Some species of the Calanus, a marine genus of the one-eyed group, are eminently social. Professor Dana found that the colour of those vast areas of what the sailors call bloody water, met with off the coast of Chili, was owing to shoals of the Calanus pontilla; and another immense area of bloody water he met with in the North Pacific was owing to a vast multitude of the Calanus sanguineus. Although this genus abounds more in individuals in the temperate seas, the species are more varied in the tropical. Those figured and described in Captain Maury’s works were mostly microscopic and very beautiful; one fished up was grey with a bunch of yellow feathers at the end of its tail. The egg-bags were purple, another was green marked with scarlet tufted antennæ longer than itself spread out at right angles from its head. This creature shone with a bright phosphorescent light, visible even when a candle was burning. These and many more were taken in tropical seas. They were remarkable for the length of their antennæ; and it was observed that no eyes were perceptible in such Crustacea as had these exaggerated antennæ; these organs of intelligence and warning were probably sufficient for their wants. When animals live without eyes on the surface of a tropical sea, it is quite conceivable that similar instruments of touch may suffice for those who live in the dark abyss below.
The Ostrapods, which form the second order of the bristly-footed Crustacea, are defended by a bivalve carapace; they have swimming limbs and a confluent eye; that is, a number of simple eyes placed under a glassy cornea.
The genus Cypris belongs to this group. Several species may be seen swimming in our streams and fresh-water pools. The body of the common Cypris (fig. 151) is enclosed between two flat oval shells, united by a hinge on the back. The little animal can open and shut the valves by means of two slender muscles, extending from its back to the shells, which are much curved above and rather flat below. There are two pairs of antennæ beneath the eye, they are perfectly transparent, many-jointed, and end in tufts of filaments. One pair projects forward and then bends gracefully backwards; the other pair are bent downwards. The mouth has no foot-jaws, and there are only two pairs of feet. Only one pair is seen in the female, for the other pair is bent upwards to support the egg sacs. The Cypris attaches her eggs to the leaves of aquatic plants by a greenish fibre. Not more than twenty or thirty eggs are deposited by one individual, while the heaps contain several hundreds; so many females contribute to form one heap. The young are hatched in the form of their parent in about four days and a half. As the pools dry up, the Cyprides bury themselves in the sand or mud at the bottom; if that remain moist they survive, if it becomes dry they perish; but the eggs remain dormant till the return of rain, when they are hatched, and the surface of the water is soon crowded with a swarm of young Cyprides.