GENERAL ZOOLOGY 



Circumesophageal 

 connective 



Subesophageal 

 ganglion 



First thoracic 

 ganglion 



First abdominal 

 ganglion 



Ventral nerve cord 



Fig. 15.7. Nervous svstem of a 

 cravfish, Aslacus: dorsal view. 

 (Redrawn with modifications 

 from W. Schmidt, 1915, ^eil- 

 schnfl fur wusenschaflliche ^o- 

 oloaie, vol. 113.) 



layer, and the hairs projecting into their cavities are comparable with the 

 sensory hairs on other parts of the body. Like the statocysts of other animals, 

 these organs contain particles called statoliths; in the crayfish the statoliths 

 are foreign bodies such as sand grains, which are lost at the time of molting 

 and must be replaced from an external source. If a freshly molted crayfish 

 is placed in water without such foreign particles, none can be found in the 

 statocysts, and the animal shows disturbances in maintenance of equilibrium. 

 If iron filings are supplied to such an animal, instead of sand grains, some of 

 these may be worked into the statocysts; the animal's reactions can then be 

 tested by subjecting it to a movable magnetic field. The reactions under such 

 conditions verify the fact that the statocysts are organs of equilibration, and 

 that sensory hairs in different parts of the organ are stimulated by contact 

 with the statoliths as the animal changes its position with reference to the 

 force of gravity. Nerve impulses set up by such stimulation pass to the brain. 



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