Slide 51:

The clam shrimp have a carapace that wraps around the animal and seems to form hinged shells like a clam. They have concentric growth rings like a mollusc. But the animal is a shrimp-like crustacean which has second antennae greatly developed for swimming. They work like oars, rowing the clam shrimp clumsily through the water. When they stop rowing, the animal falls onto its side unless it has grabbed a piece of vegetation. When disturbed it can withdraw completely within the carapace. Clam shrimps occur in all types of fresh water. Most are very small, but the specimen shown here is almost eight millimeters long.