CHAP. III.] OF SENSE ORGANS. 59 



plexity, some of the tactile structures within or around 

 the mouth may undergo a further specialization, by which 

 they and their related nerve centres become fitted to dis- 

 criminate between impressions of a slightly different 

 nature. Such ' organs of taste ' would become sensitive 

 to the more refined kind of contact yielded by certain 

 dissolved elements of the food, whose local action is 

 perhaps attended by some slight chemical change in the 

 tissues of the part. Impressions are thus produced 

 whereby the ' sapidity ' or flavour of bodies is appreciated ; 

 and such impressions gradually become associated with 

 definite related movements, partly of internal and partly of 

 external organs. 



Although this mode of impressibility doubtless exists 

 in many of the lower forms of life, still no distinct organ 

 of Taste, or specialized gustatory surface, is as yet actually 

 known to occur among invertebrate animals, except in 

 Insects and in such higher mollusca as Snails and Cuttle- 

 fishes. 



Impressions of the two orders already referred to 

 more or less distinct from one another are those by 

 which alone multitudes of the lower forms of animal life, 

 such as Polyps and various kinds of Worms, appear to 

 hold converse with the outside world. Seeing, however, 

 that tactile and gustatory impressions can only be made by 

 actual contact of external bodies with the specialized parts 

 of an organism, such impressions are not of a kind to 

 excite movements in ' quest ' of food ; although they may 

 lead to correlated motions of parts adjacent to those 

 touched, as in the acts of prehension and swallowing. 



Sight. Movements in actual quest of food may, how- 

 ever, be excited in other animal organisms by impressions 

 bringing them into relation with more or less distant 

 bodies. The way is paved for this result when some 



