SECTION X 



PHYLUM ANNULATA 



(The phylum Annulata comprises four classes of Worms — the 

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

 the Gephyrea, and the Hirudinca 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 segmental 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 cazlomoducts, for the 

 passage outwards of the reproductive elements. ) 



CLASS I.— dLETOPODA. 



The Chaetopoda, 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 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 setm (chcetce) 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 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 



