NATURE OF THE INVERTEBRATE BRAIN. 37 



lies at the root of the intellectual and instinctive life (such as it is) 

 of invertebrate animals, some are much more important than others. 

 Two are notable for their greater proportional development, viz., 

 tactile organs and visual organs. The former are soon outstripped in 

 importance by the latter. The visual sense, indeed, and its related 

 nerve-ganglia, attain an altogether exceptional development in the 

 higher insects and mollusks. 



V. The sense of taste and that of smell are developed to a much 

 lower extent. It is even difficult to point to distinct organs or im- 

 pressible surfaces as certainly devoted to the reception of impressions 

 of this kind. 



8. The sense of hearing is also developed to a very slight extent. 

 No distinct sense-organ of this kind has been discovered, except in a 

 few insects and in members of the sub-kingdom Mollusca It is, 

 however, of no small interest to find that, where these organs do exist, 

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

 but are immediately connected with one of the principal locomotor 

 nerve-centres of the body. 



9. The associated ganglia representing the single or double brain 

 are, in animals possessing a head, the centres in which all impressions 

 from sense-organs, save those last mentioned (the auditory), are re- 

 flected on to appropriate groups of muscles. This "reflection occurs 

 either at once or after the stimulus has passed through other ganglia, 

 whence it is passed along nerves to those groups or combinations of 

 muscles whose simultaneous or successive contractions give rise to 

 the organism's reply to such impressions. It may be easily under- 

 stood, therefore, that in all such animals perfection of sense-organs, 

 size of brain, and power of executing varied muscular movements, 

 are intimately related to one another. 



10. But a fairly parallel correlation also becomes established be- 

 tween these various developments and that of the internal organs. 

 An increasing visceral complexity is gradually attained. Such in- 

 creased visceral complexity carries with it the necessity for a further 

 development of nervous communications. The several internal or- 

 gans have to be brought into more perfect relation with the sensori- 

 motor nervous system, and also with one another, for all joint actions 

 in which two or more of them may be concerned. 



11. In invertebrate animals the visceral system of nerves has, 

 when compared with the rest of the nervous system, a greater pro- 

 portional development than among vertebrate animals. Its importance 

 among the Invertebrata is not dwarfed by the enormous development 

 of the brain and spinal cord, which gradually declares itself among 

 the Vertebrata. 



12. Impressions emanating from the viscera and stimulating the 

 organism to movements of various kinds, whether in pursuit of food 

 or of a mate, would, therefore, have a proportionally greater impor- 



