conclude that, in this well-known action of our commonest insect, it is scenting, not feeling, the drop of milk or grain of sugar."
Looking to the importance of this endowment in reference to the perception of food, and also looking to the situation of the organs of smell in all the vertebrate animals, there is good reason for believing that any similar organs of sense which may exist among invertebrate organisms would be found in close proximity to the mouth, so as to permit of that joint or associated activity between the sense of taste and the sense of smell which is met with in all higher forms of life.
As already pointed out, there are also obvious reasons why the principal specialized tactile organs that may present themselves in lower animals should be found in the neighborhood of the mouth; and for similar reasons, if for no other, the anterior extremity of the body, or the upper surface near this anterior extremity, is the site in which visual organs might be used with most advantage by their possessor. To an active animal, visual organs would not only be more useful at the anterior extremity of the body than elsewhere in relation to its food-taking movements, but also in reference to all other uses to which such appendages may be applied during active locomotions from place to place. To this situation of the eyes only two or three exceptions are met with among animals endowed with powers of locomotion, and these deviations are explicable by reference to the habits and modes of life of the organisms in question.[1]
The part of the body bearing the mouth, and the various sensory organs already named, is familiar to all as the "head" of the animal; and it is owing to the fact of the clustering of sense-organs on this part of the body that the head contains internally a number of nerve-ganglia in connection therewith. This aggregate mass of ganglia constitutes the brain of the invertebrate animals, which, as we shall find, differs much in different classes of animals, not only in disposition and in size, but also in respect to the relative proportion of its component parts. The size of the respective ganglia, indeed, necessarily varies in accordance with the relative importance and complexity of the several sense-endowments already mentioned—those of touch, taste, smell, and vision. The ganglia thus constituting the brain of invertebrate animals are not only connected with their own particular external organs, but, in addition, we find the several ganglia of the two sides brought into relation among themselves and with their fel-
- ↑ In some spiders the ocelli are situated rather far back on each side of the cephalo-thorax, but, as Siebold says: "The disposition and the direction of the organs are in relation with the mode of life of these animals, some of which wait for their prey hidden in chinks of a wall within silken tubes which they have constructed, while others hold themselves motionless in the centre of their webs, or wander from side to side, a mode of life which obliges them to look in all directions" ("Manuel d'Anatomie comparative," tome i., p. 308). According to Prof. Rolleston also in the crustacean genera Euphausia and Thysanopoda: "Eyes may be, contrary to the otherwise invariable rule in Arthropoda, found elsewhere than upon the head" ("Forms of Animal Life," p. cxxi.).
