SECTION X 

 PHYLUM ANNULATA 



THE phylum Annulata comprises six classes of Worms the 

 Chcetopoda or Earthworms and marine Annelids, with the Myzo- 

 stomida and Echiurida, the Archi- Annelida, the Sipunculoidea, and 

 the Hirudinea or Leeches. These in general have the elongated 

 body divided externally into a number of rings, which represent a 

 division of the internal parts into a series of segments or metameres. 

 There is usually an extensive coelome, and there is in most a system 

 of blood-vessels. The nervous system consists in most cases of a 

 cerebral ganglion, oasophageal connectives, and a double ventral 

 nerve-cord, which is segmented into a series of ganglia. The organs 

 of excretion are in the form of metarnerically arranged pairs of 

 tubes, the nephridia or segmental organs, closed internally or leading 

 from the co3lome to the exterior ; and united with these, or distinct 

 from them, are a series of paired ducts, the ccelomoducts, for the 

 passage outwards of the reproductive elements. 



CLASS I.-CHJETOPODA. 



The Chsetopoda, comprising the Earthworms, Fresh- Water 

 Worms, and Marine Annelids, are Worms the body of which, 

 unlike that of a Flat-worm or a Round-worm, is made up of a series 

 of more or less completely similar segments or metameres, each 

 containing a chamber or compartment of the body-cavity and a 

 section of the alimentary canal and other organs. At the sides 

 of each are typically a pair of muscular processes, the parapodia, 

 which do duty as limbs, bearing bundles of setce (chcetcB) or bristles, 

 and usually also certain tactile appendages, the cirri. There 

 is an extensive ccelome, incompletely divided into a series of 

 chambers corresponding to the segments by a series of muscular 

 partitions which act also as mesenteries, being attached internally 

 to the alimentary canal. The latter extends throughout the 

 length of the body ; the intestine is usually constricted, the con 

 strictions being either segmental, i.e. opposite the middle of the 

 segments, or inter-segmental, i.e. opposite the intervals between 

 the segments. There is a well-developed blood-vascular system 



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