Mating Slugs: Sperm Exchange Hypothesis - Update

A couple of nights ago, I was able to gather the data necessary to support, if not confirm, my hypothesis that the slugs of Lehmannia valentiana do indeed engage in external sperm exchange by entwining penes. As always, a hypothesis must be tested again and again, so this doesn't mean I'm going to stop collecting data. However, an analysis of the photos I took that night dispelled some of those nagging doubts I had before, thinking that perhaps my slugs did mate by intromission. So, for now at least, my conclusion is this:

Prior to copulation, the slugs of Lehmannia valentiana gradually evert the penial appendix. In a mating couple, the slug that had been the leader during the previous trail following does so a second or two before the one that had been the follower. The appendices normally do not touch, nor is anything exchanged between them; yet they are aligned. Immediately after the appendices have been everted to their full extent, the penes emerge explosively, in a single motion, entwining around each other. They remain entwined for a few seconds, exchanging sperm in a manner that is unknown at present, before gradually retracting into their respective gonopores, often leaving a strand of mucus connecting the two pores.

Couple 28:

If you look closely you can see the open gonopore of the slug on the right: its penis has completely retracted. The penis of the slug on the left has not fully retracted: you can see part of it as it's going back in.

The cord of mucus stretching between the pores.

Couple 29:

The entwined penes resemble a screw.

Both penes retracting simultaneously.

The penis of the slug on the bottom is a little slower in retracting. You can see the wide-open gonopore.

The gaping gonopore of the bottom slug; it resembles an open mouth.

I still have a question as to whether or not the end of each penis could possibly be inserted into the gonopores, as I see now that the pores can expand quite a bit. That could be a method of intromission. Nevertheless, it appears that the penes only entwine and that consequently the ends are hidden from view. I suppose that, in order to get to the bottom of this, I'll have to keep taking photos. Unfortunately the weather will get colder soon and at night the temperature will drop into the 30s. My slugs may not be as active then, and if it gets below freezing, they might die, or at best, hibernate. So I might have only a limited time to get some more shots this season, and then I'll have to wait until spring.