CHAPTER IX 



PHYLUM COELHELMINTHES (ANNELIDA) 



The Coelhelminthes or Annelida comprise a number of worm- 

 like forms which possess a coelom and are usually segmented. 

 This group contains many of the forms included under the 

 "Vermes" of Linnaeus and other early workers. It has been 

 said frequently that the Vermes constituted a wastebasket 

 into which forms which could not be placed elsewhere were 

 dumped. ]\Iuch of the advance in modern classification of the 

 animal kingdom has centered around the recognition of groups 

 of wormlike forms bearing common characteristics and establish- 

 ing for them rank as independent phyla and classes. Thus the 

 Plathelminthes,* Nemathelminthes, Molluscoidea, and Trochel- 

 minthes have been removed from the old group Vermes and 

 each has been elevated to the rank of an independent phylum. 

 There yet remains an assemblage of segmented worms, arrow- 

 worms, and gephyrean worms which seem to have enough features 

 in common to warrant their retention for the present in a single 

 major group of the animal kingdom. These forms, which 

 collectively are known as the Coelhelminthes, agree in possession 

 of a coelom and in having the nervous system of a uniform type. 

 An excretory system is lacking in members of the class Chaetog- 

 nathi and in some representatives of the other groups, but 

 characteristically a metanephridial system is found. The 

 Gephyrea, which lack segmentation, agree with the typical 

 annelids in passing through a larval stage known as trochophore. 



Class Chaetognathi 



The chaetognaths, or arrow-worms, are small forms with body 

 shape admirably adapted for life at the surface of the ocean 

 where they move about with great rapidity in search of food. 

 The body, which is almost as transparent as crystal, is usually 

 about 15 mm. long though one species reaches a length of 70 mm. 

 It is divided into three segments: head, trunk, and tail. Each 

 of these divisions is separated from adjoining segments by a 



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