ON TWO NEW GENERA OF AQUATIC OLIGOCHSETA. 293 



(2.) Setce. — The setse exist upon all the segments of the body except the first. They are 

 arranged in four couples, both of which are, in the anterior part of the body at any rate, 

 rather lateral in position. I could detect no difference of size between the setse of the 

 more dorsal and of the more ventral couples, such as I have shown to occur in 

 Phreoryctes [12]. 



The shape of the setse is in no way distinctive ; as shown in fig. 20, s, they agree with 

 those of Phreoryctes, the Lumbriculidse, and most Earthworms. 



There is, furthermore, an agreement with the two first-mentioned families in the fact 

 of there being no modification of the setse upon the clitellum or of those in the neigh- 

 bourhood of the spermathecal apertures. They are present, and are perfectly normal in 

 shape, as well as in arrangement, upon all the segments of the clitellum. 



It appears, therefore, that the setse of Pelodrilus are more like those of the Lumbri- 

 culidse than of any other family or group of Oligochseta. 



(3.) Clitellum. — The data with regard to the number of segments occupied by the 

 clitellum in the Lumbriculidse are not very numerous. With regard to Rhynchelmis, 

 Vejdovsky says (9, p. 34), " Ein solcher [wurm] ist in der Kegel im Begriif einen 

 cocon alzulegen, was sich ausserlich nach einem weisslichen Auflage am 8-16 segmente 

 kenntlich ist." I take this to imply that the Vlllth to the XVIth segments are occupied 

 by the clitellum, as he has also said (7, p. 67), " Bei Rhynchelmis der Giirtel eine 

 bedeutende Anzahl von segmenten einnimmt." 



In Phreoryctes the clitellum is much more limited, and extends over four segments 

 only, viz., from XI-XI V. 



In Pelodrilus the clitellum occupies segments XI-XIII. It is only developed on the 

 dorsal side of the body. In the region of the clitellum the body is much swollen, owing 

 to the tension caused by the genital products. 



So far, therefore, as can be said at present, Pelodrilus comes nearer to Phreoryctes 

 than to the Lumbriculidse. I shall refer to the minute structure of the clitellum under 

 the heading " Integument." 



(4.) Nephridiopores. — These are situated in front of the ventral pair of setse. 



§ Integument. 



The most interesting fact relating to the structure of the body wall in Pelodrilus is 

 its great thickness in the anterior, as compared with the posterior, segments. This is 

 frequently met with among terrestrial Oligochseta (cf., for example, figs. 4 and 3 in the 

 plate illustrating my memoir upon Moniligaster [13]), where it appears to have an 

 obvious relation to the density of the medium in which they live. Increased muscular 

 power in the anterior segments is not so much needed by worms which swim in water, 

 and is not developed. Pelodrilus, however, does not live in water, like most of its 

 allies, but in marshy land ; and its structure bears evidence of its mode of life, not only 



