262 



THE INVERTEBRATA 



body in response to excitation of the higher centres, the supra- and 

 subpharyngeal ganglia. The rapid contraction of the whole of the 

 longitudinal musculature in response to a noxious stimulus is an 

 example of this kind of reaction. On p. 200 it was shown that the 

 primary function of the primitive central nervous system is that of a 

 sensory relay. In the annelids there is added the second great func- 

 tion, that of inhibition. A Nereis, which has had the suprapharyngeal 



cm. 



Fig. 183. A, Longitudinal and B, transverse sections of Lumbricus to show 

 the musculature and its innervation. In A, segment 2 shows neurones con- 

 stituting an intrasegmental reflex arc, segments 3 and 4 show those which 

 make up an intersegmental arc. B shows the distribution in the body wall 

 of two segmental nerves and their branches, al.c. alimentary canal; cm. 

 circular muscles ; g.f. giant fibre ; l.m. longitudinal muscles. 



ganglia removed, moves about ceaselessly, showing that a function 

 of the ganglia in the normal animal is the inhibition of movement. 

 If the supra- and subpharyngeal ganglia are both removed then the 

 animal is permanently quiescent, a condition like that of a polyclad 

 turbellarian when the cerebral ganglia are removed. 



The coelom is bounded by an epithelial layer, ihQ peritoneum^ which 

 gives rise to i\\& gonads ^ which in polychaets are usually developed in 

 most of the segments, to t)\& yellow cells ^ which play a part in the work 

 of nitrogenous excretion, and to the coelomoducts by which the eggs 



