64 THE USE AND NATURE 



the scenting of food by the nose in higher animals ; and, 

 as it is by tlie odorous qualities, much more than by the 

 form of the surface, that we judge of the fitness of sub- 

 stances for food, it is more reasonable to conclude that, 

 in this well-known action of our commonest insect, it is 

 scenting, not feeling, the drop of milk or grain of sugar." 



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 that the head con- 

 tains internally a number of related nerve ganglia. This 

 aggregate mass of ganglia constitutes the * Brain ' of 

 invertebrate animals. It forms a congeries of nerve 

 centres, differing much in different classes, as we shall 

 find, not only in regard to the disposition and size, but 

 also in respect to the relative proportions of its component 

 parts. The size of the respective ganglia, indeed, neces- 

 sarily varies in accordance with the relative importance 

 and complexity of the several sense organs already men- 

 tioned — those of Touch, Taste, Smell, and Sight. 



The ganglia thus constituting the Brain of invertebrate 

 animals are not only in relation each with its own par- 

 ticuhxr sensory organs, but, in addition, we find the 

 several ganglia brought into relation among themselves 

 and with their fellows of the opposite side by means of 

 connecting or commissural fibres. They are, moreover, 

 often connected, by means of much longer commissural 

 threads, with other nerve ganglia in difi'crent parts of the 

 body. 



Hearing. — Another special sense endowment remains to 

 be referred to. This has to do with the organism's power 

 of appreciating the vibrations causing 'auditory' impressiona 

 ■ — a power which is, however, probably possessed in only a 



