344 THE EARTHWORM CHAP. 



in position. The brain of the frog is a complicated 

 structure, and the whole nervous system is hollow, there 

 being ventricles in the brain and a central canal in the 

 spinal cord ; while in the earthworm the brain consists 

 merely of a pair of cerebral ganglia, and it and the 

 ventral cord are solid. 



The whole nervous system is capable of originating 

 automatic action. It is a well-known fact that if the 

 body of an earthworm is cut into several pieces each 

 performs independent movements ; in other words, the 

 whole body is not, as in the higher animals, paralysed 

 by removal of the brain (p. 172). There can, however, 

 belittle doubt that complete co-ordination, i.e., the regu- 

 lation of the various movements to a common end, is 

 lost when the brain is removed. 



The earthworm is devoid of organs of sight or 

 hearing. It exhibits sensitiveness to bright light, 

 which may be due to direct action on the central parts 

 of the nervous system. The sense of hearing appears 

 to be absent ; but a faculty analogous to taste or smell 

 enabling the animal to distinguish between different 

 kinds of food is well developed. Groups of narrow 

 sensory cells in the epiderm, which are most abundant 

 on the prostomium and peristomium, have probably to 

 do with this faculty. 



There are two matters of general importance in con- 

 nection with the structure of the earthworm to which 

 special attention must be drawn. 



Notice in the first place how in this type, far more 

 than in Hydra, we have, as in the frog, certain definite 

 parts of the body set apart as organs (p. 30) for the 

 performance of particular functions : it is clear that 

 differentiation of structure and division of physiological 

 labour play a far more obvious and important part than 



