As an example of the larval history of the higher Crustacea, we may take the case of the Common Shore Crab, _Carcinus mænas_ (Fig. 25). The young stages are common in tow-net gatherings round the British coasts in the summer-time. The youngest larvæ (Fig. 25, A) are translucent little creatures about one-twentieth of an inch long. They have the head and front part of the body covered by a helmet-shaped carapace, with a long spine standing out from the middle of the back, and another projecting, like a beak, in front. The narrow abdomen or tail is very flexible, and can be doubled up under the body or stretched out behind; it ends in a forked telson. There are two pairs of swimming limbs, each with endopodite and exopodite, and the short antennules and antennæ are seen on either side of the rostrum. There are a pair of very large compound eyes, which are not set on movable stalks, but are under the front part of the carapace. The two-branched swimming feet are really the first and second maxillipeds (the mandibles, maxillulæ, and maxillæ, can be found in front of them), but none of the other thoracic limbs are yet developed, and, although the somites of the abdomen are distinct, there are no swimmerets. This type of larva is known as a _zoëa_, a name which was given to it when it was supposed to be an independent species of Crustacean. As a matter of fact, the zoëa just described is not quite the earliest stage of the Shore Crab, for when hatched from the egg it is without the spines on the carapace, and is slightly different in other respects. A few hours after hatching, however, it casts its skin for the first time, and becomes a fully-formed zoëa. It swims rapidly about at the surface of the sea, feeding on the minute floating animals and plants which are found there, and growing in size with repeated castings of its skin. In the later stages of the zoëa the rudiments of the hinder thoracic limbs and of the swimmerets appear as little buds. In the next stage (Fig. 25, B) all the appendages are present, the dorsal spine of the carapace has disappeared, the eyes are stalked and movable, and the animal has all the appearance of a little Crab, except that the abdomen is stretched out instead of being tucked up under the body, and the swimmerets are used as paddles in swimming. In this stage the larva, which is known as a _megalopa_, swims at the surface of the sea, but later it sinks to the bottom, and, moulting again, appears as a little Crab (Fig. 25, C), with tucked-up abdomen and swimmerets no longer adapted for locomotion. [Illustration: FIG. 26--LAST LARVAL STAGE OF THE COMMON PORCELAIN CRAB (_Porcellana longicornis_--SEE FIG. 41, p. 113). × 9. (After Sars.)]