CHAPTER XI 



PHYLUM ANNELIDA 



Chief Classes — Ch^topoda, Hirudinea or Discophora 



The Annelids or Annulata are segmented worms, in most 

 of which 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 

 caelom, 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 coelomic 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 AnneUds exhibit much 

 diversity. The Chaetopods, represented by the famihar 

 earthworms and by the marine worms, are most typical. 

 With these 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. The leeches (Discophora) are 

 divergent. Further, some zoologists include Chaetognatha 

 in this series as Annelids with three segments, and also 

 the Rotifers (Rotatoria), whose adult form somewhat 

 resembles the Trochosphere larvae of many Annelids. 

 Finally there are associated in an uncertain way Sipunculids, 



Gephyreans, Phoronids, Polyzoa, and Brachiopods. 



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