Adult and young one

Well I wonder if anyone can help me with what these vertebrate are?
They seem to be some kind of slug, but not the normal garden snail or slug. More like a sea slug. I found them under a tarpaulin with dirt. I have placed them on the broccoli leaf, possibly not appropriate, but better than a tarpaulin which was blue!!
There is a hole on their back to the right side, possibly for breathing. They curl up into a ball when disturbed.
They are shaped and marked like a leaf when stretched out, funny it reminds me a little of the green spearmint lollies we used to eat. Only their colour is wrong.