228 OLIGOCHAETA 



position of the reproductive organs is different to that of the Naids, though not, it is 

 true, precisely that of the Tubificidae. 



Phreodrilus is also not far removed from the Lumbriculidae in many points. The 

 chief resemblance is in the fact that the spermathecae open behind the male pores, 

 a resemblance which is shared by Hes2Jerodrilus. It is possible also that the periatrial 

 sac is comparable to the periatrial sperm-sac seen in Sutroa; the sigmoid setae 

 are something like those of the Lumbriculidae. Other features of resemblance are 

 dealt with in the section dealing with the general characters of the Lumbriculides 

 (see above). The genus Hesperodrilus, which has just been referred to, shows a slight 

 resemblance to the Lumbriculidae in that the first pair of nephridia occupy a consider- 

 able number of segments ; the internal and external apertures of these lengthened 

 nephridia are, however, as in the Lumbriculidae, in consecutive segments. We are at 

 present evidently only very imperfectly acquainted with the extent of the range of 

 structural variation in the family Tubificidae. 



It contains sixteen distinct genera, which are the following : 



(1) Tubifex, LAMAKCK. (9) Lophochaeta, STOLC. 



(2) Limnodrilus, CLAPAR&DE. (10) Bothrioneuron, STOLC. 



(3) Peloscolex, LEIDY. (11) Branchiura, BEDDARD. 



(4) Clitellio, CLAPAREDE. (12) Psammoryctes, VEJDOVSKY. 



(5) Hemitubifex, EJSEN. (13) Heterochaeta, CLAPAREDE. 



(6) Telmatodrilus, EISEN. (14) Embolocephalus, RANDOLPH. 



(7) Spirosperma, EISEN. (15) Vermiculus, GOODRICH. 



(8) Ilyodrilus, EISEN. (16) Hesperodrilus, BEDDARD. 



The Tubificidae thus contain a very large number of genera in proportion to the 

 species ; to anyone who has looked through VAILLANT'S work upon the Oligochaeta 

 this statement will appear to be untrue ; but it appears to me very doubtful whether 

 the large majority of the species allowed by that author are really tenable ; it will be 

 seen, in the course of the following pages, that I have eliminated, and it is hoped with 

 reason, a very considerable percentage of the described species ; out of the sixteen 

 genera ten have only one definable species each. I am very doubtful how far it is 

 really justifiable to separate Tubifex, Hemitubifex, Limnodrilus, Clitellio, Spirosperma, 

 Camptodrilus (included by VEJDOVSKY in Limnodrilus), and Psammoryctes; the 

 latter is the most distinct, principally by reason of the peculiar setae attached to the 

 spermathecae. There can be, on the other hand, no doubt about the distinctness of 

 Telmatodrilus, Ilyodrilus, Lophochaeta, Heterochaetu, Bothrioneuron, Hesperodrilus, 

 Embolocephalus, Vermiculus, and Branchiura. With regard to the first group, 

 Tubifex blancJiardi seems to ditf'er more from Tubifex rivulorum than Tubifex 



