CHAKACTEKISTICS OF THE DIGESTIVE APPARATUS. 



209 



fectly indicated. The intestine terminates in an expansion, the cloaca 

 into which the reproductive organs open. 



In bivalved mollusks like the oyster (Pig. 64) the gullet and 

 pharynx are absent and the mouth communicates directly with the 

 stomach, which is imbedded in a large glandular organ, the liver, and the 

 intestine after making a few turns passes directly through the heart. In 

 univalved mollusks like the snail the 

 gullet is long, the crop is frequently 

 present, and the stomach is some- 

 times double, the anterior portion 

 provided with teeth and serving as 

 an organ of mastication or as a giz- 

 zard (Fig. 65). A lobulated liver is 

 also here present ; the intestine is 

 convoluted, passes through the liver, 

 and usually terminates in the an- 

 terior part of the body. The highest 

 mollusks, such as the cuttle-fish 

 {Pig. 66), show a marked advance 

 in complexity, the highest, stage of 

 development of the alimentary canal 



Pig. 65.— Diagrammatic Section of Snail. 

 (Wilson.) 

 A, foot ; B. operculum : C, tentacles : D. mouth ; E. sali- 

 vary glands; F, stomach; G G, intestines; H. anus: I, liver; 

 Iv, aperture of gill-chamber: M. oviduct; N. gill-chamber: 

 0, floor of gill-chamber ; P, gill of breathing organ ; ST, heart; 

 W, cephalic, X, pedal, and Y, branchial ganglia. 



Fig. 66.— Diagrammatic Section of a 

 Female Cephalopod (Sepia offici- 

 nalis). (Huxley.) 



A, buccal mass surrounded by the lipB, and showing 

 the horny jaws and tongue ; B, oesophagus ; C, salivary 

 gland ; D. stomach ; E, pyloric caecum ; F, the funnel ; 

 G, the intestine; H, the anus; I, the ink-bag; K, the 

 place of the systemic heart; L, the liver: N, the hepatic 

 duct of the left side ; O, the ovary ; P, the oviduct ; Q, 

 one of the apertures by which the atrial system, or water 

 chambers, are placed in communication with the ex- 

 terior ; R, one of the branchise ; S, the principal ganglia 

 around the oesophagus : M, the mantle ; SH, the internal 

 shell, or cuttle-bone ; 1, 2, 3, 4, 5, the margins of the foot, 

 constituting the so-called arms of the sepia. 



being accompanied by the appearance of definite organs of circulation 

 and of the nervous system. 



In vertebrates the complexity and perfection of the alimentary canal 

 lias advanced still further, and we find in them that the buccal cavity, 

 which in fish and amphibians is single, in the reptiles is divided into two 

 divisions, — a nasal or respiratory portion and a buccal or digestive 



14 



