Part 14

n, Tentacular sheaths of lateral portion of the annular lobe.

u, The left eye.

b, The nuchal plate, continuous at its right and left posterior angles with the root of the mid-foot, and corresponding to the nuchal cartilage of Sepia.

c, Visceral hump.

d, The free margin of the mantle-skirt, the middle letter d points to that portion of the mantle-skirt which is reflected over a part of the shell as seen in fig. 1, b; the cup-like fossa to which b and d point in the present figure is occupied by the coil of the shell.

g.a. points to the lateral continuation of the nuchal plate b to join the root of the mid-foot or siphon.]

[Illustration: 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 margin of the mantle-skirt leading into the sub-pallial chamber.

g, The mid-foot or siphon.

k, The superficial origin of its retractor muscles closely applied to the shell and serving to hold the animal in its place.

l, The siphuncular pedicle of the visceral hump broken off short.

v, v, The superior and inferior ophthalmic 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.

[Illustration: FIG. 4.--View of the postero-ventral surface of a female Pearly Nautilus, the mantle-skirt (c) being completely reflected so as to show the inner wall of the sub-pallial chamber (drawn from nature by A.G. Bourne).

a, Muscular band passing from the mid-foot to the integument.

b, The valve on the surface of the funnel, partially concealed by the inrolled lateral margin of the latter.

c, The mantle-skirt retroverted.

an, The median anus.

x, Post-anal papilla of unknown significance.

g.n., Nidamental gland.

r.ov, Aperture of the right oviduct.

l.ov, Aperture of the rudimentary left oviduct (pyriform sac of Owen).

neph.a, Aperture of the left anterior renal sac.

neph.p, Aperture of the left posterior renal sac.

viscper, Left aperture of the viscero-pericardial sac.

olf, The left osphradium placed near the base of the anterior gill-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.

[Illustration: 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.]

The lobes of the fore-foot of _Nautilus_ and of the other Cephalopoda require further description. It has been doubted whether these lobes were rightly referred (by T.H. Huxley) to the fore-foot, and it has been maintained by some zoologists (H. Grenadier, H. von Jhering) that they are truly processes of the head. It appears to be impossible to doubt that the lobes in question are the fore-portion of the foot, when their development is examined (see fig. 35), further, when the fact is considered that they are innervated by the pedal ganglion. The fore-foot of _Nautilus_ completely surrounds the buccal cone (fig. 6, e), so as to present an appearance with its expanded tentacles similar to that of the disk of a sea-anemone (_Actinia_). A.G. Bourne, of University College, prepared from actual specimens the drawings of this part in the male and female _Nautilus_ reproduced in fig. 6, and restored the parts to their natural form when expanded. The drawings show very strikingly the difference between male and female. In the females (lower figure), we observe in the centre of the disk the buccal cone e carrying the beak-like pair of jaws which project from the finely papillate buccal membrane. Three tentaculiferous lobes of the fore-foot are in immediate contact with this buccal cone; they are the right and left (c, c) inner lobes, and the inferior inner lobe (d)--called inferior because it really lies ventralwards of the mouth. This inner inferior lobe is clearly a double one, representing a right and left inner inferior lobe fused into one. A lamellated organ on its surface, known as Owen's organ, probably olfactory in function (n), marks the separation of the constituent halves of this double lobe. Each half carries a group of fourteen tentacles. The right and the left inner lobes (c, c) each carry twelve tentacles. External to these three lobes the muscular substance of the mouth-embracing foot is raised into a wide ring, which becomes especially thick and large in the dorsal region where it is notably modified in form, offering a concavity into which the coil of the shell is received, and furnishing a protective roof to the retracted mass of tentacles. This part of the external annular lobe of the fore-foot is called the "hood" (figs. 2, 3, m). The median antero-posterior line traversing this hood exactly corresponds to the line of concrescence of the two halves of the fore-foot, which primitively grew forward one on each side of the head, and finally fused together along this line in front of the mouth. The tentacles carried by the great annular lobe are nineteen on each side, thirty-eight in all. They are called "digital," and are somewhat larger than the "labial" tentacles carried on the three inner lobes. The dorsalmost pair of tentacles (marked g in fig. 6) are the only ones which actually belong to that part of the disk which forms the great dorsal hood m. The hood is, in fact, to a large extent formed by the enlarged sheaths of these two tentacles. All the tentacles of the circumoral disk are set in remarkable tubular sheaths, into which they can be drawn. The sheaths of some of those belonging to the external or annular lobe are seen in fig. 3, marked n. The sheaths are muscular as well as the tentacles, and are simply tubes from the base of which the solid tentacle grows. The functional significance of this sheathing arrangement is as obscure as its morphological origin. With reference to the latter, it appears highly probable that the tubular sheath represents the cup of a sucker such as is found on the fore-foot of the Dibranchiata. In any case, it seems to the writer impossible to doubt that each tentacle, and its sheath on a lobe of the circumoral disk of Nautilus, corresponds to a sucker on such a lobe of a Dibranchiate. W. Keferstein follows Sir R. Owen in strongly opposing this identification, and in regarding such tentacle as the equivalent of a whole lobe or arm of a Decapod or Octopod Dibranch. The details of these structures, especially in the facts concerning the hectocotylus and spadix, afford the most conclusive reasons for dissenting from Owen's view. On the ventral side an extensive part of the internal surface of the muscular ring is laminated, forming the so-called "organ of Valenciennes," peculiar to the female and serving for the attachment of the spermatophores. We have so far enumerated in the female nautilus ninety tentacles. Four more remain which have a very peculiar position, and almost lead to the suggestion that the eye itself is a modified tentacle. These remaining tentacles are placed one above (before) and one below (behind) each eye, and bring up the total to ninety-four (fig. 3 v, v).

[Illustration: FIG. 6.--Male (upper) and female (lower) specimens of _Nautilus pompilius_ as seen in the expanded condition, the observer looking down on to the buccal cone e; one-third the natural size linear. The drawings have been made from actual specimens by A.G. Bourne, B. Sc., University College, London.

a, The shell.

b, The _outer_ ring-like expansion (annular lobe) of the circumoral muscular mass of the fore-foot, carrying nineteen tentacles on each side--posteriorly this is enlarged to form the "hood" (marked v in fig. 1 and m in figs. 2 and 3). giving off the pair of tentacles marked g in the present figure.

c, The right and left inner lobes of the fore-foot, each carrying twelve tentacles in the female, in the male subdivided into p, the "spadix" or hectocotylus on the left side, and q, the "anti-spadix," a group of four tentacles on the right side--it is thus seen that the subdivided right and left inner lobes of the male correspond to the undivided right and left inner lobes of the female.

d, The inner inferior lobe of the fore-foot, a bilateral structure in the female carrying two groups, each of fourteen tentacles, separated from one another by a lamellated organ n, supposed to be olfactory in function--in the male the inner inferior lobe of the fore-foot is very much reduced, and has the form of a paired group of lamellae (d in the upper figure).

e, The buccal cone, rising from the centre of the three inner lobes, and fringing the protruded calcareous beaks or jaws with a series of minute papillae.

f, The tentacles of the outer circumoral lobe or annular lobe of the fore-foot projecting from their sheaths.

g, The two most posterior tentacles of this series belonging to that part of the annular lobe which forms the hood (m in figs. 2 and 3).

i, Superior ophthalmic tentacle.

k, Inferior ophthalmic tentacle.

l, Eye.

m, Paired laminated organ on each side of the base of the inner inferior lobe (d) of the female.

n, Olfactory lamellae upon the inner inferior lobe (in the female).

o, The siphon (mid-foot).

p, The spadix (in the male), the hectocotylized portion of the left inner lobe of the fore-foot representing four modified tentacles, eight being left unmodified.

q, The anti-spadix (in the male), being four of the twelve tentacles of the right inner lobe of the fore-foot isolated from the remaining eight, and representing on the right side the differentiated spadix of the left side. The four tentacles of the anti-spadix are set, three on one base and one on a separate base.]

In the adult male nautilus we find the following important differences in the tentaculiferous disk as compared with the female (see upper drawing in fig. 6). The inner inferior lobe is rudimentary, and carries no tentacles. It is represented by three groups of lamellae (d), which are not fully exposed in the drawing. The right and left inner lobes are subdivided each into two portions. The right shows a larger portion carrying eight tentacles, and smaller detached groups (q) of four tentacles, of which three have their sheaths united whilst one stands alone. These four tentacles may be called the "anti-spadix." The left inner lobe shows a similar larger portion carrying eight tentacles, and a curious conical body behind it corresponding to the anti-spadix. This is the "spadix." It carries no tentacles, but is terminated by imbricated lamellae. These lamellae appear to represent the four tentacles of the anti-spadix of the right internal lobe, and are generally regarded as corresponding to that modification of the sucker-bearing arms of male Dibranchiate Siphonopods to which the name "hectocotylus" is applied. The spadix is in fact the hectocotylized portion of the fore-foot of the male nautilus. The hectocotylized arm or lobe of male Dibranchiata is connected with the process of copulation, and in the male nautilus the spadix has probably a similar significance, though it is not possible to suggest how it acts in this relation. It is important to observe that the modification of the fore-foot in the male as compared with the female nautilus is not confined to the existence of the spadix. The anti-spadix and the reduction of the inner inferior lobe are also male peculiarities. The external annular lobe in the male does not differ from that of the female; it carries nineteen tentacles on each side. The four ophthalmic tentacles are also present. Thus in the male nautilus we find altogether sixty-two tentacles, the thirty-two additional tentacles of the female being represented by lamelliform structures.

_Musculature, Fins and, Cartilaginous Skeleton._--Without entering into a detailed account of the musculature of _Nautilus_, we may point out that the great muscular masses of the fore-foot and of the mid-foot (siphon) are ultimately traceable to a large transverse mass of muscular tissue, the ends of which are visible through the integument on the right and left surfaces of the body dorsal of the free flap of the mantle-skirt (fig. 1, l, l, and fig. 3, k). These muscular areae have a certain adhesion to the shell, and serve both to hold the animal in its shell and as the fixed supports for the various movements of the tentaculiferous lobes and the siphon. They are to be identified with the ring-like area of adhesion by which the foot-muscle of the limpet is attached to the shell of that animal. In the Dibranchs a similar origin of the muscular masses of the fore-foot and mid-foot from the sides of the shell--modified, as this is, in position and relations--can be traced.

[Illustration: FIG. 7.--Minute structure of the cartilage of _Loligo_ (from Gegenbaur, after Furbringer)

a, Simple cells.

b, Dividing cells.

c, Canaliculi.

d, An empty cartilage capsule with its pores.

e, Canaliculi in section.]

In _Nautilus_ there are no fin-like expansions of the integument, whereas such occur in the Decapod Dibranchs along the sides of the visceral hump (figs. 15, 16). As an exception among Octopoda lateral fins occur in _Pinnoctopus_ (fig. 38, A), and in _Cirrhoteuthis_ (fig. 38, D).

In _Nautilus_ there is a curious plate-like expansion of integument in the mid-dorsal region just behind the hood, lying between that structure and the portion of mantle-skirt which is reflected over the shell. This is shown in fig. 2, b. If we trace out the margin of this plate we find that it becomes continuous on each side with the sides of the funnel. In _Sepia_ and other Decapods (not in Octopods) a closely similar plate exists in an exactly corresponding position (see b in figs. 10, 26). In _Sepia_ a cartilaginous development occurs here immediately below the integument forming the so-called "nuchal plate," drawn in fig. 8, D. The morphological significance of this nuchal lamella, as seen both in _Nautilus_ and in _Sepia_, is not obvious. Cartilage having the structure shown in fig. 7 occurs in various regions of the body of Cephalopoda. In all Glossophorous Mollusca the lingual apparatus is supported by internal skeletal pieces, having the character of cartilage; but in the Cephalopoda such cartilage has a wider range.

In _Nautilus_ a large H-shaped piece of cartilage is found, forming the axis of the funnel (fig. 8, A, B). Its hinder part extends up into the head and supports the peri-oesophageal nerve-mass (a), whilst its two anterior rami extend into the tongue-like siphon. In _Sepia_, and Dibranchs generally, the cartilage takes a different form, as shown in fig. 8, C. The processes of this cartilage cannot be identified in any way with those of the capito-pedal cartilage of _Nautilus_. The lower larger portion of this cartilage in _Sepia_ is called the cephalic cartilage, and forms a complete ring round the oesophagus; it completely invests also the ganglionic nerve-collar, so that all the nerves from the latter have to pass through foramina in the cartilage. The outer angles of this cartilage spread out on each side so as to form a cup-like receptacle for the eyes. The two processes springing right and left from this large cartilage in the median line (fig. 8, C) are the "pre-orbital cartilages"; in front of these, again, there is seen a piece like an inverted T, which forms a support to the base of the "arms" of the fore-foot, and is the "basi-brachial" cartilage. The Decapod Dibranchs have, further, the "nuchal cartilage" already mentioned, and in _Sepia_, a thin plate-like "sub-ostracal" or (so-called) dorsal cartilage, the anterior end of which rests on and fits into the concave nuchal cartilage. In Octopoda there is no nuchal cartilage, but two band-like "dorsal cartilages." In Decapods there are also two cartilaginous sockets on the sides of the funnel--"siphon-hinge cartilages"--into which fleshy knobs of the mantle-skirt are loosely fitted. In _Sepia_, along the whole base-line of each lateral fin of the mantle (fig. 15), is a "basi-pterygial cartilage." It is worthy of remark that we have, thus developed, in Dibranch Cephalopods a more complete internal cartilaginous skeleton than is to be found in some of the lower vertebrates. There are other instances of cartilaginous endo-skeleton in groups other than the Vertebrata. Thus in some capito-branchiate Chaetopods cartilage forms a skeletal support for the gill-plumes, whilst in the Arachnids (_Mygale_, _Scorpio_) and in _Limulus_ a large internal cartilaginous plate--the ento-sternite--is developed as a support for a large series of muscles.

[Illustration: FIG. 8.--Cartilaginous skeleton of Cephalopoda (after Keferstein.)

A, Capito-pedal cartilage of _Nautilus pompilius._

a points to the ridge which supports the pedal portion of the nerve-centre.

B, Lateral view of the same--the large anterior processes are sunk in the muscular substance of the siphon.

C, Cephalic cartilages of _Sepia officinalis_.

D, Nuchal cartilage of _Sepia officinalis_.]

_Alimentary Tract._--The buccal cone of _Nautilus_ is terminated by a villous margin (buccal membrane), surrounding the pair of beak-like jaws, of which the ventral projects over the dorsal. These are very strong and dense in _Nautilus_, being calcified. Fossilized beaks of Tetrabranchiata are known under the name of rhyncholites. In Dibranchs the beaks are horny, but similar in shape to those of _Nautilus_. They resemble in general those of a parrot, the lower beak being the larger and overlapping the upper or dorsal beak. The lingual ribbon and odontophoral apparatus have the structure which is typical for Glossophorous Mollusca. In fig. 9, A is represented a single row of teeth from the lingual ribbon of _Nautilus_, and in fig. 9, B, C, of other Cephalopoda.

In _Nautilus_ a long and wide crop or dilated oesophagus (fig. 10, cr) passes from the muscular buccal mass, and at the apex of the visceral hump passes into a highly muscular stomach, resembling the gizzard of a bird (fig 10, gizz). A nearly straight intestine passes from the muscular stomach to the anus, near which it develops a small caecum. In other Cephalopods the oesophagus is usually narrower and the muscular stomach more capacious, whilst a very important feature in the alimentary tract is formed by the caecum. In all but _Nautilus_ the caecum lies near the stomach, and may be very capacious--much larger than the stomach in _Loligo vulgaris_--or elongated into a spiral coil. The simple U-shaped flexure of the alimentary tract, as seen in fig. 10, is the only important one which it exhibits in the Cephalopoda. The acini of the large liver of _Nautilus_ are compacted into a solid reddish-brown mass by a firm membrane, as also is the case in the Dibranchiata. The liver has four paired lobes in _Nautilus_, which open by two bile-ducts into the alimentary canal at the commencement of the intestine. The bile-ducts unite before entering the intestine. In Dibranchiata the two large lobes of the liver are placed antero-dorsally (beneath the shell in Decapoda), and the bile-ducts open into the caecum. Upon the bile-ducts in Dibranchiata are developed yellowish glandular diverticula, which are known as "pancreas," though neither physiologically nor morphologically is there any ground for considering either the so-called liver or the so-called pancreas as strictly equivalent to the glands so denominated in the Vertebrata. In _Nautilus_ the equivalents of the pancreatic diverticula of the Dibranchs can be traced upon the relatively shorter bile-ducts.

[Illustration: FIG. 9.--Lingual dentition of Cephalopoda. A, A single row of lingual teeth of _Nautilus pompilius_ (after Keferstein). B, Two rows of lingual teeth of _Sepia officinalis_ (after Troschel). C, Lingual teeth of _Eledone cirrhosa_ (after Loven).]

[Illustration: FIG. 10.--Diagram representing a vertical approximately median antero-posterior section of _Nautilus pompilius_ (from a drawing by A.G. Bourne). The parts which are quite black are the cut muscular surfaces of the foot and buccal mass.

a, The shell.

b, The nuchal plate, identical with the nuchal cartilage of _Sepia_ (see fig. 2, b).

c, The integument covering the visceral hump.

d, The mantle flap or skirt in the dorsal region where it rests against the coil of the shell.

e, The inferior margin of the mantle-skirt resting on the lip of the shell represented by the dotted line.

f, The pallial chamber with two of the four gills.

g, The vertically cut median portion of the mid-foot (siphon).

h, The capito-pedal cartilage (see fig. 8).

i, The valve of the siphon.

l, The siphuncular pedicle (cut short).

m, The hood or dorsal enlargement of the annular lobe of the fore-foot.

n, Tentacles of the annular lobe.

p, Tentacles of inner inferior lobe.

q, Buccal membrane.

r, Upper jaw or beak.

s, Lower jaw or beak.

t, Lingual ribbon.

x, The viscero-pericardial sac.

n.c, Nerve-collar.

oe, Oesophagus.

cr, Crop.

gizz, Gizzard.

int, Intestine.

an, Anus.

nept, Aperture of a nephridial sac.

r.e, Renal glandular masses on the walls of the afferent branchial veins (see fig. 11).