CEPHALOPODA.] MOLLUSCA 677 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 Fir.. 109. Diagram to show the relations of the four nephridial sacs, the viscero- e rght apertur - .., .., glandular enlarged walls of the advehent 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 advehent 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, 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 int vt e.b.v i." 11 a o.i/. Fio. 110. Diagram representing a vertical approximately median antoro posterior section of Nautilus pompilius (from 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, b); 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 portion of the mid-foot (siphon); h, the capito-pedal cartilage (see fig. lit}); 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 ; n, tentacles of the annular lobe ; p, tentacles of the inner inferior lobe ; q, buccal membrane ; r, upper jaw or beak ; s, lower jaw or beak ; t, lingual ribbon ; x, the viscero-pericardial sac ; n.f, nerve-collar ; oe, asophagus ; cr, crop ; gizz, gizzard ; int, intestine ; an, anus ; ni, nidamental gland ; nept, aperture of a nephridial sac ; r.e, renal glandular masses on the walls of the afferent branchial veins (see fig. 109) ; a.b.v., 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 coelom 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 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. 110, #), 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 CL.ll . rent , TV> / fULct vise. per. i * apert. Fio. 111. Diagram representing a vertical approximately median antero- posterior section of Sepia ojficinalis (from a drawing by A. G. Bourne). The 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 through the black area representing the shell to the outline below it), the integument covering the visceral hump ; d, the reflected portion of the mantle-skirt forming the sac which encloses the shell ; e, the inferior margin of the mantle-skirt (mouth of the pallial chamber) ; /, the pallial chamber ; g, 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 ; n, 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 ; n.c, the nerve-collar ; cr., the crop ; gizz., 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 nephridial aperture ; visc.per.apert., viscero-pericardial aperture (see fig. 108) ; br.b., branchial heart ; app., appendage of the same ; i.s., ink- bag. 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, viscper.). 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.g., 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