CHAPTER XV 



PHYLUM ANNELIDA 



(By J. Teague Self, University of Oklahoma) 



The Phylum Annelida (a nel'i da, form of a little ring) comprises 

 an extremely large group of worms characterized by (1) the pres- 

 ence of a coelom surrounded by two layers of muscle, (2) metameres 

 or segments, (3) a ventrally located segmental nervous system, (4) 

 segmented, non jointed, chitinous appendages in most cases, (5) an 

 excretory system composed of nephridia, and (6) a nonchitinous 

 cuticle covering the body. These worms are found in almost every 

 type of free-living habitat where moisture is present. There are 

 many forms which live in the ocean, either swimming freely, bur- 

 rowing in the sand, or living in especially prepared tubes. Fresh- 

 water streams and ponds are inhabited by numerous forms of an- 

 nelids, and moist soil is usually alive with terrestrial earthworms. 

 From this it is evident that the phylum as a group has become 

 adapted to many varied living conditions and comprises one of the 

 large groups of the animal kingdom from the standpoint of num- 

 bers. In the process of adaptation the annelids have become diver- 

 sified in their anatomical features until only a very few characters, 

 such as those mentioned in the beginning of this chapter, are com- 

 mon to the entire phylum. Even then, these distinguishing features 

 are sometimes modified until only an expert can recognize them. 



The Phylum Annelida may be divided into four classes: 



Class I. Chaetopoda 



Order 1. Polychaeta 



Order 2. Oligochaeta 

 Class II. Archiannelida 

 Class III. Hirudinea 

 Class IV. Gephyrea 



Order 1. Echiuroidea 



Order 2. Sipunculoidea 



Class Chaetopoda (ke top'O da, hair and foot). — This class includes 

 the most commonly known forms of the phylum. There are marine, 



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