194 COMPARATIVE ANATOMY CHAP. 



Two digestive glands, lying in the upper part of the body, open through wide 

 apertures into the stomach. Their form can be gathered from Fig. 165. 



4. Lamellibranchia. 



In the Lamellibranchia the oesophagus, which lies under the anterior adductor, 

 widens at the anterior base of the foot to form the stomach. This descends some- 

 what into the foot. At the posterior base of the stomach lie two apertures ; one of 

 these is the pylorus, and leads into the small intestine which runs more or less 

 coiled within the base of the foot ; the other leads into a tubular diverticulum, the 

 sheath of the crystalline stylet. The large richly-branched acinose digestive gland 

 (liver) opens through several apertures into the stomach, with which it lies in 

 the anterior part of the pedal cavity. In PJwlas, Jouannetia, and Teredo, the 

 stomach has another ccecum besides the sheath of the crystalline stylet. In all 

 bivalves there is, on the inner wall of the stomach, a gelatinous cuticular structivre 

 (dreizackiger Korper, fleche tricuspide), which varies in thickness, and is continued 

 into the gelatinous crystalline stylet. This latter is secreted in concentric layers as 

 a cuticular structure by the epithelium of the sac in which it lies. A plausible 

 suggestion has recently been made as to the use of these gelatinous structures, viz. 

 that they serve for surrounding with a slimy envelope foreign particles, such as 

 sharply-pointed grains of sand, which enter the alimentary canal with the food ; in- 

 jury to the delicate walls of the intestine is thus avoided, and the travelling of such 

 particles along the digestive tract is facilitated. The point of the crystalline stylet 

 projects freely into the lumen of the intestine. In some forms it does not lie in a 

 separate sac, but in a groove (Najada, Cardium, Mytilus, Pecten, etc.). The tricuspid 

 body and the crystalline stylet appear temporarily, and are renewed periodically. 

 Similar structures have been observed in the stomachs of various Gastropods. 

 Haliotis has a stomachal ccecum which can be compared with the sheath of the 

 crystalline stylet. 



In the lower Lamellibranchs, the Nuculidce and Solenomyidcc, the stylet is either 

 very slightly developed or wanting. In the Arcidm also, it is only slightly 

 developed. 



The Scptibranchia (Poromya, Cuspidaria) are distinguished from all other 

 Lamellibranchia by the absence of coils, and the consequent shortening of the small 

 intestine (ef. on the intestine of the Lamellibranchia, Figs. 24, 25, 26, 27, 28, 

 pp. 16, 17/18, 19). 



5. Cephalopoda. 



The stomach in the Cephalopoda always lies in the dorsal portion of the 

 visceral dome in the shape of a sac with a strong muscular wall. It always has a 

 ccecal appendage (stomachal or spiral ccecum, Figs. 166, 160), which varies in shape 

 and size ; into this the digestive gland (liver) opens. This ccecum is a reservoir for 

 the secretion of the digestive gland. Food never enters it, there are even valves 

 at the point of entrance into the stomach, which allow the secretion collected in 

 the ccecum to pass into the stomach, but prevent the entrance of the contents of the 

 latter into the ccecum. 



In Nautilus, the ccecum does not open into the stomach, but into the commence- 

 ment of the small intestine, and is in the form of a small round vesicle with lamella* 

 projecting into its lumen. In Sepia and Scpiola also, the ccecum is more or less 

 round ; in Rossia, it is slightly developed ; in Loligo and Sepiotevthis, very long and 

 ending in a point ; in all Ocgopsidce and Octopoda, more or less spirally coiled at the 

 blind end. 



The well-developed digestive gland seems to arise as a paired organ, even when 



