Fam. Calanidae.—The Calanidae are exclusively marine Crustacea, and form a common feature of the pelagic plankton in all parts of the world. Some species of the genus Calanus often occur in vast shoals, making the sea appear blood-red, and they furnish a most important article of fish food. These swarms appear to consist chiefly of females, the males being taken rarely, and only at certain seasons of the year. Some of the Calanidae are animals of delicate and curious form, owing to the development of plumed iridescent hairs from various parts of their body, which may often exhibit a marked asymmetry, as in the species figured, Calocalanus plumulosus (Fig. [26]), from the Mediterranean.

Fig. [26].—Calocalanus plumulosus, × 15. (After Giesbrecht.)

Sars makes a curious observation[[39]] with regard to the distribution of certain Calanidae. He reports that along the whole route of the “Fram,” species such as Calanus hyperboreus and Euchaeta norwegica were taken at the surface, which, in the Norwegian fjords, only occur at depths of over 100 fathoms. He suggests that the Norwegian individuals, instead of migrating northwards as the warmer climate supervened, have sought boreal conditions of temperature by sinking into the deeper waters.

TRIBE II.
HETERARTHRANDRIA.

The first antennae of the male are asymmetrical, one, usually the right, being used as a clasping organ.

The males of the Centropagidae, Candacidae and Pontellidae, besides possessing the asymmetrically modified thoracic limbs of the fifth pair also exhibit a modification of one of the first antennae, which is generally thickened in the middle, and has a peculiar joint in it, or geniculation, which enables it to be flexed and so used as a clasping organ for holding the female.

Fam. 1.—Centropagidae.—These Copepods are very common in the pelagic plankton, and some of the species vie with the Calanidae in plumed ornaments, e.g. Augaptilus filigerus, figured by Giesbrecht in his monograph. The use of these ornaments, which are possessed by so many pelagic Copepods, is entirely obscure.[[40]] Certain of the Centropagidae live in fresh water. Thus Diaptomus is an exclusively fresh-water genus, and forms a most important constituent of lake-plankton; various species of Heterocope occur in the great continental lakes, and certain Eurytemora go up the estuaries of rivers into brackish water.

An excellent work on the fresh-water Copepods of Germany has been written by Schmeil,[[41]] who gives analytical tables for distinguishing various genera and species. The three fresh-water families are the Centropagidae, Cyclopidae, and Harpacticidae (see p. [62]). The Centropagidae may be sharply distinguished from the other fresh-water families by the following characters:—The cephalothorax is distinctly separated from the abdomen; the first antennae are long and composed of 24–25 segments, in the male only a single antenna (generally the right) being geniculated and used as a clasping organ. The fifth pair of limbs are not rudimentary; a heart is present, and only one egg-sac is found in the female. The second antennae are distinctly biramous.

Diaptomus.—The furcal processes are short, at most three times as long as broad; endopodite of the first swimming appendage 2–jointed, endopodites of succeeding legs 3–jointed.