282 



COMPARATIVE ZOOLOGY 



ach is characterized by sending out tubular prolonga- 

 tions, and the intestine ends in a large bladderlike 

 expansion. Scorpions have no stomachal cavity a 

 straight intestine passes directly through the body. 



In bivalve mollusks, like the clam, the mouth opens 

 into a short esophagus which leads into the stomach, 



which lies em- 

 bedded in a large 

 liver, and the in- 

 testine, describ- 

 ing a few turns, 

 passes directly 

 through the 

 heart. 94 In the 

 univalve mol- 

 lusks, like the 

 snail, the gullet 

 is long and fre- 

 quently expands 

 into a crop ; the 

 stomach is often 

 double, the ante- 

 rior being a giz- 

 zard provided 

 with teeth for 

 mastication ; the intestine passes through the liver, and 

 ends in the fore part of the body, usually on the right 

 side. 



The highest mollusks, as the cuttlefish and nautilus, 

 exhibit a marked advance. A mouth with powerful 

 mandibles leads to a long gullet, which ends in a strong 

 muscular gizzard resembling that of a fowl. 95 Below 

 this is a cavity, which is either a stomach or duodenum ; 

 it receives the secretion from a large digestive gland or 

 pancreas. The intestine is a tube of uniform size, which, 



FIG. 242. Alimentary Canal of the Oyster: a, stomach 

 laid open; d, liver; b, c, d,f, convolutions of the intes- 

 tine; g, anal aperture; n, o, auricle and ventricle; /, m, 

 adductor muscle; h, k, lobes of mouth divided to show 

 the venous canals at the base of the gills. 



