108 THE BRAIN OF INVERTEBRATES. 



with one another on the same side and also with their 

 fellows on opposite sides of the body. They are related 

 to one another either by what appears to be continuous 

 growth or by means of * commissures.' 



5. The size of the Brain as a whole, or of its several 

 parts, is therefore always fairly proportionate to the develop- 

 ment of the animal's special Sense Organs. The more 

 any one of these impressible surfaces or organs becomes 

 elaborated and attuned to tak^ part in discriminating 

 between varied external impressions, the greater will be 

 the proportionate size of the ganglionic mass concerned. 



6. Of the several sense-organs and Sensory Ganglia 

 whose activity lies at the root of the Instinctive and 

 Intelligent life (such as it is) of Invertebrate Animals, 

 some are much more important than others. Two of 

 them especially are notable for their greater proportional 

 development : viz., those concerned with Touch and 

 Vision. The organs of the former sense are, however, 

 soon outstripped in importance by the latter. The visual 

 sense, and its related nerve-ganglia, attain an altogether 

 exceptional development in the higher Insects and in the 

 highest Mollusks. 



7. The sense of Taste and that of Smell seem, as a 

 rule, to be developed to a much lower extent. In the 

 great majority of Invertebrate Animals it is even difficult to 

 point to distinct organs or impressible surfaces as certainly 

 devoted to the reception of either of such impressions. 

 Nevertheless, as we shall subsequently find, there is reason 

 to believe that in some Insects the sense of Smell is mar- 

 vellously keen, and so much called into play as to make it 

 for such creatures quite the dominant sense endowment. 

 It is pretty acute also in some Crustacea. 



8. The sense of Hearing seems to be developed to a very 

 slight extent. Organs supposed to represent it have been 



