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.
| 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). |
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| Fig. 10.—Diagram representing a vertical approximately medianantero-posterior section of Nautilus pompilius (from a drawing byA.G. Bourne). The parts which are quite black are the cut muscularsurfaces of the foot and buccal mass. | |
a, The shell. b, The nuchal plate, identicalwith the nuchal cartilage ofSepia (see fig. 2, b). c, The integument covering thevisceral hump. d, The mantle flap or skirt in thedorsal region where it restsagainst the coil of the shell. e, The inferior margin of themantle-skirt resting on thelip of the shell representedby the dotted line. f, The pallial chamber with twoof the four gills. g, The vertically cut median portionof the mid-foot (siphon). h, The capito-pedal cartilage (seefig. 8). i, The valve of the siphon. l, The siphuncular pedicle (cutshort). m, The hood or dorsal enlargementof the annular lobe ofthe fore-foot. n, Tentacles of the annularlobe. p, Tentacles of inner inferiorlobe. | q, Buccal membrane. r, Upper jaw or beak. s, Lower jaw or beak. t, Lingual ribbon. x, The viscero-pericardialsac. n.c, Nerve-collar. oe, Oesophagus. cr, Crop. gizz, Gizzard. int, Intestine. an, Anus. nept, Aperture of a nephridialsac. r.e, Renal glandular masses onthe walls of the afferentbranchial veins (see fig.11). a.b.v, Afferent branchial vessel. e.b.v, Efferent branchial vessel. vt, Ventricle of the heart. |
Posterior salivary glands are not developed in Nautilus, but on each side in the wall of the buccal mass is a gland corresponding to the anterior salivary gland of the Dibranchiata. No ink-sac is present in Nautilus.
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| Fig. 11.—Diagram to show the relations of the four nephridialsacs, the viscero-pericardial sac, and the heart and large vessels inNautilus (drawn by A.G. Bourne). | |
neph, neph, on the right sidepoint to the two nephridiaof that side (the two ofthe opposite side are notlettered)—each is seen tohave an independentaperture. x is the viscero-pericardial sac,the dotted line indicatingits backward extension. visc. per. apert, marks an arrowintroduced into the rightaperture of the viscero-pericardialsac. | r.e, r.e, point to the glandular enlargedwalls of the afferentbranchial vessels—twosmall glandular bodies ofthe kind are seen to projectinto each nephridialsac, whilst a larger body ofthe same kind depends fromeach of the four branchialafferent vessels into theviscero-pericardial sac. v.c, Vena-cava. vent, Ventricle of the heart. ao, Cephalic aorta (the smallabdominal aorta notdrawn). a.b.v, Branchial vessel. e.v.b, Efferent branchial vessel. |
Coelom, Blood-vascular System and Excretory Organs.—Nautilus and the other Cephalopoda conform to the general Molluscan characters in regard to these organs. Whilst the general visceral cavity forms a lacunar blood-system or series of narrow spaces, connected with the trunks of a well-developed vascular system, that part of the original coelom surrounding the heart and known as the Molluscan pericardium is shut off from this general blood-lymph system, and communicates, directly in Nautilus, in the rest through the renal sacs, with the exterior. In the Cephalopoda this specialized pericardial cavity is particularly large, and has been recognized as distinct from the blood-carrying spaces, even by anatomists who have not considered the pericardial space of other Mollusca to be thus isolated. The enlarged pericardium, which may even take the form of a pair of sacs, has been variously named, but is best known as the viscero-pericardial sac or chamber. In Nautilus this sac occupies the whole of the postero-dorsal surface and a part of the antero-dorsal (see fig. 10, x), investing the genital and other viscera which lie below it, and having the ventricle of the heart suspended in it. Certain membranes forming incomplete septa, and a curious muscular band—the pallio-cardiac band—traverse the sac. The four branchial afferent veins, which in traversing the walls of the four renal sacs give off, as it were, glandular diverticula into those sacs, also give off at the same points four much larger glandular masses, which hang freely into the viscero-pericardial chamber (fig. 11, r.e). In Nautilus the viscero-pericardial sac opens to the exterior directly by a pair of apertures, one placed close to the right and one close to the left posterior renal aperture (fig. 5, visc. per). This direct opening of the pericardial sac to the exterior is an exception to what occurs in all other Mollusca. In all other Molluscs the pericardial sac opens into the renal organs, and through them or the one renal organ to the exterior. In Nautilus there is no opening from the viscero-pericardial sac into the renal sacs. Therefore the external pore of the viscero-pericardial sac may possibly be regarded as a shifting of the reno-pericardial orifice from the actual wall of the renal sac to a position alongside of its orifice. Parallel cases of such shifting are seen in the varying position of the orifice of the ink-bag in Dibranchiata, and in the orifice of the genital ducts of Mollusca, which in some few cases (e.g. Spondylus) open into the renal organs, whilst in other cases they open close by the side of the renal organs on the surface of the body. The viscero-pericardial sac of the Dibranchs is very large also, and extends into the dorsal region. It varies in shape—that is to say, in the extensions of its area right and left between the various viscera—in different genera, but in the Decapods is largest. In an extension of this chamber is placed the ovary of Sepia, whilst the ventricle of the heart and the branchial hearts and their appendages also lie in it. It is probable that water is drawn into this chamber through the renal sacs, since sand and other foreign matters are found in it. In all it opens into the pair of renal sacs by an orifice on the wall of each, not far from the external orifice (fig. 29, y, y′). There does not seem any room for doubting that each orifice corresponds to the reno-pericardial orifice which we have seen in the Gastropoda, and shall find again in the Lamellibranchia.
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| Fig. 12.—Diagram to show the relations of the heart in theMollusca. (From Gegenbaur.) | |
A, Part of the dorsal vasculartrunk and transverse trunksof a worm. B, Ventricle and auricles ofNautilus. C, Of a Lamellibranch, of Chiton,or of Loligo. D, Of Octopus. | E, Of a Gastropod. a, Auricle. v, Ventricle. ac, Arteria = cephalica = (aorta). ai, Arteria abdominalis. Thearrows show the directionof the blood-current. |
The circulatory organs, blood-vessels and blood of Nautilus do not differ greatly from those of Gastropoda. The ventricle of the heart is a four-cornered body, receiving a dilated branchial efferent vessel (auricle) at each corner (fig. 11). It gives off a cephalic aorta anteriorly, and a smaller abdominal aorta posteriorly. The diagram, fig. 12, serves to show how this simple form of heart is related to the dorsal vessel of a worm or of an Arthropod, and how by a simple flexure of the ventricle (D) and a subsequent suppression of one auricle, following on the suppression of one branchia, one may obtain the form of heart characteristic of the anisopleurous Gastropoda (excepting the Aspidobranchia). The flexed condition of the heart is seen in Octopus, and is to some extent approached by Nautilus, the median vessels not presenting that perfect parallelism which is shown in the figure (B). The most remarkable feature presented by the heart of Nautilus is the possession of four instead of two auricles, a feature which is simply related to the metamerism of the branchiae. By the left side of the heart of Nautilus, attached to it by a membrane, and hanging loosely in the viscero-pericardial chamber, is the pyriform sac of Owen. This has been shown to be the rudimentary left oviduct or sperm-duct, as the case may be (E.R. Lankester and A.G. Bourne), the functional right ovi-sac and its duct being attached by a membrane to the opposite side of the heart.
The cephalic and abdominal aortae of Nautilus appear, after running to the anterior and posterior extremes of the animal respectively, to open into sinus-like spaces surrounding the viscera, muscular masses, &c. These spaces are not large, but confined and shallow. Capillaries are stated to occur in the integument. In the Dibranchs the arterial system is very much more complete; it appears in some cases to end in irregular lacunae or sinuses, in other cases in true capillaries which lead on into veins. An investigation of these capillaries in the light of modern histological knowledge is much needed. From the sinuses and capillaries the veins take origin, collecting into a large median trunk (the vena cava), which in the Dibranchs as well as in Nautilus has a ventral (postero-ventral) position, and runs parallel to the long axis of the animal. In Nautilus this vena cava gives off at the level of the gills four branchial afferent veins (fig. 11, v.c.), which pass into the four gills without dilating. In the Dibranchs at a similar position the vena cava gives off a right and a left branchial afferent vein, each of which, traversing the wall of the corresponding renal sac and receiving additional factors, dilates at the base of the corresponding branchial plume, forming there a pulsating sac—the branchial heart. Attached to each branchial heart is a curious glandular body, which may possibly be related to the larger masses (fig. 11, r.e.) which depend into the viscero-pericardial cavity from the branchial afferent veins of Nautilus. From the dilated branchial heart the branchial afferent vessel proceeds, running up the adpallial face of the gill-plume. From each gill-plume the blood passes by the branchial efferent vessels to the heart, the two auricles being formed by the dilatation of these vessels.