CHAPTER 24 



THE PHYLUM ANNELIDA AND OTHER 

 WORMLIKE COELOMATES 



The last and most advanced forms of the major worm phyla are 

 the members of the phylum Annelida. These abundant forms can be 

 found in nearly all habitats. Nearly any spadeful of garden soil has its 

 quota of earthworms, the best known representatives of this group. 

 Smaller microscopic relatives of the earthworm live in the mud at the 

 bottom of ponds and streams. In the ocean, other representatives of 

 this phylum occur from the intertidal zone to the greatest depths. These 

 latter vary in form from simple wormlike shapes to those with elaborate 

 respiratory gills which resemble handsome flowers. Another class of 

 this phylum contains the leeches which are provided with suckers. 

 Some of these are able to be temporary parasites on various vertebrates. 



The annelids show a unique combination of characters some of 

 which have undoubtedly been inherited from their more primitive 

 wormlike progenitors, others which represent a new development fore- 

 shadowing still more highly specialized invertebrates. One of these 

 new characteristics is the development of segmentation. Externally 

 this is visible as the ringlike annulations, the source of the name 

 Annelida. Thus the earthworm, which is often regarded with disdain, 

 becomes an individual of some symbolic importance in the development 

 of the animal kingdom. 



The Characteristics of the Phylum.— Like the other worms, 

 the annelids have bilateral symmetry, three germ layers, cephalization, 

 and an organ grade of construction. As unique features, these worms 

 have a true coelom, segmentation, and paired appendages. 



A true coelom, that is, one lined with mesoderm, appears to be 

 characteristic of all the more highly developed animals. Thus its first 

 appearance in this phylum marks the beginning of a body plan which 

 is retained in such highly successful groups as the arthropods and 



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