THE CRAYFISH 



223 



segment of the abdomen, form a finlike apparatus, the caudal fin. The 

 caudal fin is composed of two large swimmerets and tlio last body segment. 

 Crayfish normally swim very 

 rapidly by means of a sudden 

 jerking in a backward du-ec- 

 tion of the caudal fin. The 

 abdomen is provided with 

 powerful muscles which are 

 attached to the exoskeleton. 

 It is by these muscles that 

 the rapid swimming is ac- 

 complished. 



How the Crayfish gets in 

 Touch with its Surroundings. 

 — Several other appendages 

 besides those used for loco- 

 motion are found. Two pairs 

 of " feelers," the longer pair 

 called the antenna, the shorter 

 the antennules (little anten- 

 nae), protrude from the front 

 of the body. The longer 

 feelers appear to be used as 

 organs of touch. Certain 

 hairlike structures projecting 

 from the antennse have to do 

 with the sense of smell. The 

 smaller antennules hold at 



their bases little sacs called "ears." These "ears" have largely to do 

 with the function of balancing rather than hearing. 



Just above the antennules, projecting on stalks, are the eyes. These 

 eyes are made up of many small structures called ommatidia, each of 

 which is a very simple eye. A collection of ommatidia is known as a 

 compound eye. A little bit of the outer covering of the eye, mounted 

 under a compound microscope, shows these eye units to be shaped like 

 tiny rectangles in cross seftion. Such an eye probably does not ha\e 

 very distinct vision at a distance. A crayfish, howe\er, easily distinguishes 

 moving objects and prefers darkness to light, as may be proved by experi- 

 ment. 



Feeding. — If it is possible to have the aquarium holding the crayfish 

 in the schoolroom, the method of feeding may be watched. The pincher 

 claws (chelipeds) are used to hold and tear food, as well as for defense 

 and offense. Living food is obtained with the aid of the chelipeds. Food 

 is shoved by the chelipeds toward the mouth ; it is assisted there by 

 three pairs of small appendages called foot jaws {maxillipeds) , and to a 



Female lobster, showing eggs attached to the 

 swiminerets. From photograph loaned by the 

 American Museum of Natural History. 



