Part 15

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_.

[Illustration: FIG. 11.--Diagram to show the relations of the four nephridial sacs, the viscero-pericardial sac, and the heart and large vessels in _Nautilus_ (drawn by A.G. Bourne).

neph, neph, on the right side point to the two nephridia of that side (the two of the opposite side are not lettered)--each is seen to have an independent aperture.

x is the viscero-pericardial sac, the dotted line indicating its backward extension.

visc. per. apert, marks an arrow introduced into the right aperture of the viscero-pericardial sac.

r.e, r.e, point to the glandular enlarged walls of the afferent branchial vessels--two small glandular bodies of the kind are seen to project into each nephridial sac, whilst a larger body of the same kind depends from each of the four branchial afferent vessels into the viscero-pericardial sac.

v.c, Vena-cava.

vent, Ventricle of the heart.

ao, Cephalic aorta (the small abdominal aorta not drawn).

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.

[Illustration: FIG. 12.--Diagram to show the relations of the heart in the Mollusca. (From Gegenbaur.)

A, Part of the dorsal vascular trunk and transverse trunks of a worm.

B, Ventricle and auricles of _Nautilus_.

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. The arrows show the direction of 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.

The blood contains the usual amoeboid corpuscles, and a diffused colouring matter--the haemocyanin of Fredericque--which has been found also in the blood of _Helix_, and in that of the Arthropods _Homarus_ and _Limulus_. It is colourless in the oxidized, blue in the deoxidized state, and contains copper as a chemical constituent.

The renal sacs and renal glandular tissue are closely connected with the branchial advehent vessels in _Nautilus_ and in the other Cephalopoda. The arrangement is such as to render the typical relations and form of a renal tube difficult to trace. In accordance with the metamerism of _Nautilus_ already noticed, there are two pairs of renal organs. Each assumes the form of a sac opening by a pore to the exterior. As is usual in renal tubes a glandular and a non-glandular portion are distinguished in each sac; these portions, however, are not successive parts of a tube, as happens in other cases, but they are localized areae of the wall of the sac. The glandular renal tissue is, in fact, confined to a tract extending along that part of the sac's wall which immediately invests the great branchial afferent vein. The vein in this region gives off directly from its wall a complete herbage of little venules, which branch and anastomose with one another, and are clothed by the glandular epithelium of the renal sac. The secretion is accumulated in the sac and passed by its aperture to the exterior. Probably the nitrogenous excretory product is very rapidly discharged; in _Nautilus_ a pink-coloured powder is found accumulated in the renal sacs, consisting of calcium phosphate. The presence of this phosphatic calculus by no means proves that such was the sole excretion of the renal glandular tissue. In _Nautilus_ a glandular growth like that rising from the wall of the branchial vessel into its corresponding renal sac, but larger in size, depends from each branchial afferent vessel into the viscero-pericardial sac and forms the pericardial gland--probably identical with the "appendage" of the branchial hearts of Dibranchs.

The chief difference, other than that of number, between the renal organs of the Dibranchs and those of _Nautilus_, is the absence of the accessory growths depending into the viscero-pericardial space just mentioned, and, of more importance, the presence in the former of a pore leading from the renal sac into the viscero-pericardial sac (y, y' in fig. 29). The external orifices of the renal organs are also more prominent in Dibranchs than in _Nautilus_, being raised on papillae (np in fig. 29; r in fig. 25). In _Sepia_ the two renal sacs give off each a diverticulum dorsalwards, which unites with its fellow and forms a great median renal chamber, lying between the ventral portions of the renal organs and the viscero-pericardial chamber. In _Loligo_ the fusion of the two renal organs to form one sac is still more obvious, since the ventral portions are united. In _Octopus_ the renal sacs are quite separate.

_Gonads and Genital Ducts._--In _Nautilus_it has been shown by E. Ray Lankester and A.G. Bourne that the genital ducts of both sexes are paired right and left, the left duct being rudimentary and forming the "pyriform appendage," described by Sir R. Owen as adhering by membranous attachment to the ventricle of the heart, and shown by W. Keferstein to communicate by a pore with the exterior. The ovary (female gonad) or the testis (male gonad) lies in _Nautilus_, as in the Dibranchs, in a distinct cavity walled off from the other viscera, near the centro-dorsal region. This chamber is formed by the coelomic or peritoneal wall; the space enclosed is originally part of the coelom, and in _Sepia_ and _Loligo_ is, in the adult, part of the viscero-pericardial chamber. In _Octopus_ it is this genital chamber which communicates by a right and a left canal with the renal sac, and is the only representative of pericardium. The ovary or testis is itself a growth from the inner wall of this chamber, which it only partly fills. In _Nautilus_ the right genital duct, which is functional, is a simple continuation to the pore on the postero-dorsal surface of the membranous walls of the capsule in which lies the ovary or the testis, as the case may be. The gonad itself appears to represent a single median or bilateral organ.

The ovary forms a large projection into the genital coelom, and the coelomic epithelium is deeply invaginated into the mass of the gonad, so as to constitute an ovarian cavity communicating with the coelom by a narrow aperture. The ova originate in the epithelium, migrate below it and then, as they enlarge, project into the ovarian cavity, pushing the epithelium before them. Each ovum is surrounded by a follicular epithelium which is nourished by numerous blood-vessels, and which penetrates into the surface of the ovum in numerous folds. When mature, the ovum is contained in a membrane or chorion with a micropyle, and escapes by dehiscence of the follicle into the genital coelom and duct. In its passage to the exterior the ovum passes a glandular structure on the wall of the genital capsule, which probably secretes the gelatinous substance enclosing the eggs. In addition to this internal gland there are other accessory glands, which are not related to the genital duct or sac but are differentiations of the wall of the pallial cavity, and occur on the inner wall of the pallium in _Nautilus_, on the somatic wall in Dibranchiata. In _Nautilus_ they form a continuous mass. These produce the external envelopes of the eggs.

In the male the testis is a specialized portion of the wall of the genital coelom, and has a structure comparable to that of the ovary. The spermatozoa pass through an orifice from the cavity of the testis to the genital capsule, and thence to the spermiduct. The spermiduct is provided with a glandular pouch, and opens into a terminal reservoir known as Needham's sac or the spermatophore sac. The function of this pouch is to form the spermatophore, which is an elastic tube formed of structureless secretion and invaginated into itself. The deeper part contains the spermatozoa, the external part is called the connective, and is usually much contracted and spirally coiled. When the spermatophore is expelled into the water the connective is extended and evaginated, and the sac containing the sperms bursts. In _Nautilus_ the spermatophore when uncoiled is a little over 30 mm. in length. These spermatophores are somewhat similar to those formed in certain pulmonate Gastropods.

The eggs are laid shortly after copulation. In _Nautilus_ they are laid separately, each being about 4 cm. long and contained in two thick shells, the outer of which is partly open.

[Illustration: Fig 13.--Nervous system of _Nautilus pompilius_ (from Genebaur, after Owen).

t, t, Ganglion-like enlargements on nerves passing from the pedal ganglion to the inner series of tentacles.

t', Nerves to the tentacles of the outer or annular lobe.

b, Pedal ganglion-pair

a, Cerebral ganglion-pair.

c, Pleuro-visceral ganglionic band (fused pleural and visceral ganglion-pairs).

d, Genital ganglion placed on the course of the large visceral nerve, just before it gives off its branchial and its osphradial branches.

m, Nerves from the pleural ganglion to the mantle-skirt.]

_Nervous System._--_Nautilus_, like the other Cephalopoda, exhibits a great concentration of the typical Molluscan ganglia, as shown in fig. 13. The ganglia take on a band-like form, and are but little differentiated from their commissures and connectives--an archaic condition reminding us of _Chiton_. The special optic outgrowth of the cerebral ganglion, the optical ganglion (fig. 13, o), is characteristic. The cerebral ganglion-pair (a) lying above the oesophagus is connected with two suboesophageal ganglion-pairs, of band-like form. The anterior of these is the pedal _b, b_, and supplies the circumoral lobes and tentacles, and the funnel, a fact which proves the pedal origin of these organs. The hinder band is the visceral and pleural pair fused; from its pleural portion nerves pass to the mantle, from its visceral portion nerves to the branchiae and genital ganglion (fig. 13, d), and in immediate connexion with the latter is a nerve to the osphradium or olfactory papilla. A labial commissure arises by a double root from the cerebral ganglia and gives off a stomatogastric commissure, which passes under the pharynx immediately behind the radula and bears a buccal ganglion on either side.

_Special Sense-Organs._--_Nautilus_ possesses a pair of osphradial papillae (fig. 4, olf) corresponding in position and innervation to Spengel's organ placed at the base of the ctenidia (branchiae) in all classes of Mollusca. This organ has not been detected in other Cephalopoda. _Nautilus_ possesses other olfactory organs in the region of the head. Just below the eye is a small triangular process (not seen in our figures), having the structure of a shortened and highly-modified tentacle and sheath. By A. Valenciennes, who is followed by W. Keferstein, this is regarded as an olfactory organ. The large nerve which runs to this organ originates from the point of juncture of the pedal with the optic ganglion. The lamelliform organ upon the inner inferior tentacular lobe of _Nautilus_ is possibly also olfactory in function. In Dibranchs behind the eye is a pit or open canal supplied by a nerve corresponding in origin to the olfactory nerve of _Nautilus_ above mentioned. Possibly the sense of taste resides in certain processes within the mouth of _Nautilus_ and other Cephalopoda.

The otocysts of _Nautilus_ were discovered by J.D. Macdonald. Each lies at the side of the head, ventral to the eye, resting on the capito-pedal cartilage, and supported by the large auditory nerve which apparently arises from the pedal ganglion but originates in the cerebral. It has the form of a small sac, 1 to 2 mm. in diameter, and contains whetstone-shaped crystals, such as are known to form the otoliths of other Mollusca.

The eye of _Nautilus_ is among the most interesting structures of that remarkable animal. No other animal which has the same bulk and general elaboration of organization has so simple an eye as that of _Nautilus_. When looked at from the surface no metallic lustre, no transparent coverings, are presented by it. It is simply a slightly projecting hemispherical box like a kettle-drum, half an inch in diameter, its surface looking like that of the surrounding integument, whilst in the middle of the drum-membrane is a minute hole (fig. 3, u). Sir R. Owen very naturally thought that some membrane had covered this hole in life, and had been ruptured in the specimen studied by him. It, however, appears from the researches of V. Hensen that the hole is a normal aperture leading into the globe of the eye, which is accordingly filled by sea-water during life. There is no dioptric apparatus in _Nautilus_, and in place of refracting lens and cornea we have actually here an arrangement for forming an image on the principle of "the pin-hole camera." There is no other eye known in the whole animal kingdom which is so constructed. The wall of the eye-globe is tough, and the cavity is lined solely by the naked retina, which is bathed by sea-water on one surface and receives the fibres of the optic nerve on the other (see fig. 14, A). As in other Cephalopods (e.g. fig. 33, Ri, Re, p), the retina consists of two layers of cells, separated by a layer of dark pigment. The most interesting consideration connected with this eye of _Nautilus_ is found when the further facts are noted--(1) that the elaborate lens-bearing eyes of Dibranchiata pass through a stage of development in which they have the same structure as the eye of _Nautilus_--namely, are open sacs (fig. 34); and (2) that amongst other Mollusca examples of cephalic eyes can be found which in the adult condition are, like the eye of _Nautilus_ and the developing eye of Dibranchs, simple pits of the integument, the cells of which are surrounded by pigment and connected with the filaments of an optic nerve. Such is the structure of the eye of the limpet (_Patella_), and in such a simple eye we obtain the clearest demonstration of the fact that the retina of the Molluscan cephalic eye, like that of the Arthropod cephalic eye and unlike that of the vertebrate myelonic eye, is essentially a modified area of the general epiderm, and that the sensitiveness of its cells to the action of light and their relation to nerve-filaments is only a specialization and intensifying of a property common to the whole epiderm of the surface of the body. What, however, strikes us as especially remarkable is that the simple form of a pit, which in _Patella_ serves to accumulate a secretion which acts as a refractive body, should in _Nautilus_ be glorified and raised to the dignity of an efficient optical apparatus. In all other Mollusca, starting as we may suppose from the follicular or pit-like condition, the eye has proceeded to acquire the form of a _closed_ sac, the cavity of the closed vesicle being then filled partially or completely by a refractive body (lens) secreted by its walls (fig. 14, B). This is the condition attained in most Gastropoda. It presents a striking contrast to the simple Arthropod eye, where, in consequence of the existence of a dense exterior cuticle, the eye does not form a vesicle, and the lens is always part of that cuticle.

[Illustration: FIG. 14.--Diagrams of Sections of the Eyes of Mollusca.

A, _Nautilus_ (and _Patella_).

B, Gastropod (_Limax_ or _Helix_).

C, Dibranchiate Cephalopod (Oigopsid).

Pal, Eyelid (outermost fold).

Co, Cornea (second fold).

Ir, Iris (third fold).

Int 1,2,3,4, Different parts of the integument.

l, Deep portion of the lens.

l^1, Outer portion of the lens

Co.ep, Ciliary body.

R, Retina.

N.op, Optic nerve.

G.op, Optic ganglion.

x, Inner layer of the retina.

N.S., Nervous stratum of the retina. (From Balfour, after Grenacher.)]

The development of _Nautilus_ is still entirely unknown. Dr Arthur Willey, during his sojourn in the East Indies, made special efforts to obtain fertilized eggs, both by offering rewards to the native fishermen and collectors and by keeping the living adults in captivity, but without success.