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I have seen this type of slug only twice. Both occasions were on the rough grassy area of the field near my house. Is it a slug, for at the very end of the body it carries a shell? One normally separates slugs from snails, in that snails have shells and slugs do not. But as with all things in nature, the limits are more fuzzy than that. The division between slugs and snails, although convenient, is more a popular classification than a scientific one. In fact nearly all slugs have the remnants of a shell internally. It suggests that the ancestor of all of them was a form of snail. In my Nature Notes of July I wrote that all slugs have a mantle fold behind the head. In all common slugs this is so, but Testacella is an exception. So not only does this slug have a shell but it has no mantle. For these reasons the animal is an oddity. There are three species of Testacella in Britain and France. One rarely sees them because it seems they spend much of their time underground searching for and eating earthworms. They are probably often overlooked as just another slug. Where they are found it is often in the earthworm rich soil of gardens.
They have eyes on tentacles just like garden snails. Many a child delights in watching the eyes pulled in as a snail touches an object with its tentacle, and then unroll again, from the inside out. In the photo you can see that the right eye is half enfolded. At the back of the body emerging from the front edge of the tiny 'redundant' shell are two dark grooves or lines. I have no idea what their function might be. And indeed, perhaps the shell is not as redundant as one might think, for these grooves end there and it could suggest a sensory use. The animal can only breath through the skin. There seems to be no other breathing surface. This would make it necessary for the creature to stay moist at all times, for a moist surface is necessary for gas exchange.
It can extend itself to about 8 centimetres and when it finds an earthworm to eat, the inner part of the mouth cavity is pushed out and can engulf part of the worm. There are large teeth like structures inside the mouth and these can catch and tear at the worm. The specimen in the photo was doing this when I caught it. Unfortunately to photo it, I had to place it in an container to take it home, and it surrendered the earthworm. I then went in search of another earthworm, and meanwhile the beast escaped from my container, and I was left with a poor photograph in its memory.