Sub-class III. Entomostraca (Gnatnopoda, Woodward). -The term Entomostraca has been variously employed, and few authorities include exactly the same groups of the Crustacea under this name. By most the division is simply defined as including all those Crustacea in which the segments of the thorax and abdomen, taken together, are more or fewer than fourteen in number - the parasitic Epizoa and the Cirripedia being excluded. By Professor Rupert Jones the following definition of the Entomostraca has been given:

"Animal, aquatic, covered with a shell or carapace, of a horny consistence, formed of one or more pieces, in some genera resembling a cuirass or buckler, and in others a bivalve shell, which completely or in great part envelops the body and limbs of the animal. In other genera the animal is invested with a multivalve carapace, like jointed plate-armour; the branchiae are attached either to the feet or to the organs of mastication; the limbs are jointed, and more or less setiferous. The animals, for the most part, undergo a regular moulting or change of shell, as they grow; in some cases this amounts to a species of transformation."

The Entomostraca are divided into two great divisions, or "legions," the Lophyropoda and the Branchiopoda, with which the order Merostomata may be conveniently considered.

Division A. Lophyropoda

The members of this division possess few branchiae, and these are attached to the appendages of the mouth. The feet are few in number, and mainly subserve locomotion; the carapace is in the form either of a shield protecting the cephalothorax, or of a bivalve shell enclosing the entire body. The mouth is not suctorial, but is furnished with organs of mastication.

This division comprises the two orders Ostracoda and Copepoda.