XIII 



PHYLUM CHORDATA 



105 



in touch-cells : Pacinian corpuscles (C) consist of a terminal nerve- 

 branch surrounded by a complex laminated sheath. Touch- 

 corpuscles and Pacinian bodies are found only in the higher forms. 

 In Fishes, characteristic sense-organs are present, known as the 

 neuromast-organs or organs of the lateral line. Extending along 

 the sides of the trunk and tail is a longitudinal streak, due to the 

 presence either of an open groove or of a tube sunk in the epidermis, 

 and continued on to the head in the form of branching grooves 

 or canals (Fig. 790, A). These organs and also certain others, 

 in the form of pits or of unbranched canals, are lined with 

 epithelium (B), some of the cells of which are arranged in groups, 

 the neuromasts, and have the form characteristic of sensory cells (6) 

 produced at their free ends into hair-like processes (c) : they are 

 innervated by the 

 lateral nerve, and ^s^'" 01 



in the head, by the ...^ 



seventh and some- 

 times also the ninth 

 nerve. At their 

 first appearance in 

 the embryo the 

 organs of the lateral 

 line are distinct, seg- 

 mentally - arranged 

 patches of sensory Flo 

 epithelium in inti- 

 m a t e connection 

 with the ganglia of 

 the third, fifth, 

 seventh, ninth, and tenth nerves. Cutaneous sense-organs of the 

 lateral-line system, having at first a metameric arrangement, also 

 occur in the aquatic Amphibia. 



The function of the neuromast-organs has been shown to be 

 to enable the animal to detect vibrations in the water of too low 

 a frequency to form a sound capable of perception by the ear. 



The sense of taste has for its special organs taste-buds (Fig. 

 791), similar in general character to the end-buds in the skin, and 

 composed of groups of narrow rod-shaped "cells. In Fishes these* 

 are widely distributed in the mouth and branchial cavities, also 

 on the outer surface of the head, and in some Fishes over almost 

 the whole surface of the body. In higher Craniates they are 

 chiefly confined to the epithelium of the tongue and soft palate, 

 and are supplied mainly by branches of the glossopharyngeal. 



The olfactory organ is typically a sac-like invagination of the 

 skin of the snout, anterior to the mouth, and communicating with 

 the exterior by an aperture, the external nostril. It is paired in all 

 Craniata, except Cyclostomes, in which there is a single olfactory 



;i 



791. A, vertical section of one of the papillae of the 

 tongue of a Mammal, d. submucosa ; e. epithelium ; n. 

 nerve-fibres ; t. taste-buds. B, two taste-buds, c. covering 

 cells shown in lower bud ; d. sub-mucosa ; e. epithelium of 

 tongue ; m. sensory processes ; n. internal sensory cells 

 shown in upper bud. (From Foster and Snore's Physiology.) 



