14 



CHAPTER 



SEGMENTED WORMS 



The phylum Annelida is the second major group of animals possessing a true 

 coelom formed in the schizocoelous manner (p. 368). The annelids are 

 elongate worms whose most conspicuous characteristic is the relatively un- 

 specialized segmentation found in most species. The name of the phylum is 

 derived from the Latin annulus ("little ring") and refers to the ringed 

 appearance which the serially repeated segments give to the body. In all 

 annelids there is some differentiation at the two ends, but the main part of 

 the body is composed of segments which are usually of similar structure 

 internally as well as externally. Other annelid characteristics include the 

 extensive coelom, separated into segmental compartments by transverse parti- 

 tions, or septa; paired segmental excretory organs, the nephridia; and the 

 ventral nerve cord composed of paired ganglia at segmental intervals, con- 

 nected anteriorly with the dorsal "brain" by a pair of circumpharyngeal 

 connectives. 



The annelids, with various other animal forms, were long classified in a 

 phylum called the Vermes. Now, however, in recognition of their specific 

 distinguishing characteristics, the annelids are considered as a separate 

 major phylum. The phylum Annelida comprises four classes: the class 

 Archiannelida, a miscellaneous collection of primitive or secondarily 

 simplified marine annelids; the class Polychaeta, chiefly marine annelids 

 with many bristles, or setae,- the class Oligochaeta, which includes the 

 earthworms and many small annelids of fresh water, with bristles few in 

 number; and the class Hirudinea, the leeches. 



Annelids are typically animals that burrow or crawl upon the bottom in the 

 ocean or in fresh water. Some are free-swimming, at least during the 

 breeding season, and others are adapted to life in moist soil. Many 

 marine species build permanent tubes which they inhabit; in other species 

 the tube is temporary, like a burrow, and may be abandoned for another. 

 Some annelids are actively predaceous, seizing their prey in well- 

 developed jaws; others, notably the permanently tube-dwelling polychaetes, 

 feed upon micro.scopic particulate matter brought in by ciliated tentacles or 

 entrapped in sheets of mucus. 



The fossil record of evolution among annelids is meager indeed, being 

 limited to relatively few complete animals but featuring numerous hard parts, 

 such as chitinous jaws and calcareous tubes, as well as innumerable "worm 



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