V NERVOUS SYSTE^r 335 



body ; while in the frog it is enclosed in a neural canal 

 and is dorsal 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 surrounded by nerve-cells 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 auto- 

 matic action. It is a well-known fact that if the body of 

 an earthworm is cut into several pieces each performs in- 

 dependent 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, be little doubt that 

 complete co-ordination, i.e., the regulation 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 dis- 

 tinguish 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 connection 

 with the structure of the earthworm to which special at- 

 tention 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 js clear that differentiation of 



