=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. _Cyclops._—Numerous species, split up according to segmentation of rudimentary fifth pair of legs, number of joints in antennae, etc. =Fam. 2. Harpacticidae.=—The cephalothorax is not clearly separated from the abdomen. The first antennae are short in both sexes, both being clasping organs in the male. The second antennae have a rudimentary exopodite. The fifth pair of limbs are rudimentary and plate-shaped; a heart is absent, and the egg-sacs of the female may be one or two in number. 1. _Ophiocamptus_ (_Moraria_).—Body worm-shaped; first antennae of female 7–jointed, rostrum forming a broad plate. 2. Body not worm-shaped; first antennae of female 8–jointed, rostrum short and sharp. (_a_) Endopodites of all thoracic limbs 3–jointed. The first antennae in female distinctly bent after the second joint. _Nitocra._ (_b_) Endopodite of at least the fourth limb 2–jointed; first antennae in female not bent. _Canthocamptus._ 3. _Ectinosoma._—Body as in 2, but first antennae are very short, and the maxillipede does not carry a terminal hooked seta as in 1 and 2.