Fam. 6. Thelphusidae (Potamonidae).—Fresh-water crabs, with the branchial region very much swollen. Thelphusa (or Potamon) has nearly a hundred species distributed from North Australia, through Asia, Japan, the Mediterranean region, and throughout Africa. Potamocarcinus in tropical America.
Tribe 4. Oxyrhyncha.
This section includes the Spider-crabs and related genera, in which the carapace is triangular, with the apex in front formed by a sharply-pointed rostrum. There are two chief series, the one comprising the Spider-crabs, with much elongated walking legs, e.g. the huge Maia squinado of European seas, the yet more enormous Macrocheira kämpferi from Japan, supposed to be the largest Crustacean in existence, and sometimes spanning from outstretched chela to chela as much as eleven feet, and the smaller forms, such as Inachus, Hyas, and Stenorhynchus, which are so common in moderate depths off the English coasts. The other series is represented by genera like Lambrus (Fig. [133]), in which the legs are not much elongated, but the chelipedes are enormous.
The Spider-crabs do not burrow, and their respiratory mechanism is simple; but since they are forms that clamber about among weeds, etc., upon the sea-bottom, they often show remarkable protective resemblances to their surroundings, which are not found in the burrowing Cyclometopa. Alcock[[156]] gives a good account and figure of Parthenope investigatoris, one of the short-legged Oxyrhyncha, the whole of whose dorsal surface is wonderfully sculptured to resemble a piece of the old corroded coral among which it lives.
But besides this, the long-legged forms, such as Inachus, Hyas, etc., have the habit of planting out Zoophytes, Sponges, and Algae upon their spiny carapaces, so that they literally become part and parcel of the organic surroundings among which they live. It may, perhaps, be wondered what are the enemies which these armoured Crustacea fear. Predaceous fish, such as the Cod, devour large quantities of Crabs, which are often found in their stomachs; and Octopuses of all sorts live specially upon Crabs, which they first of all paralyse by injecting them with the secretion of poison-glands situated in their mouth. The poison has been recently found by Dr. Martin Henze at Naples to be an alkaloid, minute quantities of which, when injected into a Crab, completely paralyse it. When the Crab is rendered helpless the Octopus cuts out a hole in the carapace with its beak, and sucks all the internal organs, and then leaves the empty shell.
Many of the Oxyrhyncha are found in the abysses; among them are Encephaloides armstrongi (Fig. [132]), dredged by Alcock from below the 100–fathom line in the Indian Ocean, which has the gill-chambers (G) greatly swollen and enlarged to make up for the scarcity of oxygen in these deep regions.
Fig. [132].—Encephaloides armstrongi, × 1. The long walking legs are omitted. C, Great chela; G, one of the greatly swollen gill-chambers. (After Alcock.)
Fam. 1. Maiidae.—The chelipedes are not much larger than the other legs, but are very mobile. Orbits incomplete. A very large family, including all the true Spider-crabs, very common in the Atlantic and Mediterranean littoral. Inachus, Pisa, Hyas, Stenorhynchus, Maia, Encephaloides (Fig. [132]).
Fam. 2. Parthenopidae.—The chelipedes are much larger than the other legs. Orbits complete. Lambrus (Fig. [133]), Parthenope.