5 



7 8 THE ORIGIN OF VERTEBRATES 



the pectens or liabella, then the origin of the auditory apparatus 

 would present no difficulty. 



It is also easy to see that the formation of the parachordals from 

 entochondrites homologous with the supra-pectinal entochondrites, 

 would give a reason why the Yllth or opercular nerve is involved 

 with the Vlllth in the formation of the auditory capsule, especially 

 if the special sense-organ which gave origin to the auditory organ 

 was originally a pre-opercular sense-organ such as the rlabellum, 

 which subsequently took up a post-opercular position like that of 

 the pecten. 



The Evidence of Ammoccetes. 



As to the auditory apparatus itself, we see that the elaborate 

 oman for hearing — the cochlea — has been evolved in the vertebrate 

 phylum itself. In the lowest vertebrates the auditory apparatus 

 tends more and more to resolve itself into a simple epithelial sac, the 

 walls of which in places bear auditory hairs projecting into the sac, 

 and in part form otoliths. Such a simple sac forms the early stage 

 of the auditory vesicle in Ammoccetes, according to Shipley ; subse- 

 quently, by a series of foldings and growings together, the chambers of 

 the ear of the adult Petromyzon, as figured and described by Eetzius, 

 are formed. Further, we see that throughout the Vertebrata this sac 

 was originally open to the exterior, the auditory vesicle being first 

 an open pit, which forms a vesicle by the approximating of its sides, 

 the last part to close being known as the recess us labyrinthicus ; in 

 many cases, as in elasmobranchs, this part remains open, or com- 

 municates with the exterior by means of the ductus endolymphaticios. 



Judging, therefore, from the embryological evidence, it would 

 appear that the auditory organ originated as a special sense-organ, 

 formed by modified epithelial cells of the surface, which epithelial 

 surface becoming invaginated, came to line a closed auditory vesicle 

 under the surface. This special sense-organ was innervated from 

 a large ganglionic mass of nerve-cells, situated close against the 

 peripheral sense-cells, the axis-cylinder processes of which formed 

 the sensory roots of the nerve. 



Yet another peculiarity of striking significance is seen in connec- 

 tion with the auditory organ of Ammocoetes. The opening of the 

 cartilaginous capsule towards the brain is a large one (Fig. 154), and 



