Slugs are Mollusks that have lost their shell. Though hermaphroditic (both sexes in the one animal), they stilll need sexual conjunction with another of their kind to become fertilized. [Wasn't that delicately put?] They both exchange material.

The mollusks are important in evolution, for they invented what becomes the skeleton. With a shell, one now has a rigid piece to which one may form muscular attachment.

In closing, you might look up apophallation in a biology engine or text - a problem that slugs occasionally have.