i] STRUCTURAL AND SYSTEMATIC 35 



found in all of the great continents and of which 

 seven or eight species are known. To a more doubt- 

 ful genus Pleurophlejis are assigned a few small 

 worms which have the general appearance of Aeolo- 

 soma, but are without the coloured or colourless oil 

 drops in the skin. These worms have a very large 

 prostomium which is ciliated on the lower surface, 

 and the body is not markedly segmented externally 

 by constrictions or internally by septa. The bristles 

 are slender and hair-like, but among them are in 

 some species the shorter stouter bristles bifid at the 

 free tip, which are so general among the aquatic 

 families of the Oligochaeta. These worms are not 

 uncommon objects in pools containing weed; and 

 they are to be found usually crawling among the 

 weed. They consist as a rule of but few segments 

 to most of which a pair of nephridia belong. The 

 ovaries and the spermaries are only known in a few 

 forms and appear to be unpaired and lie respectively 

 in the fifth and sixth segments. There are 1-3 pairs 

 of spermathecae, and the sperm ducts if distinct from, 

 are at least very like, nephridia. The ova appear to 

 make their way to the exterior by a large aperture 

 in the ventral middle line of a middle segment of 

 the body. The vascular system contains uncoloured 

 blood and is greatly simplified. 



The next family to the Aeolosomatidae in zoo- 

 logical position is clearly the Naididae. These are 



32 



