52 Laboratory Guide in Zoology 



THE CLASSIFICATION OF THE ANNULATA 



The earthworm is a typical representative of the phylum 

 Annulata, nearly all the members of which have an elon- 

 gate body divided into numerous segments, with an internal 

 partition between each of the segments. There is a well- 

 developed alimentary canal, a distinct body cavity, a system 

 of blood-vessels, and a nervous system with cerebral 

 ganglia and ventral nerve cord which is double, and gener- 

 ally shows a series of ganglia along the segments. The 

 members of the phylum also have nephridia, or pairs of 

 tubes leading from the body cavity through the body wall 

 and serving as organs of excretion. 



This phylum is divided into four classes, namely : — 



Class I. Chaetopoda. The Earthworm and Nereis. 



Class II. Gephyrea. 



Class III. Archi-annelida. Polygordius and its Allies. 



Class IV. Hirudinea. The Leeches. 



Of these four classes the first and fourth contain the most 

 generally known members of the branch. The second 

 class, Gephyrea, is exceptional in its characters, in that the 

 members of it are not segmented when adult, although they 

 show the segmented condition during their development. 



