I was delighted to see this slipper lobster cruising around on last week's night dive. My girlfriend referred to it as "that big woodlouse-looking thing" but they always remind me of little tanks. I think its because their mouths and faces are covered so completely. This one's eyes are a lot more prominent than they often are and rather attractively fringed for some reason.
For those of you wondering exactly what this is; its a decapod crustacean similar to shrimps prawns and lobsters. Its front antenna are flattened into those two plates and the claws are hidden underneath - in the picture beneath you can see them in the image below.
A number of species are commercially fished and often sold under the name "bugs" like the Morton Bay Bug. I imagine they're delicious but I just enjoy watching them trundling around too much to eat one.