ANNELID WORMS 241 



polychaetes, the oligochaetes are, generally speaking, 

 without gills, though these structures are developed in 

 a few fresh-water forms. There are no jaws, except in 

 a peculiar group which is parasitic on crayfishes and 

 has no close resemblance to the earthworms. 



6. Leeches (Hirudinea) are usually found in fresh Leeches 

 water, where they swim with an undulating motion. In 

 moist regions land leeches may be found, and there are 

 even marine species. The medicinal leech (Hirudo 

 medicinalis) , formerly used to draw blood, possesses 

 jaws. Other leeches have a proboscis, but are without 

 jaws. The Hirudinea resemble the earthworms in the 

 segmented body, and also in the egg cocoon, which may 

 be found attached to plants or rocks in ponds. With 

 one exception chaetse are absent, and this separates 

 them from all the oligochsetes except a few aberrant 

 types. There are some species with external gills. 

 Simple eyes are present. 



We may infer that of the three great groups of anne- Lines of 



... , . , ....... modification 



lids, the polychsetes are the most primitive, in spite ot i n annelids 

 the fact that they include many specialized forms. The 

 leeches and earthworms are related, but represent 

 widely divergent branches of a common stock, - - both, 

 however, adapted to fresh-water and terrestrial exist- 

 ence. Of these, no doubt the oldest are fresh-water 

 forms. The earthworms have lost the jaws, the leeches 

 the chaetae ; hence it is impossible to derive either group 

 from the other. 



References 



Cambridge Natural History, Vol. II. 



BEDDARD, F. E. "Earthworms and Their Allies." Cambridge Manuals of 



Science and Literature, 1912. 

 HALL, MAURICE C. Proceedings United States National Museum, Vol. 48, 



page 187. (Account of oligochaetes, parasitic on crayfishes.) See also 



