CHAP. VII.] THE BRAIN OF INVERTEBRATES. 109 



discovered, principally in Mollusks 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 fre- 

 quently not in direct relation with the Brain, but imme- 

 diately 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 (p. 218). 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 accord- 

 ance with such a notion. 



9. Thus the associated ganglia representing the double 

 Brain are, in animals possessing 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 under- 

 stood, 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 estab- 

 lished between these various developments and that of the 

 Internal Organs. An increasing visceral complexity is 

 gradually attained ; and this carries with it the necessity 

 for a further development of nervous communications. 

 The several internal organs with their varying states are 

 gradually brought into more perfect relation with the 

 principal nerve centres 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 System ' conveying their impressions 



