DESCRIPTIONS OF GENERA AND SPECIES 237 



chamber, with its appendages of Tubifex, originates as a simple sac, comparable 

 to that in the lower forms. 



In Branchiura, the sac which clearly corresponds to the terminal chamber of 

 Ilyodrilus, no longer receives the sperm-duct ; it has become merely a caecal 

 diverticulum of the sperm-duct, which is specialized near to its termination into 

 two sections, one ciliated, the other without cilia. The function of the vas 

 deferens, with this terminal tube, is marked by a distinct narrowing of the vas deferens, 

 and by a change in minute structure ; at the junction opens the glandular sac. 



It seems, therefore, to be possible that the tube lying between the end of the vas 

 deferens and the exterior, and which corresponds to the duct of the globular sac in 

 Ilyodrilus, is the equivalent of the entire copulatory apparatus in other Tubificidae. 

 If this be so, it is then possible that the prostate of these Tubificidae is the homologue 

 of the glandular sac of Branchiura, much reduced in importance and, except in 

 Bothrioneuron, with a nearly or entirely obliterated lumen. However, it is more 

 probable that the cells of the prostate of Tubifex represent a part only of the 

 peripheral glandular layer of Branchiura. 



As BENHAM (15) and I (80) have pointed out, the prostate in Tubifex is simply 

 to be regarded as a portion of a continuous glandular covering such as exists in 

 Branchiura and Ilyodrilus; an intermediate stage is offered by Telmatodrilus, in 

 which genus the continuous glandular covering has already broken up into several 

 separate glands. This probable explanation of the various conditions of the prostate 

 in the Tubificidae does not do away with the possibility that the ' paratrium ' of 

 Bothrioneuron is the homologue of the large chamber in Branchiura; but it seems 

 more probable that this ' paratrium ' is a new structure. 



In no Tubificid is there more than a single pair of spermathecae ; and these organs 

 are always (except in Hesperodrilus, where they are in the eleventh) situated in the 

 tenth segment, i.e. that which also contains the testes. The spermathecae never have 

 any diverticula, except a rudimentary one in Hesperodrilus branchiatus, but in a few 

 species there are glands appended to the base of the pouches. Such glands, which are 

 probably to be regarded as of epidermic origin, comparable to the capsulogenous 

 glands of the genus Perichaeta, are found in Hemitubifex insignis, and in Ilyodrilus 

 sodalis and Psamnwryctes barbatus. In the latter worm the appendages are connected 

 with a sac containing a single copulatory seta. The spermathecae of the Tubificidae 

 differ somewhat in form in the different genera ; in Ilyodrilus they are globular 

 and sessile ; in the great majority of species there is a division of the spermatheca into 

 a pouch and a duct leading to the exterior; in Lophochaeta, for example, there is 

 a change in the character of the lining epithelium which marks the commencement of 



