The name of shrimps is sometimes given to members of the order Schizopoda, which differ from most of the Macrura in having swimming branches or exopodites on the thoracic legs.
The thoracic limbs have the endopodites converted, as a rule, into more or less efficient walking-legs, and the exopodites are often lost, while the abdominal limbs more generally preserve the biramous form and are, in the more primitive types, natatory.
We need only mention the Mysis-stage (better termed Schizopodstage) found in many Macrura (as, for example, the lobster), which differs from the adult in having large natatory exopodites on the thoracic legs.
The Stenopidea, another primitive group, differing from the Penaeidea in the character of the gills,, appear in the Trias and Jurassic. The Caridea or true prawns and shrimps appear later, in the Upper Jurassic, some of them presenting primitive characteristics in the retention of swimming exopodites on the walking-legs.
They may have rudimentary exopodites, and may or may not have branchial filaments or lamellae developed on their posterior faces.