Suggestion as to the Use of the Conical Porophores.
Very little is known as to the details of coition, or if you will, “copulation,” in the group of earthworms. We know the procedure in Lumbricus and in Eisenia, its near ally, and in these the two partners engage one another head to tail with the ventral surface closely in contact. Some apparatus or method is employed for the conveyance of the sperm from the male pores of the one into the spermatheca of the other, and this is assumed to be mutually contemporaneous.
In these two genera the two partners are wrapped together in a “slime sheath” derived from the secretion from the clitellum, but this is exceptional, and in other cases where the process has been studied such an envelope is absent. In several genera an extroversible “penis” is present and is inserted into the spermatheca.
One reason for the detailed account of the procedure in the family Lumbricidae is that the copulation takes place on the surface of the ground. This is also the case with Eutyphaeus waltoni, an Indian earthworm of the family Megascolecidae (as described by Bahl), but there is no “slime sheath” and the sperm-pore becomes, during copulation, everted and raised on a papilla which is inserted into the spermatheca: thus it is a penis. Whereas the Acanthodrilines and probably other families are more modest and pursue their amours in their own burrows out of sight of the enquiring zoologist,
which may account for the lack of information available as to the preedure of the exotic worms.
As to the Acanthodrilines, nothing so far as I can ascertain has been recorded. But many years ago an observation was made by one of my students, Mr. James Grant, who reported it to me. While digging in his garden, he had come across a couple of worms engaged in their amours. He noted that they were temporarily attached to one another—not by any secretion, but by the protruded “penial chaetae,” which were, however, quickly withdrawn from the spermatheca. I append a conjectural figure of what probably occurs (Fig. 6).
In the group of Acanthodrilines the male ducts do not open into the duct of the prostate as in most Megascolecidae, but have independent openings in segment xviii, whereas the two pairs of prostates open in xvii and xix respectively. In the process of copulation the two partners adopt the usual “head-to-tail” position in such a way that the two prostate pores of a side are opposite the two spermatheca pores of the other worm. The difference in the length of the segments allowing this to take place in spite of the fact that the two spermathecal pores are in consecutive segments while those of the prostates are separated by a complete segment (Fig. 6). The penial chaetae, developed in special sacs communicating with the prostate duct, are exserted and are inserted into the main sac of the spermatheca—i,e., the “ampulla”. There is no “slime sheath” such as is present when coition of Lumbricus or Eisenia takes place.
The apertures of the sperm duct are in segment xviii opening into a “seminal groove,” which passes from one prostate pore to the other of the same side. The spermatozoa then passes along this groove, both in an anterior direction and a posterior. It is probably not propelled by repeated muscular contractions as has been described by Grove and Cowley in Lubricines, where, however, the seminal groove is much longer and its relations are totally different. Here in the Acanthodrilines the spermatozoa pass forwards and backwards to reach the penial chaetae projecting, as said above, into the ampulla, and these form a bridge or guide as it were for the sperm cells to enter the “diverticulum” of the spermatheca (Fig. 6). But it may be supposed that as the two porophores are not in consecutive segments, whereas the two spermathecae are, that the two sets of organs would not be really opposite. I have measured the distances involved and have noted further that the post-elitellar segments are small—that is, shorter than those in front, may even be but half the length. I have measured the spaces under consideration in species of Maoridrilus, Octochaetus and Hoplochaetina, and find that the distance between the two porophores is equal to that separating the two spermathecae. Hence, when the two worms are in contact, the two sets of pores are precisely opposite to one another.
Probably—for we know nothing about the secretion of the large prostate glands—this secretion affords a means of conveyance for the sperms as they pass along the groove and wash them into the spermatheca. For several authors have noted that they are to be found only in the diverticulum, while the ampulla is filled with a mucus-like material. In Fig. 6 I have endeavoured to display the course of the spermatoza by a series of large dots.
And I suggest that in Conicodrilus these conical pointed porophores serve as “penes,” and are inserted into the ampullae (Fig. 7) in the same way as are the penial chaetae, which are absent in this genus. In the case of Maoridrilus, as of the Acanthodriline and other genera with penial chaetae, these serve not only to hold the two worms together, but serve as a bridge by which the sperm travels to enter the diverticula at the same time.