SENSE ORGANS. 203 



In amphibians, so far as concerns general sensibility, 

 we have the extreme opposite condition — i. e., a moist, 

 active, sensitive skin — but there are no special organs 

 of touch. 



Fishes are probably similar to amphibians in regard 

 to general sensibility. They also have a moist, sensi- 

 tive, mucous surface, and therefore general sensibility 

 well developed. In addition, many of them have special 

 organs of touch (feelers) about the mouth. 



In arthropods we have again the other extreme. They 

 all have a hard skeletal coat of mail on the outside, 

 which almost entirely cuts off general sensibility; but to 

 compensate they are endowed with very delicate special 

 organs of touch, as, for example, the long antennas of 

 insects and crustaceans. 



In mollusca we pass again to the other extreme. A 

 universal characteristic of mollusca is that they are 

 everywhere, except when inclosed in shell, covered with 

 a soft, active, mucous surface, which is endowed with 

 great sensibility. Many also, in addition, have good 

 tactile organs, such as the grasping arms of cephalopods 

 and the so-called horns or feelers of snails and other 

 gastropods. 



In echinoderms the body is again usually incased in 

 immovable shell, as in echinus; but again we find com- 

 pensation in their long, delicate tentacles, which are 

 feelers as well as locomotive organs. 



Again the pendulum swings back in cxlenterates 

 (medusae and polyps), where we find again the soft, 

 active mucous surface sensitively responsive to contact. 

 Their long tentacles also doubtless act as touch organs, 

 though perhaps imperfectly. 



Finally, in protozoa we find only general sensibility of 

 the lowest grade, to what extent conscious we can not 

 tell. From this lowest form of response to external 



