CRAYFISH 263 



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 ^^^th 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. 



Fig. 168. — A female crayfish with eggs attached to her swimmerets. (From Andrews, in 



Am. A^atur., vol. 38.) 



298. 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 mnter; 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 

 cells are transferred by the first two pairs of swimmerets of the male 

 from the opening of the vasa deferentia to the seminal receptacle of 

 the female. The seminal receptacles are cavities inclosed in folds of the 

 cuticula between the fourth and fifth pairs of w^alking legs. There the 

 sperm cells remain until the eggs are matured. At this time the latter 

 are passed out of the oviducts, which open at the bases of the third pair 

 of legs, and backward in a groove between the bases of the legs of the 

 two sides of the body, receiving sperm cells and being fertilized on the 

 way. The eggs finally become attached to the swimmerets by a gluelike 

 secretion, masses of them appearing like so many bunches of grapes 

 (Fig. 168). They remain attached during development, their aeration 

 being assisted by movements of the swimmerets. 



Cleavage is superficial and the embryo develops from a thickening 

 of the blastoderm on one side. Limb buds appear, w'hich correspond to 



