172 THE HUMAN SPECIES 



reason, that the oral ciliary organ of many Infusoria and the 

 cilia of others have tactile functions. However, there is no 

 doubt that the prehensile tentacles connected with ganglia in 

 the Ctenophora and Medusae, especially the two tentacles at 

 the base of the disc-like body of the Ctenophora, act as touch- 

 organs. The worms, with their touch-bristles and rods (setae) 

 connected with sensory nerves and their tactile papillae com- 

 posed of sensory nerve fibres penetrating into elevations of the 

 cuticular layer (nematoda) or ciliated depressions lying at the 

 side of the head (nemertinea) seem particularly well supplied 

 with touch-organs. There is no doubt, too, that the suckers 

 and oral tentacles of echinoderms serve this purpose, as also 

 those rod-like projections on the integument (tactile setae) into 

 which run nerves with ganglionic swellings (Crustacia, Myria- 

 poda, Insecta). Molluscs are in many respects similar to the 

 worms. So far as they have no hard covering to the body, the 

 whole surface of the latter is capable of transmitting tactile 

 sensations, but in addition little bristles supplied with nerves 

 exist in various parts of the body, developed in gastropoda 

 into the tentacles of the head, and in Cephalopods into long 

 mobile tentacles. 



The features distinguishing the Vertebrates from the Inver- 

 tebrates are nowhere more pronounced than in the higher sense 

 organs, while the organs of touch become in various parts of 

 the body differentiated into special structures. In fishes, in 

 addition to the feelers which many of them possess, there is the 

 "lateral line" on each side of the body throughout its length. 

 Among the higher Vertebrates a subsequent development is dis- 

 played in the touch-corpuscles in the superficial layers of the 

 tough skin which receive the terminations of the cutaneous 

 nerves (reptiles, birds and mammals). 



Certain areas of the skin in mammals, where the hair is 

 absent or scanty, are provided with papillae, for instance, in 

 some animals the alae nasi, the tip of the snout, the lip-margins, 

 the ball of the foot and toes, the nipples, mammae, glans penis, 

 clitoris, and the tip of the tail in cats, sheep, cows and pigs, but 

 not in dogs. 1 



Man's relative hairlessness is associated with an abundant 



1 Bonnet in Ellenberger's Histologie, p. 384. 



