176 COMPARATIVE ANATOMY CHAP. 



7. Degeneration of the Cephalic Eyes. 



It is becoming more and more probable that the cephalic eyes of the various 

 Mollusca are homologous structures, and that they primitively occurred in all forms. 

 They may, however, under certain biological conditions become rudimentary, and 

 even disappear, as in boring animals and those living in mud or in the deep sea 

 and in parasitic Molluscs. The Lamellibranchia and CMtonidce (?) even have 

 cephalic eyes appearing temporarily during development ; they disappear later, 

 when, covered by the shell, they are useless. They may be replaced by secondarily 

 acquired visual organs arising at more suitable parts of the body, and thus we 

 have eyes on the mantle edge in some bivalves and on the shell of some Chitonidce. 



XVI. The Alimentary Canal. 



The alimentary canal is well developed in all Molluscs, and is 

 composed of (1) the bueeal cavity ; (2) the pharynx or cesophageal 

 bulb; (3) the oesophagus or fore-gut; (4) the mid-gut with the 

 stomach ; (5) the rectum or hind-gut with the anal aperture. The 

 mouth originally lies at the anterior, and the anus at the posterior 

 end or side of the body, the latter in the mantle furrow or cavity. 

 The former always retains its original position, but the latter, as 

 central organ in the pallial complex, becomes shifted more or less far 

 forward along the right (less frequently the left) side, in the mantle 

 furrow. 



When the visceral dome grows out dorsally in such a way that 

 the longitudinal axis becomes shorter than the dorso-ventral axis, as 

 is the case in many Gastropods and Cephalopods and in Dentalium, the 

 mid-gut at least, with its accessory gland, the so-called liver, runs up 

 into this dome, filling the greater part of it. The intestine then forms 

 a dorsal loop, consisting of an ascending portion running up from the 

 fore-gut and a descending portion running down to the anus. In the 

 Gastropoda, where the anus is shifted more or less forward, the 

 descending portion bends forward to the right (rarely to the left) to 

 reach it. 



Besides this principal visceral loop, which is caused by the 

 development of the visceral dome and modified by the displacement of 

 the pallial complex, the intestine, in nearly all Molluscs, forms secondary 

 loops or coils which add to its length. These loops are found 

 principally in the tubular portion of the mid-gut which follows the 

 stomach. They are as a rule most pronounced in herbivorous 

 animals, which thus have longer alimentary canals than carnivorous 

 forms. 



The large digestive gland, usually called the liver, enters the 

 stomachal division of the mid-gut. Functionally, this organ only 

 very slightly corresponds with the vertebrate liver, if indeed it may 

 be said to correspond at all with that organ. It agrees more nearly 



