256 METAZOAN PHYLA 



ment from the wall of the ommatidium permits all of the rays entering 

 it to be reflected backward, increasing the amount of light falling upon 

 the retinula and thus giving a stronger stimulation. This probably 

 does not result in a clear image but enables the animal to distinguish 

 between light and dark. A compound eye has a great disadvantage 

 when compared with such an eye as the vertebrate eye in that the animal 

 cannot focus with it, thus limiting the distance to which vision is possible. 

 It has, however, the great advantage that it more readily perceives move- 

 ment in the visual field, since any motion almost inevitably results in a 

 stimulus being withdrawn from one retinula and applied to another. 



293. Statocyst. — At the distal end of the basal segment of each 

 antennule is a sac, lined with chitin, which is continuous with the chitin- 

 ous covering over the surface of the body; this is a statocyst. On its 

 walls are sensory hairs and in its cavity are grains of sand or other hard 

 objects known as statoliths. While the animal is in a normal position 

 there is no movement of these statoliths, and though they are in contact 

 with certain of the sensory hairs no stimulation is received. When, 

 however, the position of the animal is changed, their movement causes 

 them to come in contact with other hairs and this acts as a stimulus. 

 There results a sensation which causes the animal to respond in such a 

 way as to maintain its equilibrium. When the cuticula over the surface 

 of the body is shed, that which lines the statocyst is lost with the rest, 

 and other statoliths must be placed by the animal in the statocyst before 

 it can again function. 



294. Feeding Habits. — The food of the crayfish consists mostly of the 

 flesh of dead animals lying at the bottom of the body of water in which 

 it lives, bits of which it tears off with its large chelae. Living animals 

 which the crayfish can grasp and hold with its chelae may also serve as 

 food, such animals including snails, tadpoles, insects, and even small 

 fish. The food is held by the maxillae and maxillipeds and chewed by the 

 mandibles. Crayfishes readily devour one another when in captivity. 

 They feed at night but are most active at dusk and dawn. 



295. Behavior. — When the bottom is observed through the clear 

 water of a lake or stream, there may very frequently be seen beside a 

 stone or other object a slight depression leading to a burrow under the 

 object and presenting a very clean appearance. This appearance, which 

 gives one the impression of every particle of debris having been swept 

 away, is due to the presence of a crayfish in the burrow and the constant 

 current of water maintained by the animal in its breathing. Sometimes 

 the antennae may be seen projecting from the opening. The animal is 

 more or less in contact with the walls of the burrow. It faces the open- 

 ing, ready to receive any stimuli which may come and to emerge quickly 

 to seize any food which is presented. In this position the swimmerets 

 are waving forward and backward, and the bailer is working actively in 



