SENSE-ORGANS OF THE INTEGUMENT. 



167 



These structures function from the Amphibia onwards as organs 

 of taste, while in Fishes they probably serve as tactile organs. 

 This latter function is naturally impossible in those cases where 

 they become somewhat withdrawn inwards from the free surface of 

 the epithelium, as is the case with those situated on the tongue, where 

 they can only be reached by fluid passing in to them. 



II. TERMINAL GANGLION CELLS. 



These structures are not directly connected with the surface of 

 the epidermis, and supporting cells are wanting. 



" Tactile spots," consisting of groups of " tactile cells," are 

 met "with for the first time in tailless Amphibians, where, 

 situated mainly on small elevations, they are distributed over the 

 skin of the whole body (Fig. 136, a, a). In Reptiles they are 



FIG 



-JV 



136. A TACTILE SPOT FROM THE SKIN OF THE FROG. 

 (Modified from Merkel.) 



A 7 ", nerve, which loses its medullary sheath at N* ; a, a, nemo-epithelium ; 



b, epidermis. 



EH- 



FIG. 137. TACTILE CORPUSCLE FROM THE TONGUE OF A BIRD. 

 N, nerve ; H, outer investment, with nuclei (KH] ; S, S, septa. 



found chiefly in the region of the head, on the lips and sides of 

 the face, and on the snout, but in some cases (as in Blindworms 

 and Geckos), they extend over the whole body. 1 In Snakes and 

 Birds the tactile cells are confined to the mouth-cavity (tongue) 



1 Similar structures are also present in Crocodiles, and in the skin of the back of 

 Trionyx cellular bodies exist, which most probably are to be regarded as tactile 

 organs. 



