^C. flamma^, the box-crab. This singular animal lives on sandy and muddy bottoms offshore, from North Carolina southward. The carapace is broad and straight on the posterior side, and is curved on the anterior side, narrowing to the front. The posterior side has prominent denticulations. The body is one and a half inches thick, about four to five inches wide, and two to three inches long. The chelæ are large, broad, and flattened, and are so arranged that when flexed they fit closely together across the front. When folded, and the small legs are withdrawn under the carapace, the animal is shut up as if in a box, and resembles a shell. When in danger it closes its doors, as it were, and abandons itself to the waves, which often carry it ashore. The crested claws resemble the head of a cock. (Plate LXIV.) FAMILY ~PINNOTHERIIDÆ~ GENUS ^Pinnotheres^ [Illustration: /Pinnotheres ostreum/, the oyster-crab; male, enlarged four diameters.] ^P. ostreum^, the oyster-crab. The female of this species lives in the gill-cavity of the oyster, and is particularly abundant in oysters from the Chesapeake. The males are seldom seen, and rarely occur in the oyster, but swim freely about. They are smaller than the female, have a firmer shell, and are dark brown above, with a dorsal stripe and two conspicuous spots. The under side of the legs is whitish. The female is commensal, at least in the adult form, and its thin, whitish, transparent carapace is tinged with pink. The species ^P. maculatum^ lives in /Mytilus edulis/ (mussel) and in the smooth scallop, /Pecten magellanicus/. The oyster-crab is a true messmate, and its presence in the oyster may be advantageous in helping to provide food for its host. This crab, like the rest, holds its eggs in the posterior feet until hatched, when the larvæ leave the parent and swim about for a while. The females, at the megalops stage, enter oysters—sometimes two enter the same oyster, but seldom more than one; there it remains permanently, growing to the size of an inch or more in diameter, and becomes a degenerate. The eyes become smaller; the shell never hardens, like its allies which live in open water; its limbs and chelæ are weak; and it has no pugnacity, the protection afforded by the oyster doing away with the need for the common protective features of its kind. /Pinnixa cylindrica/, a related species, lives in the tubes of large annelid worms as a commensal. [pg288] ORDER =STOMATOPODA= GENUS ^Squilla^ [Illustration: /Squilla empusa/, the mantis-shrimp.]