PHYLUM ARTHROPOD A 29 1 



LOCOMOTION. --Locomotion is effected in two ways, walking 

 and swimming. Crayfishes are able to walk in any direction, 

 forward usually, but also sidewise, obliquely, or backward. 

 Sii'imming is not resorted to under ordinary conditions, but only 

 when the animal is frightened or shocked. In such a case the 

 crayfish extends the abdomen, spreads out the uropod and tel- 

 son, and, by sudden contractions of the bundles of flexor abdom- 

 inal muscles, bends the abdomen and darts backward. The 

 swimming reaction apparently is not voluntary, but is almost 

 entirely reflex. If turned over on its back, the crayfish either 

 raises itself on one side and topples over, or else gives a quick 

 backward flop. 



REACTIONS TO STIMULI. - - THIGMOTROPISM. - - The crayfish 

 "is sensitive to touch over the whole surface of the body, but es- 

 pecially on the chelae and chelipedes, the mouth parts, the ven- 

 tral surface of the abdomen, and the edge of the telson." (Bell.) 



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. Thigmot- 

 ropism, no doubt, is of distinct advantage, since it forces the 

 animal into a place of safety. 



CHEMOTROPISM. - - The reactions of the crayfish to food are 

 due in part to a chemical sense, 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." 

 (Bell.) The anterior appendages, however, are the most sensi- 

 tive, especially the outer ramus of the antennule. Positive re- 

 actions 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. Acids, salts, sugar, and other chemicals 

 produce a sort of negative reaction indicated by scratching the 

 carapace, rubbing the chelae, or pulling at the part stimulated. 



