ix PHYLUM ANNULATA 211 



in a small globular ovarian sac (ov. s), situated in the 

 eleventh segment. From each ovarian sac a short oviduct 

 passes inwards and backwards, and unites with its fellow in 

 a median duct which opens into a curved muscular tube, the 

 vagina (va), which opens in the middle line on the ventral 

 surface of the second annulus of the eleventh segment, i.e., 

 one segment behind the male aperture. 



The leeches are a comparatively uniform group ; but 

 some of the class differ from the medicinal leech in more 

 or less important points. Thus in one section there are no 

 jaws, and the anterior end of the body is capable of being 

 retracted within the part immediately behind it or thrust 

 forward as a proboscis or introvert. In the great majority 

 respiration takes place through the skin, as in the medicinal 

 leech ; but in one genus, Branchellion, which is an external 

 parasite on certain fishes, gills are present in the form of 

 delicate lateral outgrowths of the segments. 



The majority of the Hirudinea are inhabitants of fresh 

 water, and live, like the medicinal leech, by sucking the 

 blood of higher animals. Others are permanent external 

 parasites ; others again are carnivorous, feeding on snails 

 and other Mollusca. 



