Part 10 (1/2)
The environment in which these subterranean animals live reseht, and the consequent absence of plant-life They etable debris washed down from the surface, but the food-supply must be scanty, for the water in which they live is usually very clear and free fro to find that nearly all of theans which have been described, frorants Whether the degeneration of the eyes is the direct effect of disuse, or is due to natural selection ceasing to keep the eyes up to the standard of usefulness, is a question which has been much debated, and its anse sure of it, would settle some of the most fundamental problems of the evolution theory
At all events, we do not find in any truly subterranean species large and peculiarly modified eyes like those of many deep-sea animals, and this may be associated with the cohted by phosphorescent organisms as the deep sea is In another respect these animals differ from those of the deep sea, for they are all colourless or nearly so; while many of the inhabitants of the deep sea, as we have already seen, are brilliantly coloured
CHAPTER IX
CRUSTACEA OF THE LAND
There is every reason to believe that the Arthropoda, like the other great groups of the aniin in the sea; but they must have invaded the dry land at a very early period, and roup is divided--the Arachnids, Myriopods, and Insects--are now predominantly terrestrial in their habits The Crustacea alone have remained for the most part aquatic animals, and only in a co the existence As already mentioned, a considerable number, both of marine and of fresh-water species, are more or less amphibious in their habits Thus, the common Shore Crab of our own coasts and the Grapsoid Shore Crabs of war the rocks between, and even above, tide-marks Some Crabs, like _Ocypode_ and _Gelasi land-dwellers, since their gill cha air, and so theenus _Sesarenera are also apparently to some extent air-breathers, and one species, _Aratus pisonii_, is stated by Fritz Muller to clirove bushes and to feed on their leaves Soaeus_ of Tasmania (see Plate XX), already mentioned, are practically land animals Finally, some Amphipoda, closely allied to the Sand-hoppers of British coasts, live in dah they do not show any conspicuous modifications of structure to adapt them to this mode of life Of one of these Amphipoda, _Talitrus sylvaticus_, Mr G Smith writes: ”This species of land-hopper is widely distributed in the highlands of Tass and leaves in the forests on Mount Wellington, and in very great abundance in the beech-forests on the mountains of the west coast”
It will thus be seen that it is impossible to draw any sharp distinction between aquatic and terrestrial Crustacea, and it is chiefly from motives of convenience that we have left to be dealt with in this chapter three groups of land-dwelling Crustacea--the Land Crabs of the family Gecarcinidae, the Land Hermits (Coenobitidae), and the Land Isopods, or Woodlice (Oniscoidea)
[Illustration: _PLATE XXVI_
A WEST INDIAN LAND-CRAB, _Gecarcinus ruricola_ (REDUCED)
A LAND HERMIT CRAB, _Coenobita rugosa_ (REDUCED)]
The Gecarcinidae 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 fees are passed in the sea In the case of _Gecarcinus ruricola_ (Plate XXVI), a species very coration to the sea takes place annually during the rainy season in May The Crabs are described as co over any obstacles in their way, and even invading houses, in theirstates that ”The noise of their iment of cuirassiers” The fes which they carry attached to their abdo to hatch out The Crabs then return whence they ca 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 co the island, and is found in great nuher 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 h over the back and drag down into the burrows Froreat 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 ration hundreds may be seen on every path down steep slopes, and many descend the cliff-face itself They reradually make their way back to their accustomed homes”
In the year of Dr Andrews' first visit to the island (1898) this ration occurred in January On a subsequent visit to the island in 1908 he obtained specialopa larva (see p 70) which occurred in enorration, and also of a shtly later date It see are those of _Gecarcoidea lalandii_ A second species of Land Crab, _Cardisoma hirtipes_, found on Christ Dr Andrews says of it: ”In this island, at any rate, this species arded as a fresh-water forht 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 rees with the observations of other travellers on species of the genus _Cardiso 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 y, the stateenus _Gecarcinus_ develop without h it is ie, to state definitely that this is not the case, there is absolutely no evidence to support it, and it is an interesting exaain 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 ees of the adult except the swi to the abdomen of the parent were perfectly-formed little Crabs The specimens which he described were sent to hi, of St
Vincent, who also deals with the subject in a note published in the _Magazine of Natural History_ in the sa refers to the Crab as a _Gecarcinus_, although Guilding calls it the ”Mountain Crab,” a naives 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 soinal specimens had come into his possession They consisted of the detached abdo to them It would be by no means easy to identify the species of Crab to which a detached abdo in the whole history inconsistent with the supposition that these observations really relate to a River Crab of the family Potamonidae, 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 Gecarcinidae, and they are known to have a direct developha my attention to some of the facts here recorded
The Gecarcinidae possess well-developed gills, but in addition the gill cha, as in some other amphibious Crabs (_Ocypode_, _Gelasimus_, etc) Each cha 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 Coenobitidae are found on the coasts of all tropical seas Like the Gecarcinidae, they visit the sea periodically for the purpose of hatching off the eggs, and the larval stages are enus _Coenobita_ (Plate XXVI) reseeneral shape, and like them use the shells of Gasteropod Molluscs as portable shelters Where shells are scarce, other hollow objects are occasionally utilized; for exae 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 as investigating their habits In one instance Professor Alcock saw an individual ”so big that it see a house, and andering about recklessly, with its tail behind it all unprotected”
The Coenobites often climb into bushes in search of food, and Dr Alcock ”once found one of the the florets of a coconut, which made me wonder whether theyflowers” They are, however, by no etarians The author just quoted describes a visit to Pitti Bank in the Laccadive Archipelago, the breeding-ground of two species of terns
The ground was everywhere streith the dead bodies and clean-picked skeletons of the young birds ”We soon discovered that one great cause of the wholesale destruction of young birds was the voracity of swarain we found recently killed birds, in all the beauty of their first speckled pluhastly Crustaceans There were plenty of large Ocypode Crabs, too (_O
ceratophthale”
On Christmas Island Dr Andrews found a species of _Coenobita_ not unfrequently in the higher parts of the island far froeuntil it was noticed that they were brought by the Hermit Crabs
The species of _Coenobita_ possess a very curious adaptation for aerial respiration The soft skin of the abdomen is traversed by a network of bloodvessels and acts as a kind of lung, and a pair of contractile vesicles at the base of the abdo a specially active circulation in that part of the body The lining ill chambers also appears to aid in respiration as in other terrestrial Decapods
[Illustration: _PLATE XXVII_