178 LECTURE XV. 



capacious than in the Limtdus, and its walls are connected with a 

 calcareous frame-work supporting dense tubercles, which project into 

 the interior of the stomach around the pylorus, and are so situated 

 and moved with relation to each other as to divide and bruise the 

 alimentary matter before they pass into the intestine. In the com- 

 mon lobster these gastric teeth are three in number, the middle one 

 serving as a kind of anvil upon which the two larger lateral pieces 

 work.* This complex kind of gizzard is sometimes found to be 

 everted and protruded from the mouth, the gastric teeth being then 

 external, like those of the anterior muscular segment or proboscis of 

 the alimentary canal in the Anellides. 



The intestine in all the Crustacea is continued to the vent without 

 convolutions ; a terminal portion or rectum is sometimes, as in the 

 lobster, indicated by a slight circular valve ; the vent is situated 

 beneath the terminal segment of the abdomen, anterior to its appended 

 swimming plates in the Macroura, which would indicate that the 

 long spinal appendage of the Xiphosura was not the analogue of the 

 post-abdomen, but of its last joint in the Macroura. 



The liver is a considerable organ in all the Crustacea ; it makes its 

 appearance in the inferior species as so many caecal prolongations of 

 the intestine, continued from many points, according to the primitive 

 form which it presents in the Aphrodita. The biliary caeca extend 

 even into the limbs in the Nymphons. In the Stomapods the biliary 

 caeca, continued ten or eleven in number at equidistant points from 

 each side of the intestine, are more ramified, and are confined to the 

 thoracic-abdominal cavity, where they penetrate the interspaces of 

 the muscles, f 



In the Decapod Crustacea the liver consists of two symmetrical 

 halves, communicating by distinct ducts with the intestine ; each 

 half consists of numerous lobes, which, in the larger Macroura and 

 Brachyura, are leaf-shaped, and composed of straight tubular caeca, 

 arranged obliquely to a median canal, which forms, as it were, the leaf 

 stalk. In some species one or two pairs of long slender and simple 

 tubuli likewise communicate with the intestine. 



No other vessels are yet known to convey the chyle or nutrient 

 fluid to the circulating system than the irregular venous receptacles 

 which are in contact with the parietes of the intestine. Almost the 

 whole venous system presents the form of wide flattened sinuses, and 

 oflTers an intermediate condition between that in the Nereis and the 

 generaUy difl'used state of the venous blood through all the cellular 

 interspaces of the body in the class of insects. Through these 

 sinuses, however, the blood flows in a constant and definite course. 



* Preps. Nos. 407, 408. f Duvernoy, Annales des Sciences, vi. 



