SECTION X 



PHYLUM ANNULATA 



The phylum Annulata comprises four classes of Worms — the 

 Ghcetopoda or Earthworms and marine Annelids, the Archi- Annelida, 

 the Gephyrea, and the Hirudinea or Leeches. All of these, except 

 the Gephyrea, 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 ccelome, and there is in most a system of blood-vessels. 

 The nervous system consists of a cerebral ganglion, oesophageal 

 connectives, and a double ventral nerve-cord, which in all but the 

 Gephyrea is segmented into a series of ganglia. The organs of 

 excretion are in the form of metamerically arranged pairs of tubes, 

 the nephridia or S'^gntental organs, closed internally or leading from 

 the ccelome to the exterior ; and united with these, or distinct 

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

 passage outwards of the reproductive elements. 



CLASS I— CH^TOPODA. 



The Chsetopoda, comprising the Earthworms, Fresh- Water 

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

 like 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 oi" 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 selce {chmfce) or bristles 

 and usually also certain tactile appendages, the ciri-i. There 

 is an extensive coelome, incompletely divided into a series of 

 chambers corresponding to the segments by a series of muscular 

 partitions which act also as mesenteres, 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 



43U 



