Legless - No Legs, No Backbone
Legless invertebrates include slugs and snails (i.e. Molluscs - which also includes all shellfish, as well as octopusses and squid), earthworms and other worms, eelworms (nematodes), leeches, and lots of sea creatures like, jellyfish, sea cucumbers, sea hares, sea anemones, coral and sponge polyps - and masses more - not that any of those need worry us in the garden or great UK outdoors!
The picture above is of two Arion rufus slugs in the middle of mating - the process can last 10 hours! They are hermaphrodite so both fertilise and lay eggs. two earlier stages of mating are included in the Gallery pictures below.
Arion rufus is the redish cousin of the giant black, Arion ater, slug we all know and love - but I have to say that neither of these species are the ones to worry about. They do cause some damage to plants but mainly live on dead material including insects and small animals - it's the little devils like the Milax (keeled) species and their offspring that live in the soil that cause all the trouble.
More info later . . . .
Molluscs - Slugs & Snails
The Molluscs include the familiar slugs and snails - both on land and in the sea - but also included are creatures which 'appear' to have legs, the Octopusses, Cuttlefish and Squids (I say 'appear' to have legs because the primary use of their arms and tentacles is to catch prey, not to get about)
But we generally ony have a problem, in gardens or on the land, with Slugs & Snails - some octopusses are poisonous, even deadly (e.g. the Blue-Ringed Octopus of Australia), but your are not likely to come across them digging spuds in the veg garden . . .
Just a few pictures and words will give you all the information and advice you need.