92 THE NERVOUS SYSTEM OF VERMES. 



than in the Nemerteans ; and this is perhaps due in great 

 part to the existence in it of the numerous segmental 

 ganglia, — structures which are absent in the above-men- 

 tioned marine worms. The movements of the Nemer- 

 teans, Hke those of the Nematoids, are probably much 

 more exclusively under the control of the oesophageal 

 ganglia than are those of the segmented Earthworm — in 

 which each of the body ganglia, doubtless, has much to do 

 with bringing about the contraction of its contiguous 

 muscles in the same segment. 



The Earthworm has a more complex visceral structure 

 than is to be met with among the Nemerteans ; and it 

 presents distinct evidences of a nervous interconnection 

 between its internal organs and some of the principal 

 nerve-centres. Lockhart Clarke has described a complicated 

 gauglionic network on each side of the oesophagus, start- 

 ing from the lateral commissures and sending prolonga- 

 tions to the intestine and other parts. By means of this 

 principal visceral system of nerves, the internal organs are 

 brought into relation with one another, and with the 

 nervous system of animal life — that is, with those parts 

 of it having to do more especially with the relation of the 

 organism to its medium. 



