CHAPTER XVIII 



PHYLU:\I III. CCELHELISIIXTHES 



The Smooth and Segmented Roundworms 



The Ccelhelniinthes are distinguished from all of the worms thus far 

 considered by the presence of a coelom, or bodj' cavity located between 

 the outer body wall and the intestine. With the exception of the thorn- 

 headed worms, the digestive tract is complete, and there may or may 

 not be a closed blood circulation. Excretory vessels connect the cavity 

 of the bodj^ with the outside. The body muscles are ''epithelial muscle 

 cells" developed from the outer epithelial wall of the coelom. Sub- 

 groups exhibit distinct differences in the character of the coelom. In 

 the Annelida, to Avhich the earthworms belong, it is segmented, the 

 segments (somites) corresponding to the annulations or ringing of the 

 body wall. In the Nemathehninthes, which includes most of the par- 

 asitic species, there is no segmentation of the body cavity or annulation 

 of the body wall. In the Hirudinea, the annulated group which con- 

 tains the leeches, the coelom is but shghtly developed, and usually the 

 annulations outnumber the somites. 



The phylum Coelhelminthes has the two classes named below for 

 discussion in this work, the first containing all of the endoparasitic 

 worms which remain to be considered, while of the second, only the 

 leeches are of direct parasitic interest. 



Class I. Nemathehninthes. — Body without external or internal seg- 

 mentation. 



Class II. Annelida. — Body with" external and internal segmentation. 



Class I. Nemathelminthes 



Coelhelminthes (p. 216). — This group contains the roundworms, or 

 so-called threadworms, though not all are filiform. There are both 

 free and parasitic forms, examples of the former living under stones 

 and in other moist places. The parasitic species are by far the more 

 numerous and important. 



The bod}^ is elongated, and, in being cylindrical, differs from that of 

 the Platyhelminthes which is flat, while the absence of annulations and 

 segmentation distinguishes it from that of the Annelida. 



The class includes two parasitic orders. The first contains the typical 



