The Green Shelled Semi-Slug: the scientists who discovered this species originally wanted to name it "Ibycus felis," because it rests with its tail curled around its body, just like a sleeping cat
The scientific name of this gastropod is Ibycus rachelae, but it's also known as the green-shelled semi-slug or long-tailed semi-slug. The species was first described in 2008, and it can be found only in the montane forests of Sabah (Borneo) and Peninsular Malaysia.
The term "semi-slug" refers to an intermediate stage of evolution that occurs when a snail is gradually evolving into a slug. These gastropods still have shells that are at least partially visible, but their shells have been reduced to the point where they can no longer accommodate the snail's whole body.
There are many different species of semi-slug; their features often vary, but most of them have a noticeably reduced, receding, and/or transparent shell that is partially concealed beneath a flap of tissue known as the mantle.
This article describes another interesting characteristic of semi-slugs (including Ibycus rachelae):
... semi-slugs don’t just look weird, they act weird, too. They employ sharp projectiles called love darts in their courtship rituals, by shooting several of them at a prospective mate. The mate, in turn, shoots several love darts right back.
Researchers have found that if semi-slugs are able to lodge love darts into one another, the subsequent copulation tends to be much more successful. It’s thought that the mucus distributed by the love dart ensures greater survivability of the sperm
This is what the "love darts" look like (when magnified under SEM):
The tiny, harpoon-like structures are made of calcium carbonate, and they transmit certain hormones (via mucus) that help to increase the likelihood of reproductive success. Semi-slugs are not the only gastropods that use "love darts," however; they are also used by some other land snails and slugs.
World Wildlife Fund: Borneo's New World (PDF)
Basteria (Journal): The Slugs and Semislugs of Sabah, Malaysian Borneo (PDF)
Forest Research Institute Malaysia: Introduction to the Land Snails and Slugs of Malaysia (PDF)
Malay Peninsular Terrestrial Molluscs: Ibycus rachelae
Live Science: World's Longest Bug and 'Ninja' Slug Discovered in Borneo
Australian Geographic: Meet the Semi-Slug, a Snail without a Home