XXIV ORGANS OF SENSE 287 



Stimulus applied (a) to itself directly, (/») to the motor fibre 

 {£/. nv), (c) to the nerve-cells (Nv. C) in the central 

 nervous system, or to those {Nv. C) in the inter-muscular 

 plexus, {d) to the sensory fibre {af. nv.), or {e) to the 

 epidermic cells {Der. Epthm). 



In all probability the whole central nervous system of 

 Polygordius is capable of 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. 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 nervous system is thus an all-important means of 

 communication between the various parts of the organism 

 and between the organism and the external world. The 

 outer or sensory surface is by its means brought into 

 connection with the entire muscular system with such 

 perfection that the slightest touch applied to one end of the 

 body may be followed by the almost instantaneous contrac- 

 tion of muscles at the other. 



In some species of Polygordius the prostomium bears a 

 pair of eye-specks, but in the majority of species the adult 

 animal is eyeless, and, save for the ciliated pits (Fig. 66, 

 B, c.p), the function of which is not known, the only definite 

 organs of sense are the tentacles, which have a tactile 

 function, their abundant nerve-supply indicating that their 

 delicacy as organs of touch far surpasses that of the general 

 surface of the body. They are beset with short, fine pro- 

 cesses of the cuticle called set(z (Figs. 66 and 67, s), which 

 probably, like the whiskers of a cat, serve as conductors of 

 external stimuli to the sensitive epidermic cells. 



