INTRODUCTION. xxvii 



The visual sense, and its related nerve-ganglia, attain an altogether exceptional devel- 

 opmentin the higher insects and in the highest molluscs. 



" 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 there is reason to believe that in some 

 insects the sense of smell is marvellously 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 discovered, principally in molluscs and in a few 

 insects. It is, however, of no, small interest to find that where these organs exist the 

 nerves issuing from them are most frequently not in direct relation with the brain, but 

 immediately connected with one of the principal motor nerve-centres of the body. 

 It is conjectured that these so-called 'auditory saccules' may, in reality, have more to 

 do with what Cyon terms the sense of space than with that of hearing. The nature 

 of the organs met with supports this view, and their close relations with the motor 

 ganglia also become a trifle more explicable in accordance with such a notion. 



" 9. Thus the associated ganglia representing the double brain are, in animals pos- 

 sessing a head, the centres in which all impressions from sense-organs, save those last 

 referred to, are directly received, and whence they are reflected on to different groups 

 of muscles — the reflection occurring not at once, but after the stimulus has passed 

 through certain ' motor ' ganglia. It may be easily understood, therefore, that in all 

 invertebrate animals perfection of sense-organs, size of brain, and power of executing 

 manifold muscular movements, are variables intimately related to one another. 



"10. But a fairly parallel correlation also becomes established between these 

 various developments and that of the internal organs. An increasing visceral com- 

 plexity is gradually attained ; and this carries with it the necessity for a further devel- 

 opment of nervous communications. The several internal organs with their varying 

 states are gradually brought into more perfect relation with the principal nerve-cen- 

 tres as well as with one another. 



" 11. These relations are brought about by important visceral nerves in Vermes 

 and arthropods — those of the ' stomato-gastric systems' — conveying their impres- 

 sions either direct to the posterior part of the brain or to its peduncles. They thus 

 constitute internal impressions which impinge upon the brain side by side with those 

 coming through external sense-organs. 



" 12. This visceral system of nerves in invertebrate animals has, when compared 

 with the rest of the nervous system, a greater proportional development than among 

 vei'tebrate animals. Its importance among the former is not dwarfed, in fact, by that 

 enormous development of the brain and spinal cord which gradually declares itself in 

 the latter. 



"13. Thus impressions emanating from the viscera and stimulating the organism 

 to movements of various kinds, whether in pursuit of food or of a mate, would seem 

 to have a proportionally greater importance as constituting part of the ordinary mental 

 life of invertebrate animals. The combination of such impressions with the sense- 

 guided movements by which they are followed, in complex groups, will be found to 

 afford a basis for the development of many of the instinctive acts which animals so 

 frequently display." 



