According to Professor Gegenbaur, the Sapphirina fulgens is a true Copepod and the Mediterranean Phyllosoma is a Decapod, although it has a lacunar blood system.
Some genera of the order Copepoda inhabit salt water, others fresh, as the Cyclops quadricornis (fig. 150), which abounds in the water with which London is supplied.
[Illustration: Fig. 150. Female Cyclops:—_a_, body; _b_, tail; _c_, antenna; _d_, antennule; _e_, feet; _f_, plumose setæ of tail; B, tail, with external egg-sacs; C, D, E, F, G, successive stages of development of young.]
The genus Cyclops is a type of the bristly-footed group, distinguished by a single compound eye placed in the middle of the forehead. The head and thorax are almost entirely covered with an oval jointed buckler, which has an opening below to let the bristly limbs pass through (fig. 150); and the tail, which is five-jointed, ends in two plates furnished with bristly plumes. It is traversed by the intestine, which ends near its extremity. The brilliant little eye in front consists of a number of simple eyes placed under one glassy cornea. It rests upon the base of a cone of muscular fibres, which give it a movement of rotation upon its centre. Its upper pair of antennæ, situated below the eye, spread to the right and left. In the female they have numerous joints with a bristle at each joint; the lower pair of antennæ are short-jointed and bristled. The mouth of the Cyclops has a pair of jaws, and two pairs of foot-jaws covered with bristles. The five pairs of branching legs, which are fitted for swimming, are thickly beset with plumose tufts. In the female the egg-sacs are hung on each side of the tail (B, fig. 150) by a slender tube, through which the eggs pass from the ovary within the mother into the sacs where they are deposited in rows, and there they remain till hatched. When the larvæ come into the water the sacs drop off, and the young undergo various changes before coming to maturity, as shown in fig. 150. The Cyclops swims with great activity, striking the water with its antennæ, feet, and tail; and the rapid movement of its foot-jaws makes a whirlpool in the water which brings minute animalcules to its mouth, and even its own larvæ, to be devoured.