Most specimens of the large black slug Arion ater are jet black, but I found this handsome red form with grey tentacles crawling across a woodland path near Durham city today. The red form is much commoner in the south of England. In the image above you can see the large open breathing pore (pneumostome) that leads to the mantle cavity - a simple form of lung.
The whole slug was covered in a layer of grey mucus and ....
.... when fully extended was about 12 cm. long. This is our largest slug species.
Arion ater has very distinctive behaviour when you give it a prod. First it withdraws its tentacles under a hood and closes its pneumostome, then it gently rocks from side to side, as if in silent fury. This behaviour, combined with that distasteful mucus, probably deters a lot of predators.
After a while it will begin to extend an inquiring tentacle from under the hood, to see if the threat has gone and the coast is clear ...
.... and if it is it extends its tentacles, opens its pneumostome (to breathe a sigh of relief?) and glides away into the undergrowth.
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