XIII 



PHYLUM CHORDATA 



115 



ca 



ass 



aa 



a retina (M, r). The latter has a layer of nerve-fibres on its 

 outer, and one of rod-like visual elements (r.) on its inner surface : 

 it thus agrees with the usual types of Invertebrate retina, and not 

 with that of the paired eye. 



The organ of hearing, like that of sight,presents quite peculiar 

 features. It arises in the embryo as a paired invagination of the 

 ectoderm in the region of the hind-brain, a shallow depression being 

 formed which deepens and becomes flask-shaped, and finally, as a 

 rule, loses its connection with the external ectoderm, forming a 

 closed sac surrounded by mesoderm. At first simple, it soon becomes 

 divided by a constriction into 

 dorsal and ventral compartments. 

 The dorsal compartment is dif- 

 ferentiated into an irregular 

 chamber, the utriculus (Fig. 788, 

 u.\ and, usually, three tubes, 

 the semicircular canals. Of these 

 two, the anterior (ca.)and posterior 

 (cp.) canals, are vertical in posi- 

 tion and have their adjacent 

 limbs united, so that the two 

 canals have only three openings 

 between them into the utriculus : 

 the third or external canal (ce.*) is 

 horizontal, and opens into the 

 utriculus at either end. Each 

 canal is dilated at one of its ends 

 into an ampulla (ae., a.e, ap.\ 

 placed" anteriorly in the anterior 

 and external canals, posteriorly 

 in the posterior canal. 



The ventral compartment of 

 the auditory sac is called the 

 sacculus (s.) : it gives off pos- 

 teriorly a blind pouch, the cochlea 

 (/.), which attains considerable 

 dimensions in the higher classes ; while from its inner face is given 

 off a narrow tube, the endolymphatic duct (de.) t which either ends 

 blindly or opens on the dorsal surface of the head. The utricle 

 and saccule are sometimes imperfectly differentiated, and are then 

 spoken of together as the membranous vestibule. 



Patches of sensory cells (Fig. 789, 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 macula cwusticce and 

 cristoc acusticce (c. r.\ and to them the fibres of the auditory nerve 

 (n.) are distributed. A fluid, the endolymph, fills the whole of the 

 auditory organ or membranous labyrinth, and in it are formed 



H 2 



FIG. 788. 'External view of organ of hearing 

 of Craniata (semi-diagrammatic), aa, 

 ampulla of anterior canal ; ae, of horizontal 

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

 superior utricular sinus ; ca, anterior, ae, 

 horizontal, ap, posterior, semi-circular 

 canal; etc*, canal uniting sacculus with utri- 

 culus ; de, endolympfaatic duct ; I, cochlea ; 

 rec, utricular recess ; s, sacculus ; se, endo- 

 lymphatic sac ; sp, posterior utricular 

 sinus ; ss. superior utricular sinus ; u. 

 utriculus. (FromWiedersheim's Vertebrata). 



