PHYLUM ARTHROPODA 



287 



the orientation of the body while swimming. Statocysts are, 

 therefore, organs of equilibration. When the crayfish changes 

 its exoskeleton in the process of molting, the statocyst is also 

 shed. Individuals that have just molted, or have had their stato- 

 cysts removed, lose much of theijr powers of orientation. Per- 

 haps the most convincing proof of the function of equilibration 

 is that furnished by the experiments of Kreidl. This investi- 

 gator placed shrimps, which had just molted and were therefore 

 without statoliths, in filtered water. 

 When supplied with iron filings, 

 the animals filled their statocysts 

 with them. A strong electromag- 

 net was then held near the stato- 

 cyst, and the shrimp took up a 

 position corresponding to the re- 

 sultant of the two pulls, that of 

 gravity and of the magnet. 



MUSCULAR SYSTEM. The prin- 

 cipal muscles in the body of the 

 crayfish are situated in the ab- 

 domen, and are used to bend that 

 part of the animal forward upon 

 the ventral surface of the thorax, oviduct; 2, right "lobe of ovary; 



thus producing backward locomo- 5, left lobe opened to show central 



/ . ~ "i cavity ; 4, external opening of 



tion in Swimming. Other muscles oviduct; 5, base of third walking 



extend the abdomen in the prepara- B^ de) (From Shipley and Mac ' 

 tion for another stroke. The ap- 

 pendages are all supplied with muscles which give them the 

 power of motion. It is of interest to note that the muscles 

 are internal, and attached to the inner surface of the skeleton. 

 In man, on the contrary, the skeleton is internal and the muscles. 

 external. 



REPRODUCTION. The sexes of crayfishes are normally sep- 

 arate (dioecious). In the male the spermatozoa arise in the 

 bilobed testis (Fig. 202, 37), pass through the paired vasa defer- 



FIG. 205. Female reproductive 

 organs of the crayfish, i, right 



