1888.] STRUCTURE OF CLITELLIO. 493 



The spermatheca extends beyond the 10th segment, in the sexually 

 n-ature individual it reaches back as far as the 13th segment ; 

 where it traverses the boundary-line between the 11th and 12th 

 segments, the next septum, i. e. that which divides segments 12 and 

 13, comes into close relations with the septum dividing segments 11,12 

 (see p. 491, fig. B) ; at this point the two septa almost fuse and the 

 spermatheca passes directly from segment 11 into segment 13; 

 where it traverses the two mesenteries there is another constriction, 

 but the epitheUura does not change in character. In Spirosperma, 

 according to Eisen \ the spermatheca occupies in the same way 

 several segments. 



In C. (iter the spermatheca is not differentiated into two regions 

 and it only occupies one segment (the 11th). 



§ 3. Systematic Position and Affinities of the Genus Clitellio. 



In the remaining part of this paper I propose to discuss the 

 affinities and systematic position of Clitellio. 



In the first place, it is perfectly clear that these worms have been 

 rightly assigned by Clapartde and Vejdovsky to the Tubificidae. 

 Vejdovsky's definition of the family, translated into English, is as 

 follows : — " Normally segmented Annelids of red colour, with a 

 large number of setas which are disposed in four bundles. The 

 seta-bundles consist of 3-10 bifid setae, and occasionally, in the 

 dorsal bundles, of hair-like setae ; the lateral vascular trunks com- 

 municate directly with the ventral vessel. Testes in the 9th, 

 ovaries in the 10th segment. Vasa deferentia, one pair opening on 

 to the 10th segment. Spermathecae, one pair opening on to the 

 9th segment. Ova probably escaping between the 9th and 10th 

 segments. Spennatophores present in the spermathecae. Aquatic." 

 In all these particulars Clitellio resembles the other Tubificidae, and 

 there are no structural peculiarities that I have been able to discover 

 by which the genus can be said to be allied to any other f\imily. 



The genus Clitellio itself is briefly defined by Vejdovsky, his 

 definition being compiled from the account of the genus given by 

 Claparede in the two papers which I have already so frequently 

 had occasion to quote. 



The two facts upon which Vejdovsky lays stress in his generic 

 definition of Clitellio are the characters of the setae and the absence 

 of a prostate gland. 



So far as the first of these characters is concerned, Clitellio ater 

 agrees with Eisen's genus Hemitubifex'^, and it has a prostate gland. 

 It is, in fact, clear that the two species which I have investigated diifer 

 in so many particulars, that they cannot, as the family is at present 

 divided, belong to the same genus. 



The principal characters of the two species are as follows : — 



' G. Eisen, " Oligochretological Eesearches," Eeport of Commissioner for 

 Fish and Fisheries for 1883 (Washington, 1885), p. 922. 

 2 G. Eisen, loc. cit. p. 889. 

 Proc. Zool. ?oc.— 1888, No. XXXIV. 34 



