MOLLUSCA 



141 



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 aorta? 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, &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 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, >.<.), 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.g, 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.b). 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 amoaboid 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 areas 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 "append- 

 age" 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 

 a pore leading from the nephridial 

 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 



IG. 112. Nervous system of 

 Nautilus pompilivs (from Ge- 

 genbaur, after OwenX *, *, 

 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 osphradial branches ; 

 m, nerves from the pleural 

 ganglion to the mantle-skirt. 



