This is the 2nd part of Snail dissection in progress: Oxyloma retusa. The 1st part is here.
In the previous post I expressed my confusion about a certain muscle I had observed among the genitalia of the land snail Oxyloma retusa. I thought the muscle in question was inserting near the junction of the vagina and the penis.
Subsequent and more careful examination of the dissection revealed that I was slightly mistaken about where that particular muscle was going. But, first, here is a picture for orientation purposes. We are looking into the snail's head with the overlaying skin as well as the mantle conveniently removed out of the way. The buccal mass is where the radula resides and the ring-like organ immediately below it and encircling the esophagus may be called the snail's brain, for that's where all the nerves coming from different corners of the snail's body congregate.
Now, for a larger picture from a different angle, click on the thumbnail.
I forgot to add a scale bar; the penis plus epiphallus was about 3.8 mm long.
Find the retractor muscle entering from the right and its 2 branches that I labeled 1 and 2. Branch #1 passes in between the penis and the vagina and goes to the right (upper) tentacle. The branch #2 is what I colored green in yesterday's drawing. I thought it was inserting near the junction of the vagina and the penis. But as you can in the big picture, branch #2 actually continues further to the front of the snail's head. Nevertheless, it is also attached to the base of the penis by some fine connective tissue fibers. So, I wasn't that wrong.
The drawing below, showing the branches of the columellar muscle of Helix pomatia, is from the last volume of Libbie Hyman's incomplete series The Invertebrates, published in 1967 shortly before she died. She attributed the original drawing to a 1916 publication by someone named Trappmann. The muscle she labeled 4 is what I labeled 1, while her muscle #2, which I colored green, is the same as my #2.
Quoting from Hyman [italics mine]:
In [Helix pomatia] the [columellar] muscle, after leaving its origin on the columella, forks into right and left parts, of which the smaller right part sends a branch into each tentacle on that side and then loses itself in the tissues of the foot.So now I know which muscle I am working with. The new problem I have is that the branch (#2) that is supposed to be going to the lower tentacle (not visible in my photographs) doesn't go there in O. retusa as far as I can tell. I followed it carefully as far as it went, but it never went down to the lower tentacle; it simply lost itself on the inner side of the skin. Perhaps that is what it does in the Succineidea.
Now, click back on the thumbnail for the larger picture again and take another look at the convoluted arrangement of all the pieces around the genitalia. The branch #1 of the muscle goes in between the penis and the vagina and under the vas deferens; the branch #2 goes under the nerves from the tentacle; the vas deferens, on the other hand, has to go around the branch #1 and the nerves just to get to the other side.
Would an intelligent designer–if he was truly intelligent–be satisfied with such a messy creation? What we have here is the product of a blind evolutionary process that was tinkering with what was available to it without any concern for neatness as long as everything it came up with passed natural selection.
I will finish with a couple of quotes. The 1st is from John Maynard Smith (The problems of biology, 1986):
There are many features of animals which could be improved on, and which are as they are because of the legacy of the past...This kind of maladaptive feature would be hard to explain if we had been designed by an all-wise creator, but it is to be expected if structures change their functions in the course of evolution.Can we consider the convoluted arrangement of the inside of a snail to be maladaptive? Perhaps, but even if it weren't maladaptive, Maynard Smith's point would still be valid for the seemingly unnecessary complications that we are seeing.
The 2nd quote is from François Jacob (The possible and the actual, 1982):
In contrast to the engineer, evolution does not produce innovations from scratch. It works on what already exists, either transforming a system to give it a new function or combining several systems to produce a more complex one. Natural selection has no analogy with any aspect of human behavior. If one wanted to use a comparison, however, one would have to say that this process resembles not engineering but tinkering, bricolage we say in French.
Evolution the tinkerer, it seems, doesn't care much about neatness.