CHAPTER XIX 



EARTHWORM, OF 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, nonjointed, 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 n. Archiannelida 

 Class III. Hirudinea 

 Class IV. Gephyrea 



Order 1. Echiuroidea 



Order 2. Sipunculoidea 



Class Chaetopoda (ke top' 6 da, hair and foot). — This class in- 

 cludes the most commonly known forms of the phylum. There are 

 marine, fresh-water, and terrestrial forms ; and they all possess setae 



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