THE CRAYFISH AND ARTHROPODS IN GENERAL 223 



The antennae are usually considered the special organs of touch, 

 but experiments seem to prove that they are not so sensitive as 

 other parts of the body. The tactile hairs are plumed, and sup- 

 plied by a single nerve. Positive thigmotropism is exhibited by 

 crayfishes to a marked degree, the animals seeking to place their 

 bodies in contact with a solid object, if possible. The normal 

 position of the crayfish when at rest under a stone is such as to 

 bring its sides or dorsal surface in contact with the walls of its 

 hiding place. Thigmotropism, no doubt, is of distinct advan- 

 tage, since it forces the animal into a place of safety. 



PHOTOTROPISM. Light of various intensities in the majority 

 of cases causes the crayfish to retreat, i.e. to show negative 

 phototropism. Individuals prefer colored lights to white, having 

 a special liking for red. Negative reactions to light play an 

 important role in the animal's life, since they influence it to seek a 

 dark place where it is concealed from its enemies. 



CHEMOTROPISM. The reactions of the crayfish to food are due 

 in part to a chemical sense. Smooth hairs, with nerve bundles 

 within them, are probably chemical, and, since " The animals 

 react to chemical stimulation on any part of the body ... we 

 must assume that there are chemical sense organs all over the 

 body " (165, p. 325). The anterior appendages, however, are 

 the most sensitive, especially the outer ramus of the antennule. 

 Positive reactions result from the application of food substances. 

 For example, if meat juice is placed in the water near an animal, 

 the antennae move slightly and the mouth parts perform vigorous 

 chewing movements. The meat causes " general restlessness 

 and vague movements toward the source of the stimulation, but 

 the animals seem to depend chiefly on touch for the accurate 

 localization of food " (165, p. 326). Acids, salts, sugar, and other 

 chemicals produce a sort of negative reaction indicated by scratch- 

 ing the carapace, rubbing the chelae, or pulling at the part stimu- 

 lated. 



HABIT FORMATION. It has been shown by certain simple 

 experiments that crayfishes are able to learn habits and to modify 



