XIII 



PHYLUM CHORDATA 



109 



ca 



ass 



aa 



which attains considerable dimensions in the higher classes, while 



from its inner face is given off 

 -se a narrow tube, the endolym.- 



pliatic duct (de.\ which either 

 ends blindly or opens on the 

 dorsal surface of the head. The 

 utricle and sacculi are some- 

 times imperfectly differentiated, 

 and are then spoken of together 

 as the vestibule. 



Patches of sensory cells (Fig. 

 745, ae.) elongated cells pro- 

 duced into hair-like processes 

 (a. h.) occur in the ampulla 

 and in the utricle and saccule : 

 they are known as maculce 

 acusticce and cristce acusticcv 

 (c. r.\ and to them the fibres of 

 the auditory nerve (n.} are dis- 

 tributed. A fluid, the cndo- 

 lymph, fills the whole of the 

 auditory organ, or membranous 

 labyrinth, and in it are formed 

 otoliths of varying size and 

 number. There is every reason 

 for thinking that the labyrinth, 

 as in the lower animals, func- 

 tions as an organ of equilibration as well as of hearing. 



As the membranous labyrinth develops in the embryo it be- 

 comes surrounded 

 and enclosed by 

 the auditory cap- 

 sule, the cartilage 

 of which adapts 

 itself to the form 

 of the labyrinth, 

 presenting a large 

 excavation for the 

 utricle and sac- 

 cule and tunnel- 

 like passages for 

 the canals. The 

 auditory organ 

 does not, however, 



ce 



FIG. 744. External view of organ of hearing 

 of Craniata (semi-diagrammatic), aa, 

 ampulla of anterior canal ; ae, of horizontal 

 canal ; op, of posterior canal ; ass. apex of 

 superior utricular sinus ; ca, anterior semi- 

 circular canal ; ae, horizontal ; ap, posterior ; 

 cus, canal uniting sacculus with utriculus ; 

 de, endolymphatic duct ; ?, cochlea ; rcc. 

 utricular recess ; s, sacculus ; se, endo- 

 lymphatic sac ; sp, posterior utricular 

 sinus ; ss. superior utricular sinus ; u. 

 utriculus. (From Wiedersheim's Vertebmta.) 



fit tightly into 

 this system of 

 cavities, but be- 



FIG. 745. Longitudinal section through an ampulla. <<. e. audit' >ry 

 epithelium ; a. h. auditory hairs ; c. part of semicircular canal : 

 cr. crista acustica ; ct. connective tissue; c. <', epithelium; />. 

 nerve ; u. junction with utriculus. (From Foster and Shore's 

 Physiology.) 



