The first antennae in the male differ greatly from those in the female, being often geniculated and acting as prehensile organs.
Fig. [28].—Euterpe acutifrons, ♀, × 70. Abd.1, 1st abdominal segment; Th.5, 5th thoracic segment. (After Giesbrecht.)
Fig. [29].—First antenna of Euterpe acutifrons, ♂. (After Giesbrecht.)
Fams. 1–2. Cyclopidae and Harpacticidae, and other allied families, are purely free-living forms; they are not usually pelagic in habit, but prefer creeping among algae in the littoral zone or on the sea-bottom, or especially in tidal pools. Some genera are, nevertheless, pelagic; e.g. Oithona among Cyclopidae; Setella, Clytemnestra, and Aegisthus among Harpacticidae.
The sketch (Fig. [28]) of Euterpe acutifrons ♀, a species widely distributed in the Mediterranean and northern seas, exhibits the structure of a typical Harpacticid, while Fig. [29] shows the form of the first antenna in the male.
Several fresh-water representatives of these free-living families occur. The genus Cyclops (Cyclopidae) is exclusively fresh-water, while many Harpacticidae go up into brackish waters: for example on the Norfolk Broads, Mr. Robert Gurney has taken Tachidius brevicornis, Müller, and T. littoralis, Poppe; Ophiocamptus brevipes, Sars; Mesochra lilljeborgi, Boeck; Laophonte littorale, T. and A. Scott; L. mohammed, Blanchard and Richard; and Dactylopus tisboides, Claus.
Schmeil[[43]] gives the following scheme for identifying the fresh-water Cyclopidae and Harpacticidae (see diagnosis of Centropagidae on p. 59):—
Fam. 1. Cyclopidae.—The cephalothorax is clearly separated from the abdomen. The first antennae of the female when bent back do not stretch beyond the cephalothorax; in the male both of them are clasping organs. The second antennae are without an exopodite. The fifth pair of limbs are rudimentary, there is no heart, and the female carries two egg-sacs.