CHAPTER XI. 



ANNELIDA. 



Segmented worms in which the perivisceral cavity is a part of the 

 coelom. They almost all possess chitinous setae embedded in and 

 secreted by pits of the skin. 



The Annelida include the segmented worms. They differ from 

 the Arthropoda, which in many respects they resemble, by the 

 possession of a perivisceral division of the coelom. In other words 

 the body-cavity is a part of the coelom. In this respect and in the 

 arrangement of the central nervous system, and in the wide-spread 

 occurrence of the trochosphere larva, they approach the Mollusca. 

 They differ, however, from the Mollusca in the fact that the body is 

 segmented. This segmentation, which is exhibited by a considerable 

 number of organs, proceeds from and is based upon the mesoblastic 

 somites in the embryo. The Annelida possess a dermo-muscular 

 body-wall; that is to say, muscular tissue enters largely into the 

 composition of the integument. In consequence of this fact the 

 body-wall is always extremely contractile, and the body shrinks 

 considerably when the animals are placed in spirit. 



They all possess chitinous spines the setae, which are secreted 

 by the ectoderm and are embedded in pits of the skin. These 

 setae are very conspicuous in the class Chaetopoda, less so in the 

 classes Hirudinea, Echiuroidea, and Archiannelida. 



The alimentary canal is tubular, and generally straight : it opens 

 by a mouth which is placed on the ventral surface of the front end, 

 and by an anus which is terminal, or subterminal and dorsal. 



The head consists of the anterior -part of the body, on the ventral 

 side of which the mouth is placed. It is divided, very often by a 

 mark, into a preoral portion the prestomium and a postoral portion, 

 which is called the peristomium. The prestomium is sometimes 

 called the first segment, but in the enumeration of segments the 

 peristomium is counted as the first (Fig. 358). 



