THE CRAYFISH 127 



end of the carapace there is a prominent beak, or rostrum 

 (Fig. 67, 3), beneath which, on either side, extends an eye 

 (Fig. ,67, 4), borne on a stalk. Study of the living crayfish 

 enables one to see how well the rostrum protects the stalked 

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

 the image is formed is practically the same as in the insects. 

 Crayfishes have no simple eyes. 



The cephalothorax bears at its anterior end six slender, 

 many-jointed feelers, the shortones calledawfew>m?e (Fig. 67,6), 

 the long ones, antennce (Fig. 67, 6). The four antennules are 

 really the two 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 XIII.) This organ, with 

 its nerve-connections, constitutes the " ear " of the crayfish 

 (Fig. 67, 7), the chief function of which is to help the ani- 

 mal to balance itself during locomotion. The technical name 

 otocyst is given to it. The antennae are the inner branch of a 

 pair of double-branched appendages, of which the outer branch 

 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 append- 

 ages is a small, hard, round swelling, or papilla (Fig. 67, 8; 

 Fig. 68, II, l), on which is the opening from the "kidney," or 

 green-gland, shown in broken outline in Fig. 67. 



About the mouth are six pairs of appendages. The first, 

 the hard mandibles (Fig. 67, 9), are adapted to crushing into 

 smaller bits the food seized by the large claws. Each mandi- 

 ble bears a short palpus (Fig. 68, III, 2). Next in the series 

 are the first and second maxillce (Fig. 68, IV, V) and the first, 

 second, and third maxillipeds (Fig. 68, VI, VII, VIII). The 



