Page:EB1911 - Volume 05.djvu/717

 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.

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 Nautilusit 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