76 Instinct and Intelligence 



forming an apparatus which, as Huxley ob- 

 serves, sorts out the rays of light into as many 

 very small pencils as there are separate end- 

 ings of the fibres of the optic nerve, and in 

 part serves as the medium by which luminous 

 waves are converted into molecular nerve 

 changes. It is evident that the sensory organs 

 of crayfish are much more highly organised than 

 those of worms, or of any of the lower classes 

 of animals. Accompanying this increased com- 

 plexity of structural arrangement in these re- 

 ceptors of energy is an increased development 

 of the cerebral nervous centres, or areas from 

 which the nerve fibres supplying these organs 

 originate. Fig. n is a drawing made from a 

 section of a crayfish's brain, showing its 

 anatomical division into anterior, middle, and 

 posterior sections or lobes. 



The brain gives off fibres which form the 

 optic and antennary nerves, as well as the 

 nerves which supply the viscera and certain 

 muscular structures. The sensory organs are 

 in close connection with the nervous matter of 

 the animal's mid-brain, which seems to exercise 

 a controlling influence over the rest of the 



