TWO AQUATIC OLIGOCHiETES. 19 



or may be provided with it only on the outer surface. The 

 nuclei are roundish and contain more than one nucleolus. 



. One or two nephridial ampullae are sometimes present in the 

 main mass, but they are inconstant in position. An end-vesicle 

 may also be present ; but they are in my opinion only tem- 

 porary swellings of the canal. 



Hatai says that, in L. gotoi the nephridia are present in 

 segments VII-IX and XII to the last but one. But according to 

 my own observations they are absent in segments IX-XII and 

 in those that lie at a distance of two or more millimeters from 

 the posterior end of the body ; the latter point holds good for 

 specimens which have not suffered any injury. 



6. Nervous system. 



I can only give descriptions of the central system. 



The brain lies on the dorsal side of the mouth cavity 

 between the first and the second segment, and is held in its 

 position in the cœlom by the longitudinal muscle fibres of the 

 body wall. As seen from above it is almost square, somewhat 

 narrowed anteriorly and shallowly notched on the posterior border. 

 In a cross- section through its broadest part, it is kidney- shaped 

 and convex dorsally, measuring 0.10-0.13 mm. across and 0.04- 

 0.05 mm. dorso-ventrally. From the anterior corners of the brain 

 two lateral prostomial nerves are given off on either side into the 

 prostomium, in which they branch and subsequently enter the 

 body wall. One of these branches becomes the lateral line 

 already mentioned, which runs backwards to the hind end of the 

 body. A median prostomial nerve starts from the anterior border 

 of the brain, and is enlarged into a prostomial ganglion lying in the 

 prostomial cavity, from which nerve fibres are given off in several 

 directions. The median prostomial nerve is very slender, while 

 the lateral ones are very stout. The prostomial ganglion is short 

 spindle-shaped, 0.015-0.02 mm. long and 0.008-0.01 mm. broad. 



The peripharyngeal commissures run out from the posterior 

 corners of the latéral prostomial nerves. After giving off branches 



