CRUSTACEA. 317 



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

 swimming plates in the Macroura, which would indicate that the long 

 sword-shaped appendage of the Xiphosura was not the homologue 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 specie.- 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.* 



In the Decapod Crustacea the liver {Jig. 132, /) consists of two 

 symmetrical halves, communicating by distinct ducts with the in- 

 testine ; 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. The bile 

 is acid in the Crawfish {^Astacus Jluviatilis). 



In many Crustacea the digestive canal is surrounded by cells filled 

 by an oily or fatty matter of a yellow or blue colour : they may be 

 compared to an omentum, and probably serve as a store of nutriment, 

 to be drawn upon during the moult, or when food is scarce. 



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 

 offers an intermediate condition between that in the Nereids and the 

 generally diffused 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. 



The numerous experiments of Audouin and Milne Edwards f on 

 the circulation of the Crustacea, and the apparently favourable cir- 

 cumstances under which they were made, have very generally been 

 regarded as conclusive of the accuracy of their explanation of that 

 important function in the present class. According to these physio- 

 logists no other than the two great branchial veins terminate in the 

 heart, and, consequently, only pure aerated or arterial blood is pro- 

 pelled by it over the general system : the circulation is the same as 



* DuTemoy, Annales des Sciences, vi. p. 243. 

 t Hist Nat des Crastaces. 1829. 



