DESCRIPTIONS OF GENERA AND SPECIES 



577 



Fig. 47- 



individual into the spermatheca of the other, it cannot reach the ova of that 

 individual, but must be transferred to another or conveyed into the cocoon. If we 

 are to assume that fertilization is carried on in the way that has been studied in 

 Lumbricus, the development of the large peritoneal sacs is unintelligible ; perhaps 

 the other genus in which the female reproductive system is constructed on a similar 

 plan gives a clue ; in Heliodrilus the spermatheca is very large ; it reaches from its 

 opening on the eleventh segment as far back as the thirteenth, and is only invested 

 by a peritoneal sac at its very tip ; the sacs in this genus are greatly reduced as 

 compared with the last ; it looks as if the gradual 

 reduction of the spermatheca was accompanied by a 

 growth in the investing sac of peritoneum. Turning to 

 the remaining genera of the family, we find no one in 

 which there are true spermathecae of the kind existing 

 in Hyperiodrilus (see fig. 47), and Heliodrilus; in all 

 of them the spermathecae are peritoneal sacs, which 

 function as spermathecae because they contain sperm, 

 but which open on to the exterior and are not closed 

 sacs, as in Hyperiodrilus and Heliodrilus. The only 

 exception to this statement is afforded by Nemertodrilus j 

 in that genus the sacs in question open into the cavity 

 of the thirteenth segment no doubt an -indication of 

 their development from the septum xiii/xiv but they 

 communicate indirectly with the exterior by means of 

 paired orifices on the floor of the thirteenth segment. 

 Here, in fact, the orifices are quite independent of the 

 sacs with which they must have some relation. The 

 fact that the spermathecal sacs in the Eudrilidae are 

 peritoneal structures was first suggested by myself in 

 a paper dealing with the two genera Hyperiodrilus and 

 Heliodrilus; I pointed out that the sacs contain other organs, e.g. the true sperma- 

 thecae ; later, I found that the development of the corresponding sac in Libyodrilus 

 was, undoubtedly, from the intersegmental septa like the development of the sperm- 

 sacs ; it was necessary, therefore, to regard these sacs as peritoneal and the contained 

 cavity as coelomic ; later still, ROSA indicated the difference in the character of the 

 lining epithelium of these sacs, and of their duct leading to the exterior ; the latter 

 represents all that is left of the true spermatheca ; the extent of the part of the whole 

 spermathecal sac, which is developed as an invagination, differs in different genera. 



41 



XII 



REPRODUCTIVE ORGANS OF 

 (FEMALE) HYPERIODRILUS. 



i. Spermathecal sac. 2. Egg-sac. 

 3. Spermatheca. 4. Ovary. The 

 segments are numbered. 



