THE SENSE-ORGANS 471 



deep trough or, more usually, an enclosed tube, running just 

 below the surface ; and in this latter case, each organ may open 

 by its own pore, placed directly above it, or an entire tube 

 may open by a single common pore at one end. These canals 

 are usually filled with a clear mucous or gelatinous material, 

 secreted by the walls, and destined to protect the sensory cells. 

 In the case of the second form of development, there will be 

 produced local groups of associated, though distinct, canals, 

 each beginning superficially at an external pore and running 

 obliquely beneath the surface to terminate proximally in a bulb 

 or ampulla, in which the sense-organ is located. These organs 

 occur in localized masses in the heads of selachians, associated 

 topographically with the mucous canal type, the ampullae with 

 their nerves clustered in such a way as to resemble bunches of 

 grapes (Fig. 127). 



That a system so highly developed and so extensive in its 

 distribution as that of the lateral line organs should have 

 wholly disappeared in terrestrial vertebrates, together with its 

 nerves, is a phenomenon of so unusual a nature that numerous 

 attempts have been made by morphologists to find its direct 

 continuation among the parts of higher vertebrates. Thus, one 

 well-known theory associates these organs with the taste-buds, 

 a view arising naturally from the extreme similarity between 

 the two structures. There seem, ho\vever, to be no definite 

 data to form the logical steps between the two, and the fact 

 that the nerve supply to the taste-buds comes from the Glosso- 

 pharyngeus and not from any part of the extensive sensory 

 system associated with the lateral line organs speaks strongly 

 against this homology. As an added evidence in the same direc- 

 tion there are found in the nasal mucous membrane of many 

 fishes and of certain of the lowest urodeles (Siren) groups of 

 cells forming (f smell-buds/' extremely similar to taste-buds, and 

 yet by no possibility connected with the lateral line system. 



A second possible survival of the lateral line organs is 

 seen by some in the hair of mammals. This theory is based 

 upon a certain similarity in the early stages of development 

 of the two structures, that is, the initial procedure in both 



