CHAPTER THIRTY-THREE 



ANNELID WORMS 

 I. THE higher worms are distinguished by the seg- structure of 



r , i T / , . . i the annelid 



mentation of the body (into annuli or rings) and, except worms 

 in leeches, by the presence of bristles which can be used 

 in locomotion. Thus, an earthworm appears perfectly 

 smooth, but pass the ringers along the sides, and it feels 

 rough. Examination with a lens reveals little project- 

 ing points, which give the worm a hold on the walls of 

 its burrow, recalling the spiked shoes of the telephone 

 company's "trouble man." These bristles or spinelike 

 structures are called "chaetse," and hence the great 

 group so common in the sea, distinguished by the 

 abundance and length of the chaetse, is called Poly- 

 chceta (many bristles). In contrast with them, the 

 earthworms and their relatives are called Oligochceta 

 (few bristles). While these are the two main groups of 

 annelids, we must associate with them a third important 

 group, the Hirudinea or leeches. These may be recog- 

 nized by the flattened under side and the presence of an 

 adhesive disk or sucker, at each end of the body. There 

 is also a small group called Archiannelida, the members 

 of which have rings of cilia around the body, but no 

 bristles, and when adult are not visibly segmented. As 

 in so many other cases, the place of these animals in the 

 classification is determined by the totality of their 

 characters, and would not be suspected on superficial 

 examination. 



2. The ringed worms are not only interesting in R es em- 

 themselves, but also on account of their apparent 

 affinity with the Arthropoda, the great group which in- 

 cludes the insects and Crustacea. First of all we have 

 the segmentation of the body, so characteristic of 



23? 



