MOLLUSCA 



139 



complete internal cartilaginous skeleton than is to be found 

 in some of the lower Vertebrates. There are other instances 

 of cartilaginous endo-skeleton in groups other than the 

 Vertebrata. Thus in some capito-branchiate Chaetopods 

 cartilage forms a skeletal support for the gill-plumes, whilst 

 in the Arachnids (Mygale, Scorpio) and in Lunulus a large 

 internal cartilaginous plate the ento-sternite is devel- 

 oped as a support for a large series of muscles. 



Alimentary Tract. The buccal cone of Nautilus is ter- 

 minated by a villous margin (buccal membrane) surround- 

 ing the pair of beak-like jaws. These are very strong and 

 dense in Nautilus, being calcified. Fossilized beaks of Tetra- 

 branchiata are known under the name of Rhyncholites. 

 In Dibranchs the beaks are horny, but similar in shape to 

 those of Nautilus. They resemble in general those of a 

 parrot, the lower beak being the 

 larger, and overlapping the upper or 

 dorsal beak. The lingual ribbon and 

 odontophoral apparatus has the struc- 

 ture which is typical for Glosso- 

 phorous Mollusca. In fig. 107, A is 

 represented a single row of teeth 

 from the lingual ribbon of Nautilus, 

 and in fig. 107, B, C, of other Si- 

 phonopoda. 



In Nautilus a long and wide crop 

 or dilated oesophagus (cr, fig. 110) 

 passes from the muscular buccal mass, 

 and at the apex of the visceral hump 

 passes into a highly muscular stom- 

 ach, resembling the gizzard of a bird 

 (gizz, fig. 110). A nearly straight 

 intestine passes from the muscular 

 stomach to the anus, near which it 

 develops a small cascum. In other 

 Siphonopods the cesophagus is usually FIG. m Aiimimtery canal 

 narrower (fig. 106, ), and the mus- 



Clllar Stomach more Capacious (fig. 



106, v), whilst a very important 



feature in the alimentary tract is 



formed by the caecum. In all but 



Nautilus the caecum lies near the 



stomach, and may be very capacious 



much larger than the stomach in Loligo vulgaris or 



elongated into a spiral coil, as in fig. 106, e. The simple 



is omitted. Of, oesophagus ; 

 r.the stomach opened long- 

 itudinally; z, probe passed 

 through the pylorus ; c, 

 commencement of the cse- 

 cum ; , its spiral portion ; 

 i, intestine ; a, ink-bag ; b, 

 itsopening into the rectum. 



Fio. 107. Lingual dentition of Siphonopoda. A. A single row of lingual teeth 

 of A'au/Uiis pompilius (after Keferstein). B. Two rows of lingual teeth of 

 Sepia oflcinalis (after Troschel). C. Lingual teeth of Ekdonc tirrhosa (after 

 Loven). 



U-shaped flexure of the alimentary tract as seen in fig. 

 106, and in fig. 110, is the only important one which it 

 exhibits in the Cephalopoda, the Pteropoda (except the 

 Limacinida) agreeing with the Siphonopoda in this sim- 



plicity in consequence of their visceral hump being un- 

 twisted. The acini of the large liver of Nautilus are 

 compacted into a solid reddish -brown mass by a firm 

 membrane, as also is the case in the Dibranchiata. 

 The liver has four paired lobes in Nautilus, which open 

 by two bile-ducts into the alimentary canal at the com- 

 mencement of the intestine. The bile-ducts unite before 

 entering the intestine. In Dibranchiata the two large 

 lobes of the liver are placed antero-dorsally (beneath 

 the shell in Decapoda), and the bile-ducts open into the 

 caecum. Upon the bile-ducts in Dibranchiata are deve- 

 loped yellowish glandular diverticula, which are known 

 as " pancreas," though neither physiologically nor morpho- 

 logically is there any ground for considering either the so- 

 called liver or the so-called pancreas as strictly equivalent 

 to the glands so denominated in the Vertebrata. In Nauti- 

 lus the equivalents of the pancreatic diverticula of the 

 Dibranchs can be traced upon the relatively shorter bile- 

 ducts. 



Salivary Glands are not developed in Nautilus unless a 

 pair of glandular masses lying on the buccal cavity are to 

 be considered as such. In the Dibranchs, on the contrary, 

 one (Sepia, Loligo) or two pairs of large salivary glands 

 are present, an anterior and a posterior (Octopus, Eledone, 

 Onychoteuthis). Each pair of salivary glands has its 

 paired ducts united to form a single duct, which runs 

 forward from the glands and opens into the buccal cavity 



a.r 



FIG. 108. Diagram of the nephridial sacs, and the veins which run through 

 them, in Sepia officinalis (after VigeliusX The nephridial sacs are supposed 

 to have their upper walls removed, r.c, vena cava ; r.d.r.c, right descending 

 branch of the same ; T.S.V.C, left descending branch of the same ; t.b.a., vein 

 from the ink-bag ; r.m, mesenteric vein ; r.g, genital vein ; r.o.d, right 

 abdominal vein ; r.o-s, left abdominal vein ; r.p.d, right pallia] vein ; v.p.s, 

 left pallia! vein ; c.6, branchial heart ; x, appendage of the same ; c.r, capsule 

 of the branchial heart ; np, external aperture of the right nephridial sac ; y, 

 reno-pericardial orifice placing the left renal sac or nephridium in communi- 

 cation with the viscero-pericardial sac, the course of which below the nephri- 

 dial sac is indicated by dotted lines ; y', the similar orifice of the right side ; 

 o.r, glandular renal outgrowths ; w.fc, viscero-pericardial sac (dotted outline). 



near the radula. The anterior pair of glands when present 

 lie in the head near the buccal mass, the posterior pair lie 

 much farther back beneath the liver, at the sides of the 

 oesophagus. It is the posterior pair which alone are pre- 

 sent in Sepia and Loligo. The ink-bag is to be considered 

 as an appendage of the rectum. It is not developed in 

 Nautilus, nor in the Pteropoda ; in all Dibranchiata (even in 

 the fossil Belemnites) it is present (fig. 106, a ; fig. 103, t), 

 and has been observed to develop as a diverticulum of the 

 rectum, with spirally plaited walls which very early secrete 

 a black pigment. The spiral plaitings of the walls diminish 



