272 



THE SENSORIAL FUNCTIONS. 



of the relative positions of external objects. It is essential 

 therefore that those instruments which are more particularly 

 intended as organs of toucii. should possess this property. 



It will not be necessary to enter into a minute descri])tion 

 of these organs, because they have, for the most part, been 

 already noticed as instruments of prehension; for the sense 

 of touch is in general exercised more particularly by the 

 same ])arts which perform this latter function. Thus the 

 tentacula of the various tribes of Polypi, of Actiniae, and of 

 Annelida, are organs both of prehension and of touch. The 

 tubular feet of the Asterias and Echinus arc, in like man- 

 ner, subservient botii to tlie sense of touch, and to the fa- 

 culty of progressive motion. Tl^e feet of Insects and of 

 Crustacea are well calculated, indeed, by their jointed struc- 

 ture, for being applied to the surfaces, and to different sides 

 of bodies; but they are scarcely ever employed in this capa- 

 city; being superseded by the palpi, which are situated near 

 the mouth. When insects are walking, the palpi are inces- 



santly applied to the surface on which they advance, as if 

 these organs were especially employed to feel their way. 

 There can be little doubt, however, that, in most insects, 



