Chap. XII.] 



EAR OF CHORDATA. 



463 



which protects the cerebrum and, as such portion of 

 the cartilage forms a special investment for the ear, 

 we have in these alone, among invertebrates, that dis- 

 tinction between the outer or cartilaginous, and the 

 inner or membranous ear capsule to which we are 

 accustomed in the ears of vertebrates ; in Cephalopoda, 

 as in Crustacea and Vertebrates, an acoustic ridge or 

 crest is formed on which are set the auditory cells, and 

 in some of them, as in some of the lower Vertebrates, it 

 appears that the ear sac is permanently in communica- 

 tion with the outer world by a nar- 

 row open duct, the remnant of the 

 primitive involution of the epiblast 

 from which the organ was fashioned. 



Ampiiioxus has no known 

 auditory organ, and that which is 

 found in the Urochordata would 

 appear to .have been independently 

 developed within the limits of the 

 group (Balfour). It lies on the 

 under surface of the anterior brain 

 vesicle (Fig. 195 ; ) ; the cells of the brain form an 

 acoustic ridge, the delicate hairs oh whose cells hold 

 an otolith which projects into the cavity of the brain, 

 and is remarkable for being pigment ed. 



Though the sensory portion of the ear of higher 

 Vertebrates is at some distance from the surface of 

 the body, it is not to be supposed that it has not, like 

 all other organs of sense, its primary seat of origin in 

 the outer layer of the embryo or epiblast. In the 

 lower vertebrates the auditory capsule is closed, and 

 lies just below the skin, or sinks some way into the 

 walls of the brain case, as in Elasmobranchs, where 

 the duct either opens to the exterior by a minute 

 pore (ray), or is closed over by the skin (sharks) ; in 

 the higher forms a special auditory passage is deve- 

 loped. 



Fig. 194. Diagram 

 of the Ear of 

 Cyclas. (After 

 Simroth. ) 



