THE CRAYFISH 



133 



the posterior division of the stomach, and have the combined 

 functions of digesting food and absorbing some of the prod- 

 ucts of digestion. 



The Circulatory, Respiratory, and Excretory Systems. The 

 heart (Fig. 67, 26) is a muscular organ lying beneath the dor- 

 sal body-wall posterior to the stomach. The blood finds its 

 way into the heart through three pairs of openings furnished 

 with valves to prevent the escape of blood. When the heart 

 contracts the blood flows both forward and backward at the 



FIG. 69. Gill-Chamber and Gills of the Crayfish (Cambarus affinis). 

 Slightly enlarged 



1, gill-bailer on second maxilla; 2, first maxilliped; 3, second maxilliped with 

 first outer gill; 4, sixth outer gill; 5, last gill of inner series 



same time, forward by five tubes and backward by two tubes, 

 called arteries (Fig. 67, 29, 30, 31). These arteries branch into 

 many smaller vessels with open ends ; from the open ends 

 the blood flows over the tissues of the body in sinuses, all of 

 which connect with a still larger median cavity, the ventral 

 sinus, lying along the ventral wall of the thorax and abdomen. 

 Branches from the median ventral sinus extend out into the 

 gills. The blood is carried back to the base of the gills by 

 parallel channels, and finally carried lip to the pericardial 

 sinus in which the heart lies. The set of delicate, plume-like 



