CHAPTER XI 



PHYLUM ANNELIDA 

 Chief Classes CH^ETOPODA, DISCOPHORA 



THE Annelids or Annulata are segmented "worms? in 

 most of which th$ segmentation of the body is visible exter- 

 nally. 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. Ch&topods, have setce 

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

 ccelom, communicating with the exterior by paired nephridia. 

 The nervous system consists typically of two dorsal cerebral 

 ganglia, a commissural ring round the gullet, and a ventral 

 ganglionated chain. The gonads arise on the cozlomic epi- 

 thelium. Not infrequently the nephridia function also as 

 genital ducts. The development may be direct or indirect, 

 and if indirect it usually includes a larval Trochosphere stage. 

 In habit, form, and structure the Annelids exhibit much 

 diversity. The Chsetopods, represented on the one hand 

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

 worms, best illustrate the typical Annelid structure. 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. 



