Fig. [133].—Lambrus miersi, × 1. (After Milne Edwards and Bouvier.)
Fam. 3. Hymenosomatidae. The carapace is thin and flat; the chelipedes are neither very long nor especially mobile. There are no orbits, and the male openings are on the sternum. Characteristic of the Antarctic seas. Hymenosoma, Trigonoplax.
Tribe 5. Catometopa.
These Crabs resemble the Cyclometopa in general appearance, but the carapace is very square in outline, and its margins are never so well provided with spines as in the Cyclometopa. The position of the male genital openings is peculiar, since they lie upon the sternum, and are connected with the copulatory appendages upon the abdomen by means of furrows excavated in the sternum. The Catometopa are either littoral or shallow water forms, or else they live entirely on land. The Grapsidae are marine Crabs, Pachygrapsus marmoratus (Fig. [134]) at Naples being exceedingly common on rocks at high-water mark, over which it scuttles at a great rate; in the Mediterranean it takes the place of our common Garcinus maenas, which is not found there.
Fig. [134].—Dorsal view of Pachygrapsus marmoratus, × ⅓. (From an original drawing prepared for Professor Weldon.)
Among the land genera are Ocypoda, Gelasimus, and Gecarcinus of tropical lagoons and coastal swamps. Ocypoda often occurs in vast crowds in these regions, and digs burrows in the sand.
Fig. [135].—Gelasimus annulipes, × 1. A, Female; B, male. (After Alcock.)