ORGANISMS AND THEIR MEDIA. 2 i 7 



than on other parts of the body, is not difficult to explain. What- 

 ever the mode by which they are evoked or called into being (and 

 the most opposite views may be entertained upon this subject), it 

 seems obvious that, if organs of this kind are to be present at all, they 

 should occur in situations where they might be put to most use. In 

 an animal accustomed to active locomotions, the mouth is, with only 

 a very few exceptions, situated on that part of the body which is 

 habitually directed forward. The anterior extremity thus comes to 

 be the part of the body which is brought most into converse with its 

 environment; and, of the diverse objects impinging against it, or 

 against which it impinges, some are of a nature to serve the organism 

 as food, and some are not. A higher degree of impressibility springs 

 up, therefore, in this situation, where the parts are necessarily exercised 

 so largely with impressions corresponding with food and with others 

 having an opposite relation. It should not surprise us, therefore, to 

 find among the lower animals that the most specialized tactile or- 

 gans are found in the immediate neighborhood of the mouth. Such 

 organs may be, and are in fact, not unfrequently both tactile and pre- 

 hensile; though this is more especially the case in sedentary forms of 

 life, like the hydra, the sea-anemone, or some of the tentaculated 

 worms. The tentaculae of the latter animals would seem to be pos- 

 sessed of an extremely high degree of impressibility, if we are to 

 judge by the report of one who devoted much attention to the study 

 of this class of organisms the late Dr. Williams, of Swansea. He 

 says : " It is not easy, for those who have never enjoyed the spectacle 

 of the ' feat of touch ' performed by the tentaculated worms, to esti- 

 mate adequately the extreme acuteness of the sensibility which re- 

 sides at the extremities of the living threads with which the head and 

 sides of the body are garnished. They select, reject, move toward, 

 and recede from, minute external objects with all the precision of 

 microscopic animals gifted with the surest eagle-sight." 



But it often happens that the solid bodies serving as food are in a 

 measure soluble, so that, in animal organisms comparatively low T in the 

 scale of complexity, some of the tactile structures within or around 

 the mouth may undergo a further specialization by which they become 

 able to discriminate and respond to impressions of a slightly different 

 nature. These parts become sensitive to a more refined kind of con- 

 tact, such as may be yielded by certain dissolved elements of the food 

 substance, whose local action may be attended by some slight chemi- 

 cal change in the tissue of the organ. Impressions are thus produced 

 whereby the " sapidity " or flavor of bodies is appreciated, and such 

 impressions gradually become associated with definite related move- 

 ments. 



No distinct organ of " taste " or specialized gustatory surface is 

 known to occur among invertebrate animals, except in insects and in 

 such higher mollusca as gasteropods and cephalopods ; although such 



