298 ORGANS OP THE SENSES. 



in the pteropods, and the whole surface of the body is 

 exquisitely sensitive in the naked cephalopods where the 

 flexibility of the long arms and tentacula will enable them 

 to obtain more accurate perceptions of the forms and di- 

 mensions of outward objects. 



Among the vertebrata, as in the inferior classes, many 

 animals are covered externally with hard and insensible 

 parts which must greatly obscure their impressions of touch 

 derived from the contact of surrounding objects. The large 

 solid calcareous scales of many fishes, the smaller horny 

 scales of most ophidian and saurian reptiles, the large plates 

 of crocodilian and chelonian reptiles, the compact and dense 

 plumage of birds, the thick hides or shaggy furs of many 

 quadrupeds, and the long spines or broad horny scales of 

 others, must act like the insensible sheaths of many radiated, 

 articulated, and molluscous animals, in shielding their skin 

 from impressions of touch. Many fishes have the scales 

 so minute that their body, with relation to touch, is almost 

 as naked and sensitive as that of the amphibia above them ; 

 others have only the lower surface of their body or the 

 periphery of their mouth covered with a naked and 

 sensitive skin to compensate for the want of adaptation 

 of the arm and hands to the sense of touch, and 

 in many species of fishes the tentacula of the in- 

 ferior classes are still seen in the form of fleshy filaments 

 around or near the mouth. The long divided exsertile 

 tongue of serpents and the worm-like flexibility of their 

 trunk compensate for the want of hands as organs of touch. 

 The flexible prehensile tails of many climbing saurian and 

 mammiferous animals, the palmated feet and sensitive lips 

 of many aquatic birds and mammalia, and the delicate ex- 

 posed skin, the extended labial bristles, the long flexible 

 tongue or lips, or the extended proboscis of many quadru- 

 peds, contribute to extend this sense. The osseous foramina 

 for the branches of the fifth pair, even in animals now 

 extinct, enable us to judge of the development of these parts 

 relating to the sense of touch. But, from the cold-blooded 

 amphibious animals through all the higher forms of ter- 

 restrial vertebrata, we observe the hands to become more 

 exquisitely organized and more fitted for communicating 

 delicate impressions of the forms, dimensions, temperature, 



