 6). n, Tentacular sheaths of lateralportion of the annular lobe. u, The left eye. b, The nuchal plate, continuousat its right and left posteriorangles with the root of themid-foot, and correspondingto the nuchal cartilage ofSepia.	c, Visceral hump. d, The free margin of the mantle-skirt,the middle letter dpoints to that portion of themantle-skirt which is reflectedover a part of theshell as seen in fig. 1, b; thecup-like fossa to which b andd point in the present figureis occupied by the coil of theshell. g.a. points to the lateral continuationof the nuchal plateb to join the root of the mid-footor siphon.

Fig. 3.—Lateral view of the same specimen as that drawn in fig. 2.Letters as in that figure with the following additions—
e, points to the concave marginof the mantle-skirt leadinginto the sub-pallial chamber. g, The mid-foot or siphon. k, The superficial origin of itsretractor muscles closelyapplied to the shell andserving to hold the animalin its place.	l, The siphuncular pedicle ofthe visceral hump brokenoff short. v, v, The superior and inferiorophthalmic tentacles.

The apertures of the two pairs of renal sacs, of the viscero-pericardial sac, of the genital ducts, and of the anus, are shown in position on the body-wall of the pallial chamber of Nautilus in figs. 4, 5. There are nine apertures in all, one median (the anus) and four paired. Besides these apertures we notice two pairs of gill-plumes which are undoubtedly typical ctenidia, and a short papilla (the osphradium) between each anterior and posterior gill-plume (see figs. 4, 5, and explanation). As compared with this in a Dibranchiate, we find (fig. 25) only four apertures, viz. the median anus with adjacent orifice of the ink-sac, the single pair of renal apertures, and one asymmetrical genital aperture (on the left side) except in female Octopoda and a few others, where the genital ducts and their apertures are paired. No viscero-pericardial pores are present on the surface of the pallial chamber, since in the Dibranchiata the viscero-pericardial sac opens by a pore into each nephridium instead of directly to the surface. A single pair of ctenidia (gill-plumes) is present instead of the two pairs in Nautilus. The existence of two pairs of ctenidia and of two pairs of renal sacs in Nautilus, placed one behind the other, is highly remarkable. The interest of this arrangement is in relation to the general morphology of the Mollusca, for it is impossible to view this repetition of organs in a linear series as anything else than an instance of metameric segmentation, comparable to the segmentation of the ringed worms and Arthropods. The only other example which we have of this metamerism in the Mollusca is presented by the Chitons. There we find not two pairs of ctenidia merely, but sixteen pairs (in some species more) accompanied by a similar metamerism of the dorsal integument, which carries eight shells. In Chiton the renal organs are not affected by the metamerism as they are in Nautilus. It is impossible on the present occasion to discuss in the way which their importance demands the significance of these two instances among Mollusca of incomplete or partial metamerism; but it would be wrong to pass them by without insisting upon the great importance which the occurrence of these isolated instances of metameric segmentation in a group of otherwise unsegmented organisms possesses, and the light which they may be made to throw upon the nature of metameric segmentation in general.

Fig. 4.—View of the postero-ventral surface of a female PearlyNautilus, the mantle-skirt (c) being completely reflected so as toshow the inner wall of the sub-pallial chamber (drawn from natureby A.G. Bourne).
a, Muscular band passing fromthe mid-foot to the integument. b, The valve on the surface ofthe funnel, partially concealedby the inrolled lateralmargin of the latter. c, The mantle-skirt retroverted. an, The median anus. x, Post-anal papilla of unknownsignificance. g.n., Nidamental gland. r.ov, Aperture of the right oviduct.	l.ov, Aperture of the rudimentaryleft oviduct (pyriform sacof Owen). neph.a, Aperture of the left anteriorrenal sac. neph.p, Aperture of the left posteriorrenal sac. viscper, Left aperture of theviscero-pericardial sac. olf, The left osphradium placednear the base of the anteriorgill-plume.
The four gill-plumes (ctenidia) are not lettered.

The foot and head of Nautilus are in the adult inextricably grown together, the eye being the only part belonging primarily to the head which projects from the all-embracing foot. The fore-foot or front portion of the foot has the form of a number of lobes carrying tentacles and completely surrounding the mouth (figs. 2, 3). The epipodia incline towards each other posteriorly so as to form an incomplete siphon (fig. 4), a condition which is completed and rendered permanent in the tubular funnel of Dibranchiata. The epipodial nature of the funnel is well seen in young embryos, in which this organ is situated laterally and posteriorly between the mantle and the foot.

Fig. 5.—View of the postero-ventral surface of a male Pearly Nautilus, the mantle-skirt (c) being completely reflected so as to show the inner wall of the sub-pallial chamber, and the four ctenidia and the foot cut short (drawn from nature by A.G. Bourne). pe, Penis, being the enlarged termination of the right spermatic duct; l.sp, aperture of the rudimentary left spermatic duct (pyriform sac of Owen). Other letters as in fig. 4.