152 



GENERAL PRINCIPLES OF ZOOLOGY. 



tions of carbonate or of phosphate of lime, whose function 

 is not yet explained. They usually float free in the centre 



FIG. 76. Auditory vesicle of a Mollusk (Pterotrackea). N, auditory nerve ; //=, auditory 

 cells with the central cell; 2, ciliated cells; M'z, ciliated cells; Of, otolith. (After 

 Claus.) 



of the vesicle, and are often held in place by bunches of 

 cilia which project from the non-sensitive epithelial cells. 



Auditory Pit. Every auditory vesicle develops from a 

 pitlike invagination of the skin, and consequently is for a 

 time an auditory pit. Therefore it is not surprising that in 

 many animals the organ has stopped at the lower stage of 

 development ; for example, the crayfish has an open audi- 

 tory organ or auditory pit. On the other hand, the auditory 

 vesicle may assume a great variety of forms. In the 

 vertebrates, for example (Fig. 77), it is divided by a con- 



FIG. 7 7._Diagram of the human labyrinth. 7, utriculus with the semicircular canals ; S, 

 sacculus connected with the cochlea (C) by the canalis reuniens ; R, recessus labyrinthi ; 

 y, blind sac of the cochlea; A", apex of the cochlea. 



striction into the sacculus and the utriculus; the sacculus 

 is provided with a spirally-wound blind sac, the cochlea. 



