262 METAZOAN PHYLA 



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

 it can again function. 



296. 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. 



297. Behavior. — When the bottom of a lake or stream is observed 

 through clear water, 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 

 the opening at the anterior end of the gill chamber, resulting in the main- 

 tenance of a current forward through the chamber. The legs are often 

 moved quietly backward and forward, serving to wave the gills back and 

 forth and thus aid in respiration. Some crayfish live in burrows in the 

 ground which reach down to the water level. When a pond or stream 

 dries up, the crayfish digs its burrow deeper and the earth excavated by 

 the animal is brought to the entrance and built up into a characteristic 

 mud chimney, which may be capped over with mud. 



When attacked the crayfish defends itself with its chelae and resists 

 being dragged from the burrow, but if the object under which it is hidden 

 is raised, the animal is ready to dart away in the turbid cloud which is 

 spread through the water and so escape. If food comes near enough to 

 the opening of the burrow that its presence is detected, the animal 

 emerges, walking by means of its walking legs, seizes the food, and 

 immediately backs into the burrow again. 



The crayfish is able to walk in any direction. It can also dart back- 

 ward, the movement being the result of an extension of the abdomen and 

 a spreading of the telson and the uropods, followed by a sudden flexion 

 of that part of the body. The resistance of the water drives the animal 

 rapidly backward. Since this action carries the animal only a short 

 distance, it is often repeated, and thus the crayfish makes a series of 

 backward darts. 



