Page:Popular Science Monthly Volume 9.djvu/237

Rh than on other parts of the body, is not difficult to explain. Whatever 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 organs are found in the immediate neighborhood of the mouth. Such organs may be, and are in fact, not unfrequently both tactile and prehensile; 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 tentaculæ of the latter animals would seem to be possessed 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 estimate adequately the extreme acuteness of the sensibility which resides 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 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 contact, such as may be yielded by certain dissolved elements of the food substance, whose local action may be attended by some slight chemical 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 movements.

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