Passage 5    The phylum Annelida, named for the Latin word anellus, meaning “ring”, includes earthworms, leeches, and other similar organisms. In their typical form, these animals exhibit bilateral symmetry, a cylindrical cross section, and an elongate body divided externally into segments (metameres) by a series of rings (annuli). They are segmented internally as well, with most of the internal organs repeated in series in each segment. This organization is termed metamerism. Metameric segmentation is the distinguishing feature of this phylum, and provides it with a degree of evolutionary... Show more

Passage 5   

The phylum Annelida, named for the Latin word anellus, meaning “ring”, includes earthworms, leeches, and other similar organisms. In their typical form, these animals exhibit bilateral symmetry, a cylindrical cross section, and an elongate body divided externally into segments (metameres) by a series of rings (annuli). They are segmented internally as well, with most of the internal organs repeated in series in each segment. This organization is termed metamerism. Metameric segmentation is the distinguishing feature of this phylum, and provides it with a degree of evolutionary plasticity in that certain segments can be modified and specialized to perform specific functions. For example, in some species certain of the locomotor parapodia, or feet, may be modified for grasping, and some portions of the gut may evolve digestive specializations.    
The gut is a straight, muscular tube that functions independently of the muscular activity in the body wall. The Annelida resemble the nematodes, another worm phylum, in possessing a fluid-filled internal cavity separating the gut from the body wall. In both phyla, this cavity is involved in locomotion. However, in the annelids, this space is formed at a much later time during the development of the embryo, and presumably evolved much later as well. This fluid-filled internal space is called a true coelum.    
The annelid excretory and circulatory systems are well developed, and some members of the phylum have evolved respiratory organs. The nervous system offers a particular example of metameric specialization. It is concentrated anteriorly into enlarged cerebral ganglia connected to a ventral nerve cord that extends posteriorly and is organized into repeating segmental ganglia.    
This phylum includes members bearing adaptations required for aquatic (marine or freshwater) or terrestrial habitats. They may be free-living entities or exist as parasites. Among the best known are the earthworm Lumbricus, the water leech Hirudo, and the marine worm Nereis.    

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