The Mysis has two pairs of jaw-feet differing little from feet; five pairs of thoracic feet, all thin and divided into two branches, which increase in length as they are nearer the tail, and are all provided with a ciliated appendage to adapt them for swimming. In the female, broad horny plates, attached to the two last pairs of legs, are bent under the body so as to form a kind of pouch, destined to lodge the eggs and the young during the first period of their lives, whence their name, ‘Opossum Shrimps’: the young are crowded in this pouch, and acquire their adult form before they come into the water. The circulation of the white blood of the Mysis was discovered by Mr. Thompson: the pulsations of the heart are so rapid that they resemble vibrations. There are many species of these small shrimps. [Illustration: Fig. 149. Lucifer, a stomapod crustacean.] The genus Lucifer is one of the most singular of the crustaceans from its almost linear form (fig. 149), the excessive length of the anterior part of the head, the extreme shortness of the thorax, the smallness of the carapace or shell, and the great development of the tail, which is more than three times as long as the thorax. The thin eye-stalks, which are of exaggerated length, extend at right angles from the top of the long cylindrical part of the head, and terminate in large, staring, dark-coloured eyeballs covered with a multitude of facettes. The two pairs of antennæ are placed between and below the eye-stalks. The undermost pair, which are the shortest, have a little lamellar appendage at their base: in some Lucifers, when viewed in front, it looks like a cross. The salient mouth is placed at the base of the long organ that carries the eye-stems. It has strong toothed mandibles, two pairs of jaws with plates attached to each jaw, and three pairs of foot-jaws. The tail is very narrow, consisting as usual of seven rings movable on one another; but they are quite abnormal, for each of the rings is at least as long as the thorax; the last has five plates spreading like a fan. All the bristly feet, which seem to hang loosely down from the animal, are fitted for swimming; those of the tail have long ciliated plates in their basal joints. These creatures are small, and inhabitants of warm seas. _Amphipoda._