136 A COURSE ON ZOOLOGY. 



project into the stomachic cavity, and a*e so arranged 

 as to crush the food again just as it passes into the intes- 

 tine which opens from the bottom of the stomach. 



The intestine is a straight tube leading to the pos- 

 terior end of the body. 



The liver is largely developed, and forms two masses 

 of a yellowish color, divided into lobes and lobules on 

 each side of the stomach. 



The respiratory apparatus consists of gills or branchiae 

 that are located under the thoracic carapace, which 

 forms a sort of respiratory chamber on each side, com- 

 municating with the exterior by two orifices. One of 

 these openings serves for the admission of water, and is 

 located on the outside between the base of the feet and 

 the edge of the shell ; the other is near the mouth, and 

 is employed for the exit of the water. A plate-like ap- 

 pendage of the second pair of jaws moves continually be- 

 fore this orifice, and sets up a current by which water 

 is continually caused to flow through the cavities. 



FIG. 74. 



GILLS OF CRAYFISH EXPOSED (after Huxley). 



The heart is situated just under the carapace of the 

 thorax, and consists of a single fleshy pocket, having six 

 openings, by which the oxygenated blood coming from 



