74 Instinct and Intelligence 



of the animals we have referred to are propor- 

 tionate to the completeness with which the 

 protoplasmic elements of their nervous system 

 has responded to the divers modes of energy 

 acting on it, that is to the environment. 



Passing from the Annelida to the higher class 

 of the Arthropoda, which includes the Crustacea 

 and Insecta, we find the instinctive movements 

 of these orders of animals to be in harmony 

 with the ideas we have endeavoured to inculcate 

 in the preceding pages, which in our opinion 

 are competent to explain the phenomena in 

 question. 



We may refer to the crayfish as representing 

 the Crustacea. These animals abound in many 

 of our streams and harbours; they are rarely 

 more than three or four inches long, and in 

 appearance are not unlike small lobsters; 

 they may often be seen walking along the 

 bottom of shallow streams by means of four 

 pairs of jointed legs. 1 At other times a cray- 

 fish appears at the mouth of a burrow he has 

 excavated in the banks of a stream; he lies at 

 the entrance to his hole, barring it with his 



1 The Crayfish, by T. H. Huxley, pp. 6, 9. 



