Brachyuran crabs are marvellous feats of engineering. Basically, they are macrurans (lobsters and their decapod allies) with the abdomen flipped underneath. They rest on their nether regions.
You may have known that, but did you know how they breathe? Gills, yes, but the anatomy of those, and their ventilating mechanism, are neat. You can see the gills in the above Red Rock Crab, Cancer productus, in which the top of the carapace has been flipped up from the back end. They are the soft-looking orangey things on either side of the midline. This is actually the exuvium of the crab--the crab has backed out of itself and gone on to bigger things. (When they molt they shed the entire exoskeleton, including the complex chitinous surfaces of their feathery gills).
The gills extend up from the leg bases into the gill chambers within the carapace. Water flows over them, not from the mouth inward, but the reverse. It is sucked in at the leg bases, flows upward and forward within the gill chambers, and exits as a potent stream from the mouth. You can see this in a live one by lifting the face out of the water, or within an aquarium setting by introducing a non-toxic dye next to a crab's (or lobster's or crawdad's) legs to see where it pops out.
What is the source of the water current? What draws it over and through the gills and shoots it forward? Appendages known as a gill-bailers, or scaphognathites. These are wispy, Pringle Chip-shaped structures that extend from the second maxillae (one of six sets of mouthparts) and teeter-totter near the front of the gill chambers, pulling water in and then passing it forward. Marvellous indeed!
So now you know. Next time you're in a seafood restaurant, check out the doomed crabs in the tank, and try to resist the urge to explain to your dinner-mates the fascinating functioning of the scaphognathite. (Pronounced "gill-bailer.")