Touch. 433 



seem to be specialized organs. Star-fishes and Echinoderms also have 

 similar tactile organs. Earth-worms are very sensitive to contact, the 



FIG. 172. Tactile apparatus from the 

 skin of larva of Corethra (a two-winged 

 insect of the gnat form ). 



Tb Tactile bristle. 



N. Nerve. 



G. Ganglion. 



whole skin being an organ of 

 touch. As a rule, the animals 

 above the worms are all sensi- 

 tive to touch on the exposed 

 parts of the body, and gener- 

 ally they have specialized or- 

 gans for purposive touch. 

 Crustaceans and Articulates gen- 

 erally have long tactile bristles. 

 When a bristle is touched, it 

 vibrates down to its roots, where 

 it connects with a nerve fibre and 

 Fia. 172. transfers the stimulation to it. 



In the Cuttle fishes, Octopus, &c. , the long tentacles are very delicate 

 organs of touch. In the mammals, the snout, including tongue, &c. , is 

 specialized into a touch organ, and many, such as Squirrels, Bears, 

 Monkeys, &c. , have the touch sense specialized in their hands. The wing 

 of the Bat has become exquisitely sensitive to varying pressure in the 

 air as shown by its avoidance of obstructions in its rapid flight, even 



FIG. 173. Organ of Touch of Onchid- 

 ium. Diagrammatic section. 



( After Semper.) 



a a .Two layers of the cuticle. 



a. A biconvex thickened portion of 

 the cuticle. 



6. Enlarged epithelial cells. 



6'. Ordinary epithelial cells. 



c. Cellular body. 



d.-Cells. 



n. Nerve. 



when made blind. There is a 



naked mollusk ' living on the 



shores of some tropical seas 



called* the Qnchidium, which 



possesses some remarkable sense 



organs both of touch and sight. 



The organ of touch is shown in 



fig. 173. It is strikingly like FlG 173 



a simple eye and carries a suggestion of an easy possibility of being 



developed into one. ( See figs. 183, 184.) 



The tactile papillae which are common to the vertebrates are not found 

 in the invertebrates. In Lizzards they occur under the toes. In. the 

 chameleons and some ant-eaters the tail is the organ of touch and con- 



