CHAPTER XL 



PHYLUM ANNELIDA. 

 Chief Classes CH^ETOPODA, DISCOPHORA. 



THE Annelids or Annulata include segmented "worms" in 

 most of iv hi ch the segmentation of the body is visible externally. 

 The head usually consists of a pre-oral "prostomium " and a 

 post-oral peristomium. The body wall has several layers 

 of muscles, and many, e.g. Chcetopods, have setce embedded 

 in the skin. In most, there is a well-developed cculom, com- 

 municating with the exterior by paired nephridia. The 

 nervous system consists typically of dorsal cerebral ganglia, 

 a commissural ring round the gullet, and a ventral gang- 

 lionated chain. The gonads arise on the ccelomic epithelium. 

 Not infrequently the nephridia function also as genital ducts. 

 The development may be direct or indirect, and the?i includes 

 a larval Trochosphere stage. 



In habit, form, and structure, the Annelids exhibit much 

 diversity. The Chaetopods, represented on the one hand 

 by the familiar earthworm, and on the other by the marine 

 worms, best exhibit the structure upon which the Annelid 

 type is founded. With these, however, may be included the 

 aberrant Echiuridae, e.g. Echiurus and Bonellia. A few 

 primitive forms (Archi-Annelida), and the Myzostomata 

 (parasitic on Crinoids), may also be appended to the 

 Chaetopod class. The leeches (Discophora) are probably 

 Annelids which have diverged in consequence of a peculiar 

 half -parasitic habit. Finally, some zoologists include 

 Sagitta (Chsetognatha) in this series as an Annelid with 

 three segments, and also the Rotifers (Rotatoria), whose 

 adult form somewhat resembles the Trochosphere larvae of 

 many Annelids. 



