CHAP, vi.] OF TOUCH. 407 



In the echinoderms, the tentacula situated in the neighbourhood 

 of the buccal orifice, and receiving the nerves, are considered as 

 tactile organs. 



The mollusks have as instruments of touch cutaneous cells 

 with setiform prolongations, disseminated where the body is not 

 covered with hard pieces. 1 



When we can trace the cutaneous nerves of the mollusks, in 

 the transparent species, for example, we see that these nerves are 

 clear, pale, offering here and there ganglionary expansions. It 

 seems then that here the sense of touch is exercised by grey 

 fibres. 



The articulated appendages or palpi situated near the mouth 

 in the crustaceans, the arachnida, and the insects, are tactile 

 organs- In the crustaceans there are, on the antennae and other 

 appendages, filiform prolongations, " tactile baculi," which also 

 exist in the myriapods, the insects. 2 Besides these special organs, 

 we are forced to admit, in the crustaceans, the arachnida, and the 

 insects, sensitive organs disseminated in the skin. In effect, in 

 spite of the envelopment, the varnish of chitine, which covers 

 them, these animals feel strongly the contact of the exterior 

 bodies at any point whatever of their own bodies. The soft 

 membrane which lines this bed of chitine is therefore sensitive, as 

 is also the subungual dermis of man. 



The general structure of the skin is perceptibly the same in 

 all vertebrated animals, but nevertheless the manner of termi- 

 nation of the nervous fibres varies. Often the fibres end in 

 expansions, some of which are called corpuscles of Paccini ; 

 others, corpuscles of touch. The first are observed in man, 

 birds, &c. In the corpuscle of touch, the sensitive fibre con- 

 ducts to an ovoid nucleus. The corpuscules of touch, in man, 

 have a special form, which is nevertheless met with also in the 

 hand of the monkey, and in the lingual papillae of the elephant. 

 Every one knows that these corpuscles are extremely numerous 

 1 Gegenbaur, loc. tit., pp. 122, 302, 479. IKd, p. 361. 



