TYPE MOLLUSC A. 349 



It leads into a muscular pharynx (Fig. 152, h), upon the floor 

 of which lies the characteristic molluscan radula, while into 

 its cavity the ducts of one or two pairs of salivary glands 

 open. Succeeding the pharynx is a tubular oesophagus (os) 

 which in some forms is provided with a lateral diverticulum, 

 the crop, and which terminates below in the large pyriform 

 stomach (s). The intestine leaves the stomach close to the 

 entrance of the oesophagus, and a pouchlike structure, in some 

 forms prolonged into a spiral caecum {cw), is to be found ■ 

 either communicating with the stomach close to this point or 

 else opening into the proximal portion of the intestine {Nauti- 

 lus). Into this caecum the two ducts from the large digestive 

 glands, or so-called liver, open, their walls being in the ma- 

 jority of cases provided with sacculations arranged in bunches 

 and constituting the pancreas, a structure which in Loligo 

 (Fig. 153,^) is imbedded in the thickened walls of the ducts or 

 else, as in Octopus, attached to the digestive gland in the region 

 where its ducts arise. From its origin in the stomach the 

 intestine passes ventrally, the entire tract having thus a 

 V-shaped arrangement, and opens into the mautle-cavity on 

 the summit of a papilla situated a short distance from the 

 dorsal end of the infundibulum. From each side of the anal 

 papilla a fleshy appendage arises, the anal valve, which in 

 some forms may be drawn down so as to completely close the 

 anal opening. 



In connection with the posterior portion of the digestive 

 tract there is found in all Cephalopods except Nautilus a sac- 

 like gland (Fig. 152, i) which secretes a dark pigment and is 

 known as the ink-bag, the animal discharging the ink into 

 the surrounding water to conceal its retreat when alarmed. 

 It arises as a saclike diverticulum of the rectum close to its 

 termination and, elongating, becomes differentiated into a duct 

 of considerable length opening into the terminal portion of 

 the rectum and closed by a circular band of muscle-fibres 

 which surround it near its opening. The more or less globu- 

 lar extremity of the diverticulum becomes differentiated into 

 (1) a cavity which serves as a reservoir for the inky secretion 

 manufactured in (2) a special glandular region, traversed by a 

 series of trabeculae lined by the secreting cells. 



