THE CRAYFISH. 137 



the gills may penetrate into its cavity. This heart is 

 contained in a sort of sac or sinus that receives directly 

 the blood coming from the gills. The heart then forces 

 the blood it contains into the arterial vessels. These 

 are an anterior aorta, for the head, eyes, and other 

 processes, and a posterior aorta, which soon divides into 

 a dorsal branch and the ventral branch, and these pene- 

 trate into the interior of the organs, and reach the lower 

 surface of the body, where they distribute the blood in 

 front, behind, to the feet, and the neighboring organs. 

 There are no veins ; after circulating in the arteries, the 

 blood penetrates into interorganic spaces, sorts of com- 

 municating chambers, situated between the organs and 

 the muscles, and thus returns to the gills. 



The blood of the crayfish is colorless or somewhat 

 bluish. The nervous system is ganglion ic, consisting, as 

 we have seen in the chapter on generalities, of a cerebral 

 ganglion, joined by an cesophageal chain with a double 

 chain of ganglia, extending from one end of the body to 

 the other. 



The organs of sense cannot be well understood, ex- 

 cepting the eyes, which are two in number and stalked. 



The sense of touch is exercised by the antenna, and 

 perhaps also by the palps that form part of certain buc- 

 cal processes. 



Crayfish live in fresh water, preferring rocky and 

 shallow streams. They hide under stones and in holes, 

 which they leave rarely except when in search of their 

 food, consisting of mollusks, fish, worms, shreds of rotten 

 flesh, etc. They are very voracious, and gorge themselves 

 with all animal matters coming within their reach. 



Crayfish lay eggs which remain attached in clusters 

 to their abdominal false feet. These eggs are developed 



12* 



