466 Dynamic TJieory. 



Here there is a sort of ear-drum covering the cavity in which is the stone, 

 the hairs, the endolymph and the cells, connecting with the auditory 

 nerve. This drum answers to our fenestra ovalis. 



Fig. 205. Ear of Unio (Fresh water Clam). 

 a. Nerve. 6. Cells. 



c Cilia. d.~ Otolith ( or ear stone ). 



In the sepia or cuttle-fish ( a cephalopod mollusk ) 

 there is a cavity in the cartilage of the head in which 

 there is a little membranous sac in which the fibres of 

 the auditory nerve begin something equivalent to fig. 

 206. The development of the ear in this animal ap- 

 pears to be inferior to that of the eye in the same 

 animal. The amphioxus, the present representative of the lowest type 

 of the vetebrate, is destitute of ears of any kind. 



The ear of the Hag-fish ( myxine ), of the order of the cyclostomi, con- 

 sists of a single membranous sac which, however, is shaped like a tube 

 t bent into a ring. If we call one part of this tubular ring the vestibule, 

 the rest of it would represent a semicircular canal. ( The development 



FIG. 205. 



FIG. 206. Auditory Vesicle in section of a Mollusk Heteropod; Pterotrachea. 

 a to 6. Receptive sensory cells answering to the drum called Macula Acustica ( hear- 

 Hz. Cells in the same. Cz. Central cell. [h 



ing spot). 



En. Space filled with fluid, Endolymph. 



Ot. Otolith suspended in same. 



.2V. Auditory nerve. ( Glaus.) 



of the semicircular canal as observed in the chick is as follows : From 

 the original ear sac, a pocket-like process pushes out. In the middle of 

 this pocket the two side walls opposite each other approach until they 

 come together and coalesce, forming an island around which the un 

 coalesced portion of the walls forms a tube, each extremity of which joins 

 the original sac.) (Haeckel.) 



