480 ZOOLOGY. 



canal, the extremity of which terminates on each side in the 

 intestine near the pylorus. 



567. Nothing is known of the manner in which the 

 chyle passes from the intestine into the circulatory apparatus. 

 The blood is colourless, or slightly bluish or lilac, and 

 coagulates readily. This liquid is set in motion by a heart 

 placed on the median line of the back (c. Fig. 427), and 

 composed of a single cavity. Its form varies, and its con- 

 tractions drive the blood into the arteries, by which it is dis- 

 tributed to all parts of the body. The veins are replaced by 

 the lacunae which the various organs leave between them, 

 and which are lined b}^ a thin layer of cellular tissue; they 

 terminate in vast sinuses, situated near the base of the limbs 

 (s, Fig. 512), and from these cavities the blood proceeds to 

 the respiratory organs, then returns to the heart by dis- 

 tinct canals named branchio-cardiac (c b, Fig. 512). 



Fig. 512. Circulatory Apparatus of a Crab.* 



568. The Crustacea are almost^all essentially aquatic 

 animals : thus their respiration takes place almost always by 

 means of branchiae, and when these organs are wanting, 

 it is the skin of certain parts of the body (most generally of 

 the limbs) which takes their place. Thus in the crabs, 

 lobsters, and all Crustacea of analogous organization, the 

 branchiae consist of a considerable number of small cylinders, 



* Vertical section of the thorax of a crustaceous animal, showing the 

 course followed by the blood ; c, the heart ; s, venous sinuses ; b, branchiae ; 

 va, vessel carrying the venous blood to the branchiae ; ve, vessel receiving 

 the blood after its passage through the capillary network of the branchiae ; 

 vb, branchio-cardiac vessels ; /, arch of the flancs : &t, sternum ; ce, cellule 

 of the flancs ; p, base of the limbs. 



