530 THE TYMPANIC CAVITY. 



The evolution of the accessory parts of the ear would be very difficult 

 to explain on Darwinian principles if the views of Hunt and Urbantschitsch 

 were correct ; and the accepted doctrine, originally proposed by Huschke 

 (No. 389), according to which these structures have originated by a ' change 

 of function' of the parts of the first visceral cleft, may fairly be held till 

 more conclusive evidence has been brought against it than has yet been 

 done. 



Tunicata. The auditory organ of the Tunicata (fig. 306) is 

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



FIG. 306. LARVA OF ASCIDIA MENTULA. (From Gegenbaur ; after Kupffer.) 



Only the anterior part of the tail is represented. 



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

 neural tube; n. hinder part of neural tube; ch. notochord ; A", branchial region 

 of alimentary tract ; d. cesophageal and gastric region of alimentary tract ; O. eye ; 

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



It consists of two parts (i) a prominence of the cells of the floor 

 of the brain forming a crista acustica, and (2) an otolith pro- 

 jecting 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 



