AUDITORY ORGANS. 437 



placed on the under surface of the anterior vesicle of the brain. It 



Fig. 306. Larva of Ascidia mentula. (From Gegenbaur ; after KupfEer.) 



Only the anterior part of the tail is represented. 



N'. anterior swelling of neural tube; N. anterior swelling of spinal portion of 



neural tube ; 71. hinder part of neural tube ; eh. uotochord ; K. branchial region of 



alimentary tract; d. oesophageal and gastric region of alimentary tract; 0. eye; 



a. otolith ; o. mouth ; s. papilla for attachment, 



consists of two parts (1) a prominence of the cells of the floor of the 

 brain forming a crista acustica, and (2) an otolith projecting into 

 the cavity of the brain, and attached to the crista by delicate hairs. 



The crista acustica is formed of very delicate cylindrical cells, and 

 in its most projecting part is placed a vesicle with clear contents. 

 The otolith is an oval body with its dorsal half pigmented, and its 

 ventral half clear and highly refractive. It is balanced on the highest 

 point of the crista. 



The crista acustica would seem to be developed from the cells of 

 the lower part of the front vesicle of the brain. The otolith however is 

 developed from a single cell on the dorsal and right side of the brain. 

 This cell commences to project into the cavity of the brain and its 

 free end becomes pigmented. It gradually grows inwards till it 

 forms a spherical prominence in the cavity of the brain, to the wall of 

 which it is attached by a stalk. At the same time it travels round 

 the right side of the vesicle of the brain (in a way not fully ex- 

 plained) till it reaches the summit of the crista, which has become in 

 the meantime established. 



The auditory organ of the simple Ascidians can hardly be brought 

 into relation with that of the other Chordata, and has most probably 

 been evolved within the Tunicate phylum. 



Bibliography. 



InverteWata. 



(384) V. Hen sen. "Studien ub. d. Gehororgan d. Decapoden." Zeit.f. wiss. 

 ZooL, Vol. xiii. 1863. 



(3815) 0. and K. Hertwig. Das Nervensystem u. d. Sinneaorgane d. Medusen, 

 Leipzig, 1878. 



