CH. II] THE CRAYFISH 59 



fish 1 there is no crossing of nerve fibres from right to left 

 sides of the body and vice versa, and thus that the right 

 portion of the central nervous system controls the right 

 half of the body and similarly the left, mutatis mutandis. 

 The brain (supra-oesophageal ganglia) is the seat of all 

 voluntary as distinguished from reflex actions ; it also is 

 responsible for the maintenance of equilibrium and for the 

 use of the abdomen in swimming, and moreover controls 

 by inhibition the purposeless activity of the lower nerve 

 centres. The suboesophageal ganglion mass coordinates 

 the feeding and locomotor movements and sets up peculiar 

 rhythmic swinging of the limbs if the brain is removed. 

 In the absence of these first two ganglion masses all co- 

 ordination is lost and the limbs move in an aimless way, 

 even to the extent of interfering with each other as if not 

 members of the same individual ; for example, the chelae 

 will rob each other of food and generally play at cross 

 purposes. 



The vision of Crustaceans and of all Arthropods has 

 given rise to much discussion and it is not our intention 

 to go into this vexed question. But it may be pointed 

 out that even though the compound eye may produce on 

 the retinal cells a mosaic of numerous small pictures fitted 

 one to another, it by no means follows that the impression 

 received by the animal in any way corresponds with this 

 mosaic. We ourselves, in common with all Vertebrates, 

 receive through the optical media of our eye an inverted 

 picture upon the retina, but the sensation produced is that 

 of an erect and not inverted object. 



1 Ward, Proc. Roy. Soc. xxvm. p. 379, and Journ. Physiol. n. p. 214. 



