Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/706

Rh 678 MOLLUSCA [CEPHALOPODA. 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 ex tension 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 nephridia, since sand and other foreign matters are found in it. In all it opens into the pair of nephridial sacs by an orifice on the wall of each, not far from the external orifice (fig. 108, 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 Lamelli- branchia. The single tube-like nephridium and the peri cardium of the Pteropoda also communicate by an aperture. The circulatory organs, blood-vessels, and blood of Nauti lus 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. 109). It gives off a cephalic aorta anteriorly, and a smaller abdominal aorta posteriorly. The diagram, fig. 105, 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 charac teristic of the Anisopleurous Gastropoda (excepting the Zygobranchia). 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 recently been shown to be the rudimentary left oviduct or sperm-duct, as the case may be (Lankester and Bourne, 37), the functional right ovi-sac and its duct being attached by a membrane to the opposite side of the heart. The cephalic and abdominal aortas of Nautilus appear, after running to the anterior and posterior extremes of the animal respectively, to open into sinus-like spaces surround ing the viscera, muscular masses, &amp;lt;fcc. 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 investiga tion of these capillaries in the light of modern histological knowledge is much needed. From the sinuses and capil laries 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 advehent veins (fig. 109, v.c.}, which pass into the four gills without dilating. In the Dibranchs at a similar posi tion the vena cava gives off a right and a left branchial advehent vein (fig. 108, r.s.v.c, r.d.v.c), each of which, traversing the wall of the corresponding nephridial sac and receiving additional factors (fig. 108, v.tj, v.p.d, v.a.d, v.b.a), dilates at the base of the corresponding branchial plume, forming there a pulsating sac the branchial heart (fig. 104, x; and fig 108, c.6). Attached to each branchial heart is a curious glandular body, which may possibly be related to the larger masses (r.e in fig. 109) which depend into the viscero-pericardial cavity from the branchial advehent veins of Nautilus. From the dilated branchial heart the bran chial advehent vessel proceeds, running up the ad-pallial face of the gill-plume (vi, vc, fig. 104). From each gill- plume the blood passes by the branchial efferent vessels (v, fig. 104) to the heart, the two auricles being formed by the dilatation of these vessels (v, v in fig. 104). The blood of Siphonopoda contains the usual amoeboid cor puscles,, 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 nephridial sacs and renal glandular tissue are closely connected with the branchial advehent vessels in Nautilus and in the other Siphonopoda. The arrangement is such as to render the typical relations and form of a nephridium difficult to trace. In accordance with the metamerism of Nautilus already noticed, there are two pairs of nephridia. Each nephridium assumes the form of a sac opening by a pore to the exterior. As is usual in nephridia, 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 arese 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 advehent vein. The vein in this region gives off directly from its wall a complete herbage of little venules, which branch and ana stomose with one another, and are clothed by the glandular epithelium of the nephridial sac. The secretion is accumu lated 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 nephridial sacs, consisting of calcium phosphate. The presence of this phosphatic calculus by no means proves that such was the sole ex cretion of the renal glandular tis sue. In Nautilus a glandular growth like that rising from the wall of the branchial vessel into its corresponding nephridial sac, but larger in size, depends from each branchial advehent vessel into the viscero-pericardial sac, prob ably identical with the &quot;append age&quot; of the branchial hearts of Dibranchs. The chief difference, other than that of number between the ne phridia 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 import ance, the presence in the former of Fjo 112. -Nervous system of a pore leading from the nephridial Nautilus pompiirus (from Ge sac into the viscero-pericardial sac (y, y in fig. 108). The external orifices of the nephridia are also more prominent in Dibranchs than in Nautilus, being raised on papillae (np in fig. 108 ; r in fig. 103). In Sepia, according to Vigelius (38), the two nephridia give off each a diverticulum dorsalwards, which unites with its fellows and forms a great median renal chamber, lying between the ventral portions of the nephridia and the viscero-pericardial chamber. In Loligo the fusion genbaur, after Owen), t, t, ganglion-like enlargements on nerves passing from the pedal ganglion to the inner series of tentacles; f, nerves to the ten tacles of the outer or annular lobe ; 6, pedal ganglion-pair ; a, cerebral ganglion-pair); c, pleuro - visceral ganglionic band (fused pleural and visce ral ganglion-pairs) ; d, genital ganglion placed on the course of the large visceral nerve, just before it gives off its branchial and its ospliradial branches ; m, nerves from the pleural ganglion to the mantle-skirt.