CRA YFISH 257 



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. 



Crayfishes respond positively to contact stimuli and seek to place 

 themselves in such a position that as much of the body as possible is in 

 contact with a firm surface. Chemical substances dissolved in the water 

 also act as stimuli. Food not only causes a movement of the .animal 

 toward it but also excites chewing movements, and if meat juices are 

 added to the water, vigorous movements of such a type result. Chemicals 

 which are not normal to the water in which the crayfish is living may cause 

 it to rub its legs together or to scratch the surface of its body with them. 

 If the chemicals are in considerable concentration, however, the animal 

 may endeavor to escape entirely from the stimulus. 



Simple experiments have led to the general opinion that the behavior 

 of the crayfish is in part instinctive and in part habitual. An instinct is 

 an action involving an inherited association of reflexes all tending toward 

 a certain end. A habit is an action of similar character, but it is acquired 

 during the lifetime of the individual by the continued repetition of a 

 particular action. 



296. Reproduction. — Pairing of the sexes may take place either in 

 the spring or in the fall. If at the former time, the young become well- 

 developed before winter; if at the latter, the eggs may not be laid until 

 the following spring. The author, however, has observed a female 

 crayfish with very recently hatched young as late as the latter part of 

 October in an extremely warm fall season. During pairing the sperm 



