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.
Phyllopoda.
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.
The Branchipes stagnalis, which may be taken as a type of the second order, has a perfectly transparent segmented body nearly an inch long, eleven pairs of pale red gill-feet, antennæ of bluish green, and a long tail ending in red bristles. The head has two large eyes on movable stems, and a sessile black oculus between them. Filiform antennæ spring from the upper part of the head; the other pair, like two large horns, are turned downwards. The last ring of the swimming tail has two plates with ciliated appendages.
The Artemia salina differs very little from the Branchipes. It abounds so much in the brine pans at Lymington and other salt works, as to give a red tinge to the nearly concentrated brine, the temperature of which is so high that no other animal could live for a moment in it.
Pycnogonoïdea or Spider Crabs.
Some of the Spider crabs hook themselves to fishes, while others live under stones, or sprawl with their long hairy legs over sea-weeds, and feed on the gelatinous matter these weeds afford. The throat with its members, and the head soldered to its first ring, forms nearly the whole animal. It has a pair of antennæ and four rudimentary eyes, set on a tubercule. A proboscis-like projection extends from the front; the mouth is furnished with cilia and one pair of foot-jaws. Four pairs of long hairy legs proceed from the throat, spread widely on each side, and end in a hooked claw. The stomach, which occupies the centre of the animal, sends off five pairs of long closed tubes like rays; one pair enters the foot-jaws, the others penetrate the legs. This digesting system is in a state of perpetual vermicular motion, which, as well as the movements of the animal itself, aërate its transparent blood through the skin, by keeping it in circulation. So this insignificant-looking creature has a very curious and complicated mechanism.[[39]]
Fossil Crustacea.
Analogues to the Anomura are found in the Chalk formation, but the Macrura are the prevailing forms. Extinct species of lobster, crawfish, and shrimps are met with in the secondary strata, from the Chalk to the Coal measures. In the Coal formation all these higher forms disappear, but then the gigantic King Crab, or Limulus, is found accompanied by the minute Entomostracan forms in infinite variety of species.