XIII 



PHYLUM CHORDATA 



109 



ass 



c/j 



which attains considerable dimensions in the higher classes, while 



from its inner face is given off 

 se a narrow tube, the endobjm- 



phatic 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 ma cuke 

 acustica? and cristce acusticcc 

 (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, 



fit tip-htlv into FIG. 745. Longitudinal section through an ampulla, a. e. auditory 



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



this SVStem Of er. crista acustica ; ct. connective tissue; c. i, epithelium ; /. 



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



Cavities, but be- Physiology.) 



ce 



FIG. 744. External view of organ of hearing 

 of Craniata (semi-diagrammatic), an, 

 ampulla of anterior canal ; ae, of horizontal 

 canal ; ap, of posterior canal ; ... apex of 



' superior ntricular sinus ; ca, anterior semi- 

 circular canal ; ae, horizontal ; ap, posterior ; 

 cus, canal uniting sacciilus with utriculus ; 

 de, endolymphatic duct ; /, cochlea ; rec. 

 utricular recess ; s, sacculus ; se, endo- 

 lymphatic sac ; sp, posterior utricular 

 sinus ; gs. superior utricular sinus ; u. 

 utriculus. (From Wiedersheim's Vertebrata.) 



