14-0 



MOLLUSCA 



in relative size as the volume of the sac increases. Its 

 outer surface acquires a metallic iridescence similar to that 

 of the integuments of many fishes. The opening of the 

 ink-sac is in the adult sometimes distinct from but near to 



vent 



FIG. 109. 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 indicat- 

 ing its backward extension ; viac.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 advehent branchial vessels, two small 

 glandular bodies of the kind are seen to pro.ject into each nephridial sac, 

 whilst a larger body of the same kind depends from each of the four branchial 

 advehent vessels into the viscero-pericardial sac; v.c., vena cava ; vent, 

 ventricle of the heart ; at>., cephalic aorta (the small abdominal aorta not 

 drawn) ; a.b.i', advehent branchial vessel ; e.v.b., efferent branchial vessel. 



the anus (Sepia) ; in other cases it opens into the rectum 

 near the anus. The ink-bag of Dibranch Siphonopoda is 

 possibly to be identified with the adrectal (purpuriparous) 

 gland of some Gastropoda. 



Ccelom, Blood-vascular System, and Excretory Organs. 

 Nautilus and the other Siphonopoda conform to the 



> , 

 a b.v. 



FIG. 110. Diagram representing a vertical approximately median antero 

 posterior section of Naittilus jw/npilins (frum a drawing by A. G. Bourne). 

 The parts which are quite black are the cut muscular surfaces of the foot and 

 buccal mass, a, the shell ; b, the nuchal plate identical with the nuchal 

 cartilage of Sepia (see fig. 90, 6) ; c, the integument covering the visceral 

 hump ; d, the mantle flap or skirt in the dorsal region where it rests against 

 the coil of the shell ; e, the inferior margin of the mantle-skirt resting on the 

 lip of the shell represented by the dotted line ; /, the pallial chamber with 

 two of the four 'gills ; g, the vertically cut median pcrtion of the mid-foot 

 (siphon); h, the capito-pedal cartilage (see fig. 116); i, the valve of the 

 siphon ; I, the siphuncular pedicle (cut short) ; m, the hood or dorsal enlarge- 

 ment of the annular lobe of the fore-foot ; 71, tentacles of the annular lobe ; 

 p, tentacles of the inner inferior lobe ; q, bnccal membrane ; r, upper jaw or 

 beak ; s, lower jaw or beak ; t, lingual ribbon ; x, the viscero-pericardial sac ; 

 n.c, nerve-collar ; oe, cesophagus ; cr, crop ; qizz, gizzard ; int, intestine ; an, 

 anus ; ni, nidamental gland ; ncpt, aperture of a nephridial sac ; r.e, renal 

 glandular masses on the walls of the afferent branchial veins (see lig. 109); 

 0.6.11., afferent branchial vessel ; e.b.v, efferent branchial vessel ; vt, ventricle 

 of the heart. 



general Molluscan characters in regard to these organs. 

 Whilst the general body-cavity or co;lom 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 ccelom surrounding the heart and known 

 as the Molluscan pericardium becomes shut off from this 

 general blood-lymph system, and communicates, directly in 

 Nautilus, in the rest through the nephridia, with the exte- 

 rior. In the Siphonopoda this specialized pericardial cavity 

 is particularly large, and has been recognised 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. 110, .r), investing the genital and other viscera which 

 lie below it, and having the ventricle of the heart sus- 

 pended in it. Certain membranes forming incomplete 

 septa, and a curious muscular band the pallio-cardiac 

 band traverse the sac. The four branchial advehent veins, 

 which in traversing the walls of the four nephridial sacs 

 give off, as it were, glandular diverticula into those sacs, 

 also give off at the same points four much larger glandular 



app. 



Vr.b. 



f 



FIG. 111. Diagra 



pi isterior secti 



>ram representing a vertical approximately median antero 

 .ion of tie-pia oflrcinolis (from a drawing by A. G. Bourne). Thi 



The 



i i ep cns o . . . 



lettering corresponds with that of fig. 110, with which this drawing is intended 

 to be compared, a, shell (here enclosed by a growth of the mantle) ; 6, the 

 nuchal plate (here a cartilage) ; c (the reference line should be continued 



maiine-sim ij loriiiiug me sac uinea encloses me alien , t, me jmeiiui in ^ 

 of the mantle-skirt (mouth of the pallial chamber) ; /, the pallial chamber ; 

 17, the vertically cut median portion of the mid-foot (siphon); i, the valve of 

 the siphon ; m, the two upper lobes of the fore-foot ; 11, the long prehensile 

 arms of the same ; o, the fifth or lowermost lobe of the fore-foot ; p, the third 

 lobe of the fore-foot ; q, the buccal membrane ; v, the upper beak or jaw ; s, 

 the lower beak or jaw ; t, the lingual ribbon ; x, the viscero-pericardial sac ; 

 7i.c, the nerve-collar ; cr., the crop ; aizz., the gizzard ; an, the anus ; c.t., the 

 left ctenidium or gill-plume; vent, ventricle of the heart; a.b.v., afferent 

 branchial vessel ; e.b.v, efferent branchial vessel ; re, renal glandular mass ; 

 n.n.a, left uephridial aperture; visc.per.apert., viscero-pericardial aperture 

 (see fig. 108); Ir.ti., branchial heart; app., appendage of the same; i.s., ink- 

 masses, which hang freely into the viscero-pericardial 

 chamber (fig. 109, 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 

 nephridial aperture (fig. 101, viscjier.). 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 nephridia, and through them 

 or the one nephridium to the exterior. In Nautilus there 

 is no opening from the viscero-pericardial sac into the 

 nephridia. Therefore the external pore of the viscero-peri- 

 cardial sac may possibly be regarded as a shifting of the 

 reno-pericardial orifice from the actual wall of the nephridial 

 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.ff., Spondylus) open into the nephridia, whilst in other 

 cases they open close by the side of the nephridia on the 

 surface of the body. The viscero-pericardial sac of the 



