SECT. vii. DECAPOD CRUSTACEA. 193 



gill chamber ; a few of them are shown in fig. 147. Each 

 gill chamber has two openings; the water is admitted by 

 a slit in the base of the claw feet, and ejected by another 

 into the mouth. But the act of breathing is regulated 

 by a plate on the second pair of jaws, so connected with 

 the exterior pair of foot-jaws that, when the crab applies 

 the latter to its mouth, the plate shuts the slit, the water 

 in the gill chamber is ejected by the mouth, and in order to 

 admit a fresh supply, the crab must open the foot-jaws 



Fig. 147. Section of a Crab. 



again, so that they are in constant motion. There are 

 plates called whips on all the appendages of the crab, 

 from the last pair of foot-jaws to the fourth pair of 

 walking feet inclusive, which ascend and descend verti- 

 cally between the bunches of gills to sweep particles of 

 sand or other foreign matter out of them. 



The heart of the crab, as in all the Decapods, is placed 

 under the skin of the back next to the throat ; and the 

 blood, which is white or bluish, flows from the heart 

 through a complicated system of vessels, and, having 



VOL. II. O 



