Though the exterior is more uniform than in most groups of Crustacea, the bivalved shell or carapace may be strongly calcified and diversely sculptured (fig.
The bivalved carapace has a jointed rostrum, and covers only the front part of the body, to which it is only attached quite in front, the valve-like sides being under control of an adductor muscle.
The group may be defined as follows: Sessile solitary Coelomata with bivalved shells usually of unequal size and arranged dorso-ventrally.
In this suborder the head is more or less distinct, the rest of the body being in general laterally compressed and covered by a bivalved test.
The Copepoda have normally a segmented body, not enclosed in a bivalved shell-covering, the segments not exceeding eleven, the limbs not branchial.
In the outward appearance of the adults there is great want of uniformity, one set having their limbs sheltered by no carapace, another having a broad shield over most of them, and a third having a bivalved shell-cover within which the whole body can be enclosed.