Metazoa Lumbricus. 245 



with the maintenance of individual life we have yet to treat 

 of the system which regulates the action of the various 

 tissues and organs of the body, and maintains communica- 

 tion between the external world and the internal economy. 

 In the worm the nervous system has arrived at a much higher 

 stage of differentiation than that attained in the Hydrozoa. 

 The nerve fibres are not irregularly diffused, but collected 

 in definite strands or nerves, and the cells, in which, pre- 

 sumably, the nervous changes originate, are grouped to- 

 gether into ganglia. In the last section we saw that probably 

 the first stage in the development of a definite nervous system 

 consisted in the gathering of the nerve fibres into two lateral 

 cords, with branches to the different organs, and the aggre- 

 gation of the nerve cells on either side of the body into a 

 mass at the anterior end of the cord of that side. This was 

 followed by the approximation of the two ganglia to form 

 a brain. Lastly the two cords also approached each other 

 on the ventral side and united to form a ventral chain with 

 lateral branches. When segmentation of the body ensued, 

 ganglia were formed on the cords in each segment to serve 

 as centres for local nerve impulses. Hence we find the 

 nervous system of Lumbricus shows a pair of closely united 

 ganglia above the oesophagus, and therefore named supra- 

 cesophageal, from which spring two circum-oesophageal 

 commissures, which surround the gullet and unite under- 

 neath it in two sub-cesophageal ganglia. From the sub- 

 cesophageal ganglia the double cord, with its pairs of 

 ganglionic swellings, passes backwards along the ventral 

 surface, piercing each successive septum, and giving off a 

 nerve to it on either side in its passage. The two united 

 ganglia, which occupy the centre of the ventral wall of each 

 somite, also give off nerves to the organs in the neighbour- 

 hood. 



Sense organs and sensation, The earthworm can 

 scarcely be said to possess any sense organs, but it possesses 

 a generalised sensitiveness, due to the plentiful distribution 



