126 NERVOUS SYSTEM OF VERTEBRATES. 



that the point at which the canal opens in the adult is the place 

 from which the organ develops. Starting as pits at these points 

 the tubes grow inward until they reach their final location. At 

 the inner end of the tube there is developed a sense organ of the 

 same type as the pit organ, but more complex in structure. The 

 organs are supplied by branches of the nerves which supply the 

 adjacent canals and are regarded as modified pit organs. The 

 vesicles of Savi are small isolated sacs lying beneath the epidermis 

 which contain pear-shaped hair cells and are also innervated by 

 the same system of nerves as the canal organs. In ganoids also 

 are similar organs called nerve sacs. Both are probably degen- 

 erated or at least modified organs of the same system with the 

 canal and pit organs. All this system of organs has been com- 

 monly given the name of the lateral line system, because the lateral 

 line canal is its most conspicuous part. Some years ago the term 

 neuromasts was proposed by Wright for the organs whose sense 

 cells are pear-shaped hair cells, as distinguished from the rod- 

 shaped cells found in the organs which are now known to be taste 

 organs. This is a much more satisfactory name than the term 

 lateral line organs. 



It was shown in the chapter on embryology that the neuromast 

 system develops in close connection with the ear. An examina- 

 tion of the ear in all its relations has led to the conclusion that it 

 is fundamentally a part of the same system of organs as the 

 neuromasts. The evidence of this in part will be given in detail 

 in the present chapter but may be summarized here as follows, 

 (i) The sense cells in the ear of all vertebrates are hair cells 

 similar to those of the canal organs. (2) The sense cells are 

 enclosed in canals which during development sink in from the 

 surface as do the sensory canals and remain open to the surface 

 for some time by way of the ductus endolymphaticus. (3) The 

 hair cells of both neuromasts and ear respond to vibrations in 

 the fluid which fills the canals. The cells of the ear of many fishes 

 and of higher vertebrates respond to vibrations of a more rapid 

 rate (more than 100 per second) than those to which the canal 

 organs respond. (4) The sense cells of the ear are supplied 

 by nerve fibers which in their development and their central endings 



