ORGANISMS AND THEIR MEDIA. 221 



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 loco- 

 motion, 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 com- 

 ponent 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 bi-ain 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- 



1 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 Thymnopoda : "Eyes may be, contrary to the otherwise invariable rule in Arihro- 

 poda, found elsewhere than upon the head" (" Forms of Animal Life," p. cxxi.). 



