A LAND HERMIT CRAB, Cœnobita rugosa. (REDUCED)

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The Gecarcinidæ are abundant in the tropics of the Old and New Worlds. Some of the species at least, probably all, visit the sea at intervals for the purpose of hatching off the eggs carried by the females, and the larval stages are passed in the sea. In the case of Gecarcinus ruricola ([Plate XXVI].), a species very common in the West Indies, the migration to the sea takes place annually during the rainy season in May. The Crabs are described as coming down from the hills in vast multitudes, clambering over any obstacles in their way, and even invading houses, in their march towards the sea. Stebbing states that "The noise of their march is compared to the rattling of the armour of a regiment of cuirassiers." The females enter the sea to wash off the eggs which they carry attached to their abdominal appendages, or rather, probably, to allow the young to hatch out. The Crabs then return whence they came, and are followed later by the young, which, having passed through their larval stages in the sea, leave the water, and are found in thousands clinging to the rocks on the shore.

On Christmas Island, in the Indian Ocean, Dr. C. W. Andrews studied the habits of another Land Crab, of which the proper name seems to be Gecarcoidea lalandii. He says: "This is the commonest of the Land Crabs inhabiting the island, and is found in great numbers everywhere, even on the higher hills and the more central portion of the plateau. In many places the soil is honeycombed by its burrows, into which it rapidly retreats when alarmed. These Crabs seem to feed mainly on dead leaves, which they carry in one claw held high over the back and drag down into the burrows. From their enormous numbers, they must play a great part in the destruction of decaying vegetable matter and its incorporation into the soil."

"Once a year, during the rainy season, they descend to the sea to deposit [or, rather, to hatch out] their eggs, and during this migration hundreds may be seen on every path down steep slopes, and many descend the cliff-face itself. They remain on the beach for a week or two, and ... afterwards gradually make their way back to their accustomed homes."

In the year of Dr. Andrews' first visit to the island (1898) this migration occurred in January. On a subsequent visit to the island in 1908 he obtained specimens of a large Megalopa larva (see [p. 70]) which occurred in enormous quantities in the sea shortly after the migration, and also of a small Crab which appeared in similar numbers at a slightly later date. It seems practically certain that these larvæ and young are those of Gecarcoidea lalandii. A second species of Land Crab, Cardisoma hirtipes, found on Christmas Island, has very different habits from the foregoing. Dr. Andrews says of it: "In this island, at any rate, this species must be regarded as a fresh-water form, and, in fact, when a specimen was seen it might be taken as an indication that fresh water was not far off. It lives in deep holes in the mud at the sides and bottom of the brooks." Dr. Andrews tells me that he never saw this species at or near the sea (in marked contrast to Gecarcoidea), and this agrees with the observations of other travellers on species of the genus Cardisoma, so that the breeding habits remain unknown. There is every probability, however, that in this case, also, the young stages are passed in the sea.

The student will find, in many textbooks on zoology, the statement that some Land Crabs of the genus Gecarcinus develop without metamorphosis. Although it is impossible, with our present knowledge, to state definitely that this is not the case, there is absolutely no evidence to support it, and it is an interesting example of the way in which erroneous statements sometimes gain currency in science.[3] It is based upon the fact that in 1835 Professor J. O. Westwood described the early stages of "a West Indian Land Crab," in a paper "On the Supposed Existence of Metamorphosis in the Crustacea," published in the Transactions of the Royal Society. Professor Westwood found that the embryos extracted from the egg possessed all the appendages of the adult except the swimmerets, and that young specimens clinging to the abdomen of the parent were perfectly-formed little Crabs. The specimens which he described were sent to him by the Rev. Lansdown Guilding, of St. Vincent, who also deals with the subject in a note published in the Magazine of Natural History in the same year. Neither Westwood nor Guilding refers to the Crab as a Gecarcinus, although Guilding calls it the "Mountain Crab," a name which Patrick Browne in 1756 gives to the Gecarcinus ruricola of Jamaica. So far as I am aware, the first writer to refer to Westwood's Crab as a Gecarcinus, was Professor T. Bell, who in his "British Stalk-eyed Crustacea," published in 1853, states that some of the original specimens had come into his possession. They consisted of the detached abdomens of female Crabs, with eggs and young adhering to them. It would be by no means easy to identify the species of Crab to which a detached abdomen belonged, and there is nothing in the whole history inconsistent with the supposition that these observations really relate to a River Crab of the family Potamonidæ, of which at least one species, Pseudothelphusa dentata, is known to occur on the island of St. Vincent. As we have already seen, some of these River Crabs are quite as much land animals as the Gecarcinidæ, and they are known to have a direct development.

The Gecarcinidæ possess well-developed gills, but in addition the gill chambers are modified for air-breathing, as in some other amphibious Crabs (Ocypode, Gelasimus, etc.). Each chamber is capacious and vaulted, and the lining membrane is thick and richly supplied with bloodvessels, and is folded so as to divide off the upper part of the chamber as a sort of pocket.

The Land Hermit Crabs of the family Cœnobitidæ are found on the coasts of all tropical seas. Like the Gecarcinidæ, they visit the sea periodically for the purpose of hatching off the eggs, and the larval stages are marine. The species of the genus Cœnobita ([Plate XXVI].) resemble the marine Hermit Crabs in general shape, and like them use the shells of Gasteropod Molluscs as portable shelters. Where shells are scarce, other hollow objects are occasionally utilized; for example, large individuals will sometimes carry about the shell of a broken coconut, and a specimen has been seen to walk off in a cracked test-tube discarded by a naturalist who was investigating their habits. In one instance Professor Alcock saw an individual "so big that it seemed to have given up hope of finding a house, and was wandering about recklessly, with its tail behind it all unprotected."