Fam. 1. Calappidae.—Cephalothorax rounded and crab-like. The abdomen is hidden under the thorax, the antennae are small, and the legs normal in position. The afferent openings to the gill-chambers lie in front of the chelipedes. Male openings on coxae of last pair of legs. Calappa (Fig. [128]) circumtropical, and extending into the warmer temperate seas. Matuta (Fig. [129]) from the Indo-Pacific.
Fig. [130].—Dorsal view of Ilia nucleus, × 1. (From an original drawing prepared for Professor Weldon.)
Fam. 2. Leucosiidae.—Similar to the above, but the afferent openings to the gill-chambers lie at the bases of the third maxillipedes. Male openings on the sternum. This family contains a great number of forms, with headquarters in the tropical littoral, but extending into the temperate seas. Ilia in the European seas. I. nucleus (Fig. [130]) common in the Mediterranean. Ebalia in the Atlantic, North Sea, and Indo-Pacific. Leucosia in Indo-Pacific.
Fam. 3. Dorippidae.—Cephalothorax short and square. The abdomen is not hidden under the thorax; the antennae are large, and the last two pairs of legs are held dorsally, and have terminal hooked claws. Dorippe, littoral in Mediterranean and Indo-Pacific. Cymonomus (Fig. [127]) from deep-sea of Atlantic and Mediterranean.
Fam. 4. Raninidae.—Similar to Dorippidae, but the cephalothorax is elongated, and the legs usually have the last two joints very broad. Several genera, chiefly in the deeper littoral zone. Ranina dentata in the Indo-Pacific.
Tribe 3. Cyclometopa.
In these Crabs the carapace is circular rather than square; its frontal and lateral margins are produced into spines and there is no pointed rostrum. The mouth is square, and the third maxillipedes are greatly flattened and form a lid-like expansion over the other oral appendages. This group includes the common Shore-crab of our coasts (Carcinus maenas), the swimming Crabs with expanded pereiopods (Portunus, Lupa, etc.), the Edible Crab (Cancer pagurus), and many others.
Corystes cassivelaunus is a Crab of doubtful affinities. It is sometimes placed among the Oxyrhyncha, but, as Gurney[[154]] has pointed out, the Megalopa shows Portunid characters, and the resemblance to the Oxystomata in the front of the carapace and in the mouth may be secondary. The respiratory arrangement of this Crab has already been mentioned in comparing its structure with that of the Mole-crab Albunea. The form of the antennal tube can be gathered from the figure of the Megalopa stage (Fig. [125], p. 183). It should be noted that when the Crab is buried in the sand with only the tip of the antennal tube projecting, the water is sucked down and enters the branchial cavities anteriorly, the antennal tube being continued by a tube formed from the third maxillipedes and the forehead; the water is exhaled at the sides of the branchial cavities beneath the branchiostegites. Thus in Corystes the normal direction of the current is reversed, but when the Crab is not buried, and is moving over the surface, it breathes in the usual manner, taking in the water at the sides of the branchiostegites and exhaling it anteriorly by the tube. The related Atelecyclus, found like Corystes very commonly at Plymouth, uses two methods of breathing: when it is in the surface-layers of sand it makes use of its antennal tube, which is, however, much shorter than in Corystes; but when it burrows deeper, where the antennal tube is no use, it folds its chelipedes and also its other legs, which are densely covered with bristles, so as to form a reservoir of pure water underneath it free from sand, which it passes through the gill-chambers in the usual manner (see Garstang, loc. cit. p. 186).
The respiratory adaptations in Lupa hastata and their convergence towards those of the Oxystomatous Matuta have been already touched upon (pp. 186, 187).