STATOCYSTS 



167 



of the statocyst and stimulate the cells in that region. By 

 this means the organism is informed of the orientation of its 

 body in space. 



389. No statocysts are found in either nereis or the earth- 

 worm, but they are present in some other Annelids. 



390. On the upper surface of the basal portion of the anten- 

 nules of the crayfish there is a small opening which leads into a 

 statocyst. The inside of the sack is Hncd with sensory hairs, 

 upon which rests the statolith. In this case the statolith is 

 composed of grains of sand cemented together by a secretion 



Fig. 84. — Diagram of the internal 

 ear (labyrinth) of one of the lower 

 vertebrates. 11, Utriculus with three 

 semicircular canals; s, sacculusj /, 

 lagena. 



Fig, 84a. — Diagram of the laby- 

 rinth of a mammal showing the 

 cochlea. 



of the epidermis. The sand is introduced into the sack by the 

 animal itself after each ecdysis, for the Hning of the sack *' sheds " 

 like the remainder of the cuticula, and its contents are cast 

 out at the same time. 



391. Statocysts are practically wanting in insects. 



392. The Vertebrate Organ of Equilibration. — The internal 

 ear of vertebrates consists of a membranous sack, the labyrinth, 

 which is lined internally with a layer of cells of ectodermal origin. 

 At certain places in this lining there are groups of sensory cells, 

 which have a close resemblance to the sensory cells of the stato- 

 cysts just described. The labyrinth is filled with a fluid and 



