No. 437.] SENSE ORGANS OF FISHES. 



appear in one species as canal organs, in another as pit organs 

 and in still another as naked sense organs, and the separation of 

 the latter from the terminal buds of the third type is often a 

 matter of difficulty. This has led many of the students of these 

 organs to deny the validity of the distinction between neuro- 

 masts and terminal buds, classing both as variants of one type. 



In 1870 Schulze discovered that the neuromasts, whether 

 enclosed within canals or exposed upon the surface, are charac- 

 terized by the presence of specific sensory cells, the pear cells or 

 hair cells, which extend only part way through the sensory 

 epithelium, while the specific sensory cells of the terminal buds, 

 like those of taste buds, extend from the external to internal 

 limiting membrane of the epithelium. This has been generally 

 confirmed and receives further support and interpretation from 

 the recent demonstration that neuromasts and terminal buds 

 receive distinct and strictly characteristic innervation. 



All neuromasts, whether canal organs, pit organs or naked 

 organs, are innervated by fibers which are separable from all 

 other types of nerve fibers, with separate ganglia and roots, and 

 all of which terminate in the tuberculum acusticum or in the 

 cerebellum. The sense organs of the internal ear have the same 

 general structure as the neuromasts of the skin and are likewise 

 innervated from the tuberculum acusticum, so that the whale 

 system is termed the acustico -lateral system of nerves and sense 

 organs. The tuberculum acusticum and its derivative, the cere- 

 bellum, are morphologically intimately related to the general 

 cutaneous centers of the dorsal horn, and the whole acustico-lat- 

 eral system is in all probability phylogenetically derived from the 

 general cutaneous system. 



This probability is strengthened by the results of recent 

 physiological experiments upon this system. The lateral line 

 system of fishes as a whole is undoubtedly concerned in the 

 maintenance of bodily equilibrium, and the method of stimulation 

 here is closely similar to that of ordinary tactile nerves, and 

 doubtless derived from it. The function of orientation in space 

 is especially localized in the semicircular canals of the internal 

 ear, of like phylogenetic origin with the lateral line canals, and 

 this part of the system persists in all terrestrial vertebrates. 



