You could describe scuds (members of the order Amphipoda) as “shrimplike sowbugs.” Like sowbugs (in the order Isopoda), they have two pairs of antennae; they lack a carapace (a covering “back” like a crayfish has); their eyes are not on stalks; and they have several body segments with legs, gills, and other appendages. But unlike sowbugs, their arched bodies are flattened sideways, like shrimp, and the gills arise on the thorax segments (not on the abdomen). The various appendages have different purposes: armlike gnathopods at the front for feeding, followed by leglike pleopods for swimming, waving water across the gills, and other types of locomotion. Although some Missouri sowbugs live on land and others in water, our scuds are all aquatic.
To identify scuds to species, and often even to family, you must be prepared to dissect them under a microscope and note many details, for example, of the appendages, including their structures and comparative lengths.