146 GENERAL ZOOLOGY 



and arachnids ; that is, a series of somites placed one after 

 the other, with all appendages jointed. 



The Cephalothorax. The shell covering the dorsal and lat- 

 eral surfaces of the anterior region of the body is distinct 

 from the hard parts elsewhere on the body, and is termed the 

 carapace. At the anterior dorsal end of the carapace there 

 is a prominent beak, or rostrum, beneath which, on either 

 side, extends an eye, borne on a stalk. Study of the living 

 crayfish enables one to see how well the rostrum and the 

 squame, mentioned below, protect the stalked movable 

 eye. The eye is compound ; the manner in which the image 

 is formed is practically the same as in the insects. Cray- 

 fishes have no simple eyes. 



The cephalothorax bears at its anterior end six slender, 

 many-jointed feelers, the short ones called antennules (Fig. 

 83), the long ones, antennse. The four antennules are really 

 the four branches of a single pair of appendages coming 

 from a short stem which is attached to the body. On the 

 upper surface of one of the segments of this stem is a small 

 hole surrounded by a number of bristles. The hole opens 

 into a cavity which contains several small grains of sand 

 placed there by the crayfish itself after every molt (the 

 process of molting is explained in Chapter XVI). This 

 organ, with its nerve connections, constitutes the "ear" of 

 the crayfish (Fig. 83), the chief function of which is to help 

 the animal to balance itself during locomotion. The techni- 

 cal name otocyst is given to it. The antennae are the inner 

 branches of a pair of double-branched appendages. The 

 outer branch of each is short, flat, and triangular, and lies 

 just below the eyes, where it serves a protective function. It 

 is called the squame. On the lower, or basal, segment of the 

 stem of this pair of appendages is a small, hard, round swell- 

 ing, or papilla (Fig. 84, II, 1), on which is the opening from the 

 "kidney," or green gland, shown in broken outline in Fig. 83. 



