Small Crustaceans, having the head and thorax covered by a carapace, and furnished with five pairs of natatory feet. Usually there are two caudal locomotive appendages. A distinct heart is sometimes absent (as in the Cyclo-pidae) but is sometimes present. Both marine and fresh-water Copepods are known.
The larvae of the Copepods are Naupliiform, with unpaired eyes, three pairs of limbs (the future antennae and mandibles), and two terminal setae. Next the maxillae are produced, and then three other pairs of limbs (the foot-jaws and the two front pairs of natatory feet). At the next moult, the larva assumes the Cyclops form, but has at first much fewer limbs and somites.
In the Cyclops (fig. 145, c), which is one of the commonest of the "Water-fleas," the cephalothorax is protected superiorly by a carapace, and the abdominal somites are conspicuous. In front of the head is situated a single large eye, behind which are the great antennae and the antennules. The feet are five pairs in number, each consisting of a protopodite and a segmented exopodite and endopodite, usually furnished with hairs, and forming an efficient swimming apparatus. The young pass through a metamorphosis, and are not capable of reproducing the species until after the third moult or change of skin. The female Cyclops carries externally two ovisacs, in which the ova remain till they are hatched. A single congress with the male is apparently sufficient to fertilise the female for life.
The Copepoda, or Oar-footed Crustaceans, are all of small size, and are of common occurrence in fresh water in all parts of Europe. Many forms also live in the sea, sometimes in immense numbers. Thus Cetochilus is so abundant in the North and South Atlantic, as to communicate a ruddy tinge to the ocean, and to serve as one of the principal articles of diet of the whale. By good authorities the Ichthyophthira are regarded as merely Copepoda peculiarly modified to suit a life of parasitism.
The Crustaceans included in this division have many branchiae, and these are attached to the legs, which are often numerous, and are formed for swimming. In other cases the legs themselves are flattened out so as to form branchiae. The body is either naked, or is protected by a carapace, which may enclose either the entire body, or the head and thorax only. The mouth is provided with organs of mastication.
The Branchiopoda comprise the Cladocera, the Phyllopoda, and probably the Trilobita, though this order departs in many respects from the first two groups. The Merostomata may be considered along with these, though these, too, are in many respects peculiar.