TTTBIFEX. 



347 



cells similar to those described above, but distally the 

 peritoneum is again composed of flattened cells. The 

 distal end of this tube expands rather suddenly to form 

 a small, contractile vesicle, which opens to the exterior 

 at the nephridiopore. The tube is ciliated to its distal 

 end, and a tuft of specially long cilia projects into the 

 vesicle. The latter is somewhat pear-shaped when fully 

 expanded, narrowing considerably as it approaches the 

 nephridiopore, which is situated on the ventral surface 

 of the body, a short distance in front of the ventral setae 

 on either side. 



REPRODUCTIVE ORGANS. 



Tubifex rivulorum is hermaphrodite, the ova and 

 spermatozoa maturing at the same time. The worms 

 exhibit fully-developed reproductive organs during the 

 autumn and early winter, that is, from October to 

 December. The first cocoons are laid about the beginning 

 of November, and can be found in large numbers in the 

 mud for the next two months, after which time their 

 formation ceases. The mature worms show well marked 

 differences in external appearance from those in which 

 the reproductive organs are not well developed, especially 

 in the anterior segments. ' The sexual organs occupy 

 segments 9-16 or 17 in quite mature worms, and 

 these segments are considerably swollen and of a dull 

 whitish colour, due mainly to the large size of the sperm 

 sac and to the large ova which are present. During May, 

 June and July it is comparatively common to find worms 

 which present somewhat this appearance, and it would be 

 easy to make an error and suppose that such worms were 

 mature if they were not more minutely examined. 

 Further investigation shows that this whiteness and 



