CRUSTACEA OF THE LAND
There is every reason to believe that the Arthropoda, like the other
great groups of the animal kingdom, had their origin in the sea; but
they must have invaded the dry land at a very early period, and most of
the classes into which the group 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 comparatively few cases have they succeeded in adapting
themselves completely to an air-breathing 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 warmer seas
voluntarily leave the water and scramble about among the rocks between,
and even above, tide-marks. Some Crabs, like _Ocypode_ and _Gelasimus_
(see Plate XV.), have gone farther towards becoming land-dwellers, since
their gill chambers are adapted to serve as lungs for breathing air,
and some species may even be drowned by keeping them in water. The
marsh-dwelling or fresh-water Crabs of the genus _Sesarma_ (see Plate
XXIII.) and allied genera are also apparently to some extent
air-breathers, and one species, _Aratus pisonii_, is stated by Fritz
Mueller to climb mangrove bushes and to feed on their leaves. Some
Crayfishes, like the _Engaeus_ 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 damp
places on land, although 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 Tasmania, being
found under logs 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 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 larvae 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 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 development.
[3] I am indebted to Mr. J. T. Cunningham for calling my attention to
some of the facts here recorded.
The Gecarcinidae 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 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 marine. The species of the genus _Coenobita_ (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."
The Coenobites often climb into bushes in search of food, and Dr. Alcock
"once found one of them busy, like a large bee, among the florets of a
coconut, which made me wonder whether they may not sometimes play a part
in fertilizing flowers." They are, however, by no means exclusively
vegetarians. 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 strewn with 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 swarms
of large Hermit Crabs (_Coenobita_), for again and again we found
recently killed birds, in all the beauty of their first speckled
plumage, being torn to pieces by a writhing pack of these ghastly
Crustaceans. There were plenty of large Ocypode Crabs, too (_O.
ceratophthalmus_), aiding in the carnage."
On Christmas Island Dr. Andrews found a species of _Coenobita_ not
unfrequently in the higher parts of the island far from the sea, and he
remarks that the occurrence of large marine shells high up on the hills
seemed very puzzling until 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 abdomen serve as accessory hearts in
promoting a specially active circulation in that part of the body. The
lining membrane of the gill chambers also appears to aid in respiration
as in other terrestrial Decapods.
[Illustration: _PLATE XXVII_
THE COCONUT CRAB, _Birgus latro_. (MUCH REDUCED)]
The "Robber Crab" or "Coconut Crab" (_Birgus latro_--Plate XXVII.)
also belongs to the family Coenobitidae, and has attracted much notice
from its relatively gigantic size and its singular habits. Although
resembling _Coenobita_ closely in essential structure, _Birgus_ differs
from it and from most other Hermit Crabs in not making use of a portable
shelter, perhaps owing to the difficulty of obtaining one of suitable
size. The necessary protection for the abdomen is obtained by a
redevelopment of the shelly plates (terga) on the upper surface of the
abdominal somites. The abdomen is carried doubled underneath the body to
protect the soft under-surface, and the animal, when threatened, seeks a
shelter for its vulnerable hinder part in the nearest hole or cranny.
The swimmerets are absent in the male sex, and are present only on one
side of the abdomen in the female. This unsymmetrical development of the
appendages is interesting as indicating the derivation of the Robber
Crab from ancestors adapted to living in the unsymmetrical shells of
Gasteropod Molluscs. The last pair of abdominal appendages, which in
other Hermit Crabs serve to hold the body in the shell, are here much
reduced in size, and quite useless for that purpose. The carapace is
very broad posteriorly, owing to the great development of the branchial
cavities, which are much too capacious for the very small gills. As in
the true Land Crabs, the lining membrane of the gill cavity is thick and
spongy, and traversed by numerous bloodvessels; but in this case its
efficiency as a lung is added to by numerous tufted papillae, which
increase the surface exposed to the air.
As in other Hermit Crabs, the last two pairs of legs are shorter than
the others, and they end in small chelae. The last pair are very slender,
and are usually carried folded up within the gill chambers, which they
possibly serve to keep clear from foreign bodies. The penultimate pair
of legs are stouter, and the two pairs in front of these are long
walking legs. The chelipeds are very strong and are of unequal size.
When attacked, the animal defends itself, not, as might have been
expected, with its chelipeds, but with the first pair of walking legs,
the sharp points of which form very efficient weapons.
The statement that the Robber Crab climbs lofty trees was first made by
the Dutch naturalist Rumphius, in the beginning of the eighteenth
century. Its accuracy has been often doubted or denied since then, and
only finally put beyond dispute by a photograph taken on Christmas
Island by Dr. Andrews, which shows one of these Crabs in the act of
descending the trunk of a sago-palm. It seems not impossible that the
habits of the animal may vary to some extent in different localities,
and that where food is abundant on the ground the tree-climbing habit
may be in abeyance. If this were so, it would explain the very definite
statements made by some observers, that _Birgus_ does _not_ climb
trees.
In localities where coconut palms abound, _Birgus_ feeds largely on the
nuts, tearing off the fibrous outer husk and breaking open the shell by
hammering with its powerful claws at one of the "eye-holes." According
to Darwin in his "Naturalist's Voyage," the pincers of the penultimate
pair of legs are used for extracting the contents of the nut, but this
observation does not seem to have been confirmed. In spite of its name
of "Coconut Crab," however, _Birgus_ by no means feeds exclusively on
coconuts. On Christmas Island, where until recently there were no
coconut palms, the Crabs are exceedingly abundant, and, according to Dr.
Andrews, they "eat fruits, the pith of the sago-palm and the
screw-pines, dead rats and other carrion, and any of their fellows that
may have been injured.... They are excellent scavengers, and have a
curious habit of often dragging their food long distances before
attempting to eat it. I have seen a Crab laboriously pulling a bird's
wing up the first inland cliff, half a mile or more from the camp whence
it had stolen it."
Large specimens of the Robber Crab may be at least a foot in length of
body when the abdomen is straightened out. Their great strength is
illustrated by the fact, related by Darwin, that specimens placed in a
strong biscuit-tin, of which the lid was secured by wire, escaped by
turning down the edges with their claws, and in doing so actually
punched holes quite through the tin.
The breeding habits and mode of development of the Robber Crab have
often formed the subject of inquiry by naturalists, but it is only
recently that Dr. Willey has been able to prove definitely that the
female visits the sea for the purpose of hatching off the eggs, and that
the young are hatched in the zoea stage. The larvae obtained by Dr.
Willey have been described by Mr. Borradaile, who finds that, as was to
be expected, they closely resemble those of _Coenobita_. There appears,
however, to be no such simultaneous migration of the Crabs towards the
sea as has been described in the case of the Gecarcinidae. The statement,
quoted by Darwin, that _Birgus_ visits the sea every night for the
purpose of moistening its branchiae, cannot be universally applicable,
since the Crabs are often found, as on Christmas Island, at distances
from the sea which put a nightly journey to it out of the question.
Of all Crustacea, the most completely adapted to terrestrial life are
the Land Isopods, or Woodlice, which may be found in every garden. It is
true that most species are found in damp places, although some that
inhabit the sandy deserts of Asia and Africa must be content with a very
slight degree of humidity; and in no case is their dependence on
moisture greater than, for instance, that of many Insects and Arachnids
which are regarded as typically terrestrial animals. Since there is
reason to believe that the Woodlice have been derived from marine
ancestors--they show no special affinities to the fresh-water Isopoda,
like _Asellus_--it is interesting to find that the most primitive forms,
which have departed least from the general Isopod type, are commonly
found on or near the seashore. The "Sea-slater," _Ligia oceanica_ (Fig.
63), which is abundant in rocky places on our own coast, is one of the
most primitive forms. It has a broad, flattened, greenish-brown body,
about an inch long, and it runs quickly, creeping into narrow crevices
of the rocks, so that it is not easy to catch. The antennules, as in the
other land Isopods, are very minute, but the antennae are long, and have,
besides the five segments which form the "peduncle," a "flagellum" of
about twelve short segments. The uropods or tail appendages are long,
each with two slender, pointed branches. On the under-side of the
abdomen can be seen the five pairs of pleopods, each with two plate-like
branches attached to a very short peduncle. As in most aquatic Isopods,
the plates of the pleopods are soft and thin, and appear adapted to act
as gills, although the outer plate of each pair is somewhat stiffer than
the inner. The Sea-slater is generally found just above high-water mark,
probably always within reach of the salt spray, and it is said sometimes
to enter the water of rock-pools.
[Illustration: FIG. 63--THE SEA-SLATER (_Ligia oceanica_). ABOUT TWICE
NATURAL SIZE. (After Sars.)]
In almost every garden there may be found, under flower-pots and the
like, a Woodlouse, about two-thirds of an inch long, of a brown colour,
with yellowish blotches arranged in a row on each side of the back. This
is _Oniscus asellus_, a species widely distributed in Europe and North
America. It has the antennae shorter than in _Ligia_, and the flagellum
is composed of only three segments. The uropods are quite short. The
endopodites of the pleopods are membranous gill-plates, which serve for
respiration in the moist air in which these animals generally live. The
exopodites are stiff plates which cover and protect the delicate
endopodites; it is probable that they also aid in respiration, for they
contain a system of minute channels, filled with air, where the cuticle
is separated from the underlying cells. As these channels are nowhere
open to the outside, the air must find its way in by diffusion through
the cuticle.
[Illustration: FIG. 64--STRUCTURE OF THE BREATHING ORGANS OF _Porcellio
scaber_. (From Lankester's "Treatise on Zoology," after Stoller.)
A, Exopodite of first pleopod, showing the tuft of air-tubes
("pseudo-tracheae"), seen through the transparent cuticle; B, vertical
section through same; C, part of section more highly magnified. _art_,
Point of attachment of exopodite to peduncle; _c_, cuticle; _gr_,
grooved area of cuticle; _hy_, hypodermis, or layer of cells under the
cuticle; _n_, nucleus of hypodermis cell of air-tube; _o_, external
opening; _tr_, air-tubes]
Even more abundant than _Oniscus asellus_, and often found together with
it, is _Porcellio scaber_ (see Fig. 20, p. 51). It is usually of a dark
bluish-grey, but occasionally it is irregularly mottled with a lighter
colour. The flagellum of the antenna has only two segments. The most
interesting difference from _Oniscus_, however, is found in the
pleopods. If the under-side of the living animal be examined with a
pocket lens, a white spot will be seen on each exopodite of the first
two pairs of pleopods. When the structure of the pleopods is
investigated by means of microscopic sections (Fig. 64), it is found
that the white spots are tufts of fine branching tubes radiating into
the interior of the exopodite from a slit-like opening on the outer
edge. These tubes arise by an in-pushing of the integument, and they are
lined throughout by a delicate continuation of the external cuticle.
During life they are filled with air, and they serve to aerate the blood
circulating in the interior of the appendage.
[Illustration: FIG. 65--_Armadillidium vulgare._ x 2-1/2. (After Sars.)]
Another Woodlouse common in England is _Armadillidium vulgare_ (Fig.
65), a little slaty-grey species with a very convex body, which rolls
itself into a ball when touched. Like the last-mentioned species, it has
two segments in the flagellum of its short antennae, and it has tufted
air-tubes in the exopodites of the first two pairs of pleopods. It is
often mistaken for an animal of widely different structure, which it
superficially resembles--the Pill Millipede (_Glomeris marginata_). The
latter, however, may easily be recognized by having either seventeen or
nineteen pairs of walking legs (instead of seven pairs), set close
together in the middle line of the body, and by lacking the plate-like
pleopods. The resemblance between the two animals can hardly be regarded
as a case of "mimicry," since there is no reason to believe that either
benefits by its likeness to the other. As in so many other cases of
"convergent resemblance" between animals of different structure, it does
not seem possible to get beyond the vague suggestion that a similarity
in habits may have led, in some way that we do not understand, to a
similarity in appearance.
The presence of air-tubes in the pleopods of many Woodlice raises some
questions which are of importance with reference to the classification
of the Arthropoda as a whole. The Six-legged Insects, most Spiders and
many of their allies, the Centipedes and Millipedes, and the worm-like
_Peripatus_, all breathe air by means of fine tubes which penetrate
throughout the body, and bring the air into close contact with the
tissues. These tubes, which are known as "tracheae," arise as ingrowths
of the outer layer of the embryo, and are lined by a delicate
continuation of the external cuticle. It has been held by some
zoologists that so peculiar a system of breathing organs must indicate a
common descent of the animals that possess them, and accordingly it has
been proposed to separate the Insects, Arachnids, Myriopods, and
_Peripatus_, as a group, Tracheata, from the Crustacea and some other
Arthropods which have no tracheae. The air-tubes of the Woodlice,
however, are precisely like tracheae in structure and function, and only
differ from the tracheae of the other groups in the fact that they are
confined to the appendages, and do not penetrate into the body. Since
the Woodlice are a small and highly specialized branch of the Crustacea,
we can hardly suppose that they derive their tracheae from any ancestral
type which they had in common with the widely different Arachnids, for
example; and if tracheae have been evolved independently in these two
groups, there seems no reason why those of the Insects may not have
arisen independently of either. This is only one example out of many
which go to show that, in attempting to reconstruct the genealogy, or
phylogeny, as it is called, of the animal kingdom, we must constantly
admit the possibility of "convergent evolution."
Although Woodlice are very common animals, comparatively little is known
of their habits. They seem to live chiefly on vegetable food, and
sometimes damage seedlings and tender plants in gardens and greenhouses,
but occasionally they are carnivorous, and even cannibalistic, in their
habits. A few species live as "guests" in ants' nests, and one of these,
the little blind white _Platyarthrus hoffmannseggii_, is common in many
localities in this country. Why the ants tolerate their presence we do
not know, for they do not seem to render any service to their hosts, as
do the plant-lice and some other insects that are kept by the ants for
the sake of the secretions which they yield.
The Woodlice, like some other Isopoda, have a peculiar method of
moulting. Instead of the whole exoskeleton being cast off at one time,
as in other Crustacea, that of the hinder half of the body is moulted
first, and it is only after two or three days, when the new cuticle has
hardened, that the exoskeleton of the anterior half follows. As a result
of this arrangement, it occasionally happens that specimens are found
with the fore part of the body differing in colour from the hind part,
owing to the one having been moulted more recently than the other.
Woodlice occur in most regions of the globe, and one of the most
remarkable features of their geographical distribution is the extremely
wide range of certain species. This is probably due, at least in many
cases, to their accidental transport by human agency. Thus, _Porcellio
scaber_, so common in this country, is also found in great abundance in
New Zealand; but Professor Chilton notes that it is usually found near
buildings, and only rarely in the native bush, so that there can be no
doubt that it has been introduced by artificial means.