[Illustration: 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.
The eggs, when laid, are deposited in a receptacle between the back and the shell of the female Daphnia, and after the young come into the water they undergo no transformations. Between each brood the Daphnia moults, and the egg receptacle is thrown off with the exuvia. After several changes of skin the young Daphniæ come to maturity and lay eggs, which produce successive generations of females throughout the spring and summer; but in the autumn males appear, and then the eggs are retained in the receptacle of the female and are not hatched till spring. If the female should moult after this, the case with the eggs in it is cast off with her outer skin, which then becomes a protection to the eggs during the winter, and they are hatched in spring, producing females.
The second order of gill-footed Crustacea are called Phyllopoda, because they have gills like the leaves of a book attached to their lamelliform swimming feet. Their bodies are divided into many segments, and they form two groups, one of which has a carapace, the other has not. The Apus cancriformis is an example of the first. It is about two inches and a half long, and is a large animal compared with the others of its class. Its head and thorax are covered by an oval carapace, and its cylindrical body is composed of thirty articulations. It has a compound movable eye in the middle of its forehead, and a sessile eye on each side of it. All the members that follow the apparatus of the mouth have a foliaceous form, and are in constant motion even when the animal is at rest. The Apus has sixty pairs of jointed legs; the number of joints in these and in the other appendages is estimated to be not less than two millions. However, the instruments chiefly used for locomotion are the first pair of feet, which are very long and serve for oars; with these the animal can swim freely in any position, but when they are at rest it floats on the surface of the stagnant water in which it lives, and the fin feet maintain a constant whirlpool in the water, which brings the small animals on which it feeds to its mouth.