6 4 



Descriptive Zoology. 



extend back into the abdomen, being attached to the an- 

 terior edges of the abdominal rings in the upper part of the 

 abdomen. When these muscles shorten they pull the an- 

 terior edge of each tergum under the posterior edge of the 

 preceding tergum, and thus straighten the abdomen. The 

 extensor muscles lie above the intestine. Below the intes- 

 tine, and filling out most of the space of the abdomen, are 

 the flexor muscles. These are very complicated. Like the 



Extensor muscle. 



Abdominal artery. 



Tergum. 



Intestine 



Flexor muscles. 



Epimerum. Ventral V %\J// X Pleurum. 



abdominal artery. ^Pwpf Nerve cord 



Sternum. 

 IQ, one of the swimmerets. 



FIG. 46. CROSS SECTION OF ABDOMEN OF CRAYFISH. 



From Huxley's Crayfish. 



extensor muscles, they originate in the thorax, and extend 

 back and are inserted on the sternums ; and when they 

 shorten, they flex the abdomen, giving the powerful stroke 

 by which the animal swims backward so rapidly. 



Food of the Crayfish, and Mode of Eating. The crayfish 

 lives largely on worms, the larvae of insects, with which 

 most waters abound, snails, etc. The crayfish is carnivorous 

 by choice, yet by necessity may be almost omnivorous. It 

 is a greedy eater, and does not disdain carrion. It is de- 



