It is now understood that they are Euthyneurous Gastropods adapted to natatory locomotion and pelagic life.
They may, however, be natatory as in many Ostracoda and Copepoda, or prehensile, as in some Copepoda.
4) the abdominal appendages are constantly divided into an anterior group of three natatory " swimmerets " and a posterior group of three limbs used chiefly in jumping or in burrowing.
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.
In those Copepods in which the palps of the mandibles as well as the antennae are biramous and natatory, the first three pairs of appendages retain throughout life, with little modification, the shape and function which they have in the nauplius stage, and must, in all likelihood, be regarded as approximating to those of the primitive Crustacea.