134 OLIGOCHAETA 



duct opens on to a papilla, which itself projects into the interior of a terminal 

 sac. This is, of course, highly suggestive of the penis in the Tubificidae and in some 

 others. I figured, but did not describe, this structure in my paper quoted above. 

 On this hypothesis we have, of course, to account for the spermathecal diverticula, 

 whose presence is so marked a feature of the Megascolicidae. An obvious suggestion 

 is that they are the sacs which formerly gave rise to the copulatory setae. 



2. Spermathecae formed from mesoblastic structures. — The Eudrilidae are charac- 

 terized by the possession of spermathecae of a different kind to those that have 

 been hitherto considered. In a few genera, as already described, there are sperma- 

 thecae of the ordinary kind, only differing from those of the majority of the 

 Oligochaeta in being situated posteriorly. In most genera, however, the place of 

 these organs is taken by sacs which are developed from the septa, and whose cavity 

 is therefore coelomic. I describe the variations of these more particularly under the 

 family Eudrilidae, inasmuch as they are peculiar to that family and appear to 

 have no relation to any other organs found among other earthworms. These sacs 

 are generally large and generally unpaired, paired only in Eudrilus, Pareudrilus, 

 and Hemertodrilus j if true spermathecae are present, these sacs involve them ; 

 if such spermathecae are not present, they open on their own account on to the 

 exterior. Often they communicate with the sac that in the Eudrilidae generally 

 envelops the ovaries, and they contain therefore both sperm and ova. That there 

 should be organs performing the same function, but morphologically quite distinct, 

 in a group which is so comparatively limited as that of the Oligochaeta, is perhaps 

 the most remarkable fact in the economy of that group, and is not paralleled by any 

 other structure. , 



I will take as an example of these structures the spermathecal sac of Libyodrilus 

 (PI. iv. fig. i). When the worm is dissected, a large sac, extending through several 

 segments (xiii-xviii), is seen to lie dorsally covering the gut. It is of a brownish- 

 yellow colour, and is of an irregular elongated form, with numerous furrows on the 

 surface which appear to indicate the possibility of distension. It has three pairs of 

 diverticula reaching for some way down the sides of the gut. From the anterior of 

 these arise the oviducts, one on each side. Anteriorly the sac bifurcates, embracing 

 the oesophagus and reuniting below it. It then again divides so as to surround the 

 nerve cord ; the two branches reunite below the nerve cord before opening by the 

 single median pore on to the exterior on segment xiii. The oviduct does, not, as it 

 appears to do, open into this spermathecal sac. A small roundish projection marks 

 its attachment to the sac ; this round body is the egg-sac, and has the structure 

 characteristic of that organ in other Oligochaeta. Its lumen does not communicate 



