CHAPTER XI. 



SEGMENTED WORTylS OR ANNELIDA. 

 Chief classes CH^ETOPODA, DISCOPHORA. 



THE Annelida do not form a well defined phylum, but in- 

 clude segmented worms, in which the segmentation of the 

 body is usually visible externally. There is usually a well 

 developed body cavity, which communicates with the exterior 

 by paired nephridia or segmental organs. The nervous 

 system consists typically of dorsal cerebral ganglia, a com- 

 missural ring round the gullet, and a ventral ganglionated 

 chain. Not infrequently the nephridia function also as genital 

 ducts. The development is either indirect, when it includes a 

 larval Trochosphere stage, or direct. 



In habit, form, and structure the Annelids exhibit much 

 diversity of type. 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. It seems, however, that with 

 these we may also include the aberrant Echiuridae e.g., 

 Echiurus and Bonellia. A few forms of primitive type 

 (the Archi-Annelida), and the Myzostomata, which are 

 degenerate parasites found on Crinoids, may also be 

 appended to the class Chaetopoda. The divergent 

 leeches (Discophora) are probably Annelids which have be- 

 come modified in consequence of a peculiar habit. Finally, 

 some zoologists provisionally include Sagitta (Chaetognatha) 

 in this series as an Annelid with three segments, and also 

 the Rotifers (Rotatoria), since their adult form somewhat 

 resembles the Trochosphere larvae of many Annelids. 



According to Lang, the Chaetopods are derived from a 



