DESCRIPTIONS OF GENERA AND SPECIES 227 



opening on to the eleventh segment. Oviducts open on to boundary-line XI/XII. 

 Spermathecae one pair in X '. Penial setae sometimes present. 



This family of Oligochaeta is plainly derived from (or has given origin to) the 

 Naidomorpha. Indeed, the two are united by VAILLANT into one family, Naididae. 

 The principal annectent genus is .Ilyodrilus, formerly confounded with Tubifex, but 

 now known, through the researches of EISEN (12) and STOLC (3), to be distinct. 

 Ilyodrilus approaches the Naidomorpha principally by virtue of the structure of the 

 nephridia and of the spermiducal glands, by the development of the ova, and by the 

 relations of the intestinal vascular network. As in many Naids, a dark glandular 

 swelling is developed upon the nephridium, immediately behind the septum; within 

 this swelling the nephridial tube forms a network. 



The spermiducal gland differs from that of most other Tubificidae in having neither 

 ' prostate ' nor penis ; with it communicates a very short vas deferens, which is again 

 a character found among the Naids. Ilyodrilus is, however, unmistakeably a Tubificid. 

 The position of the sexual organs shows this, as well as the complicated vascular system. 

 Nevertheless, it serves to indicate the nearest affinities of the family in one direction. 

 In two species of Hesperodrilus there is a slight cephalization, so characteristic of the 

 Naidomorpha. Affinities with other families of Oligochaeta are not yet clearly apparent. 



The remarkable genus Pkreodrilus, perhaps, indicates a passage to the Lumbri- 

 culidae. Its principal characters will be found below ; I separate it a little from the 

 other Tubificidae. In my account of the anatomy (21) of this worm I pointed out 

 that it ought to be regarded as the type of a new family ; I am not now disposed 

 to separate it so widely. There is, however, it must be admitted, a considerable series 

 of differences which distinguish the genus Phreodrilus from the Tubificidae ; but 

 rather greater differences distinguish the genus from other families of the aquatic 

 Oligochaeta ; and there are certain important resemblances to the Tubificidae, which 

 are, perhaps, closer than any resemblances shown to the other groups of Oligochaeta ; 

 the chief resemblances to the Tubificidae are : 



(1) Dorsal setae capilliform. 



(2) Spermiducal gland elongated without any thick glandular investment (but 



also without prostates). 



(3) Supra-intestinal vessel present. 



The two first characters, of course, are also those of the Naidomorpha. But, 

 although I originally dwelt upon the likeness which Phreodrilus shows to that 

 family, I am not now so impressed with this view of its affinities. It has, for example, 

 yet to be proved that Phreodrilus is capable of multiplication by gemmation. The 



1 The position of the generative organs is abnormal only in Hesperodrilus (q. r.}. 



G g 2 



