26 



NERVOUS SYSTEM AND SENSE-ORGANS 



Single epidermal cells or groups of such cells are specialized 

 for the reception of stimuli leading to sensations of touch, but 

 in such forms as Ccelenterates and Annelids many scattered 

 cells of the kind probably minister to other senses besides 



that of touch. And it must be re- 

 membered that even the special sense- 

 cells of hearing and sight are derived 

 from the skyi, which is in fact the 

 primitive sense-organ. Cells which arc 

 regarded as tactile, from some of the 

 lowest animals, are represented in fig. 

 1029. 



The firm external covering with 

 which the bodies of Arthropods are 

 clothed is naturally a hindrance to the 



Fig. 1030. Tactile Organs of Insects, . r * T "U U J 1 



greatly enlarged. On the right is a group of TCCeptlOn OI Stimuli by tllC Underlying 



r s ^r res ' and Dne> epidermis. The difficulty is got over 



by the existence of little pores in the 



hard investment. Under each pore is an enlarged sense-cell, 

 placed at the base of a stiff tactile bristle, with which external 

 bodies come into contact (fig. 1030). 



In aquatic Vertebrates the sense-cells of the skin are in direct 

 contact with the surrounding medium, although they are not 



infrequently protected by being 

 lodged in pits, grooves, or canals 

 s - c - which open at intervals to the ex- 

 terior. But in terrestrial Verte- 

 . T brates there are special end-organs 

 of touch which have sunk below the 

 epidermis, though they remain suf- 

 ficiently near to the surface to be 

 stimulated when the body comes into 



from the bill of a Duck, much enlarged; S. C., sense- COntaCt with SUrrOUnding" objects, 

 cells; N.F., nerve-fibre; Sh., fibrous sheath. " 



Such are the groups of touch-cor- 

 puscles which are to be found in the skin of the Frog, and 

 around the edge of the Duck's bill (fig. 1031). The latter animal 

 feeds upon small worms, &c., which live in the mud that is 

 strained through its bill, and such special arrangements are 

 clearly necessary to aid in the discrimination between what is 

 edible and what is not. Another example is afforded by the 



Fig. 1031. Organs of Touch 

 A, Small piece of the skin of a Frog, in vertical 



