Page:EB1911 - Volume 05.djvu/715

 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.

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.

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