Slugs and snails and puppy dog tails (Ok, no puppy dog tails)

So, this is the first in a series of posts otherwise known as “Darwin says Hi!”

These will cover the awesome diversity of the natural world, mostly things which have made me do an O.O face when I first saw them.

Without further ado, lets jump into the subject for today: Gastropods, specifically the ones better known to you and me as slugs and snails.  Slugs are effectively snails without shells.  They do have a mantle (the shell of a snail), but it is vestigial.  Their anatomy is very similar, so I will use the example of a snail.  Below is the generalised anatomy of a Pulmonate snail (This is a general term for a land snail)

Note location of the anus, vagina and penis relative to the head

Yes, a snail quite literally poops on its head, yes it has a penis on the back of its head, yes the vagina is also on the back of the head.  It might be assumed that this is from the evolution of the shell over time, and this was assumed for a while, but it is now thought that torsion (the twisting of the organs from the left to right side of the snail, and the movement of the anus from the back to the front) is a seperate evolutionary event to the development of the shell (coiling).  In addition to pooping on its head, the entrance to the lungs (called a pneumostome) is in the vicinity of the anus, and the other excretory pore.  The picture below shows the torsion which occurs during the development of slugs and snails.  This occurs in the second phase of development, known as the veliger laval stage.

Torsion in a developing snail

Whilst I previously said that the torsion and coiling evolved seperately, there is an argument that the torsion is a result of ancestral coiling. The fossil record appears to show that coiling occured prior to torsion.  This could then mean that the shell evolved as a defense mechanism, and the further that a snail is able to withdraw its head into its shell, the more effective the shell is as a defense. Without torsion, the snail is unable to to withdraw its head fully. The evolutionary benefits of being able to pull completely into the shell outweigh the disadvantages of excreting in the region of the head.  Usually air is expelled along with the excrement, allowing for it to clear the head area, especially the pneumostome, and, even allowing for ocassional fouling of the head region, I think I would prefer pooping near my head to having my head bitten off by a predator.