THE CRAYFISH 133 



pair, which are called antennules (Fig. 75, i), consist each of 

 two many-jointed filaments and act as organs of touch, and 

 smell or taste. In the base of each antennule is a cavity con- 

 taining a calcareous particle ; this structure, the statocyst, is an 

 organ of equilibrium, enabling the animal to maintain an upright 

 position in the water. 



Each antenna of the second pair (Fig. 75, 2) is a much longer 

 jointed filament, which serves both as a tactile organ and for 

 detecting changes in the chemical constitution of the water. 



Enemies may also be located by means of the two compound 

 eyes (Fig. 75, 28) which are placed on stalks and can be moved 

 in all directions. They resemble in structure those of the in- 

 sects. The crayfish uses them to locate the insect larvae, snails, 

 small fish, tadpoles, and other small moving animals that it uses 

 as food. 



Locomotion. Ordinarily the crayfish creeps along upon its 

 stiltlike legs. There are five pairs of these, but the first pair, 

 the pinchers (Fig. 75, 9), are seldom used to walk with, being 

 held in readiness as weapons of offense and defense. When 

 alarmed, the crayfish walks backwards, and if it becomes neces- 

 sary for it to escape quickly, it bends its flexible, scoop-shaped ab- 

 domen underneath the body and thus swims backward very 

 rapidly in jerks. At the end of the abdomen are two fin-shaped 

 appendages which, like blunt oars, aid in propelling the animal 

 when the abdomen is bent forward under the body. The vital 

 organs of the crayfish are mostly in the cephalothorax, the ab- 

 domen being filled with the muscles used in swimming. 



Food and Digestion. - - The food that the crayfish needs to 

 keep its body going and growing consists by preference of small 

 living animals, but these may be flavored with pieces of plants 

 and other animal and vegetable substances to be found in quiet 

 waters. The large pinchers are used to hold and cut the food 

 into pieces, and the small pinchers on the second and third 

 pairs of legs (Fig. 75, 10 and //) carry the pieces to the mouth. 

 Here the six pairs of mouth parts work together, the two pairs of 



