jg, PHYLUM ANNELIDA 



well-formed appendages except in connection with the mouth (the 

 parapodia do not qualify as such) and the absence of a continuous 

 thick chitinous exoskeleton. Typical members of the phylum 

 possess chcTtiE. Some of the above characters are discussed more 

 fully in the next chapter. 



CLASS I—CHMTOPODA 



Ch«Tt£e are present and the coelom is well developed. There are 

 two Orders. 



ORDER I — POLYCHyETA 



The chaetae are many and parapodia are characteristic ; the 

 head is comparatively well developed, with cirri, tentacles, and 

 jaw apparatus ; gills are frequent ; the sexes are usually separate, 

 fertilisation is external, and the larva is a trochosphere. 



The polychaetes are marine, and are divided ecologically into 

 the errant forms, which are typically free swimming, though they 

 sometimes live in temporary burrows, and the sedentary forms, 

 which are subdivided into those which are tubicolous, that is, 

 live in formed tubes which they seldom or never leave, and those 

 which burrow in soft sand. The body, especially the head region, 

 is generally modified to suit the mode of life. Nereis is a typical 

 errant genus, Serpida lives in little tubes of calcium carbonate 

 attached to rocks, and Arenicola, the lug worm, is a burrowing 

 form living between tide-marks. 



ORDER II — OLIGOCH^TA 



The characters are in general the reverse of those of the poly- 

 chsetes. A clitellum is present. They are terrestrial or freshwater 

 in habitat, Lumhricus being an example of the first, and Nais, 

 common in ponds, of the second. 



CLASS II—HIRUDINEA 



These are the leeches. They have 32 segments and two ventral 

 suckers. Chatae are absent except in some primitive forms ; 

 the ccelom is reduced ; they are hermaphrodite, with a chtellum 

 and a rudimentary cocoon and direct development. Hirudo is 



