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.
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.
Fig. 151. Cypris.
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.
Fig. 152. Section of Daphnia pulex.
The Cladocera is the first order of the gill-footed Crustacea: their body is defended by a bivalve carapace; they have from four to six gill-footed limbs, one compound eye, and two pairs of antennæ, one pair of which is large and adapted for swimming. The Daphnia pulex, or Arborescent Water-flea, of which [fig. 152] is a section, is a common form of this tribe. It is very abundant in pools and ditches, coming in groups to the surface in the mornings and evenings in cloudy weather. The bivalve shell is transparent, flexible, and open below; it ends behind in sharp toothed peaks. The eye placed in front is moved by four muscles, and on each side of it are the great antennæ, which are jointed, branched, and garnished with feathery filaments, and are the chief organs of locomotion. This animal has no foot-jaws, but it has a nervous system and a heart, whose pulsations are repeated two or three hundred times in a minute, and the blood is aërated by gills at the extremities of six pairs of bristly feet situated behind the mouth, and only used for respiration and prehension.