CRUSTACEA OF THE SEASHORE
The tract of seashore which is laid bare by the retreat of the tide
offers on most coasts a rich collecting-ground to the student of
Crustacea. In places where shelving, weed-covered rocks run out to sea,
innumerable Crustacea have their home in the rock-pools, or lurk in
crannies awaiting the return of the tide. On sandy beaches, at first
sight apparently barren of life, a closer search will reveal a whole
fauna, amongst which burrowing Crustacea of various orders are
prominent. Further, the shore collector will find from time to time
stray specimens of forms that have their proper habitat beyond low-tide
mark, and occasionally their remains are thrown in quantities on the
beach by storms. It is convenient, therefore, to treat the Crustacea of
the shore as a sample of those inhabiting the shallower waters of the
ocean. In these shallower waters--down to the limit where light no
longer penetrates from above, where vegetable life ceases, and where the
strangely modified inhabitants of the deep sea begin to appear--the
sea-bottom is perhaps the most densely populated of all parts of the
earth's surface. Nowhere, at all events, do we find so wide a range of
animal forms, from the simplest organisms (Protozoa) up to
highly-organized Vertebrates. Nowhere, perhaps, is the struggle for
existence more keen, and it is not without justice that some naturalists
have regarded the shallow waters of the sea as "one of the great
battle-fields of life," where, in the long course of evolution, the main
branches of the animal kingdom have had their origin.
Conspicuous among the animals of this region are Crustacea of all sorts
and sizes. To identify all the species that may be obtained in a single
haul of the dredge in British seas would sometimes be a hard task even
for the most expert student of the group. Our present purpose, however,
is not to compile a faunistic catalogue, but merely to give some idea of
the endless diversity of form, and to note a few of the "shifts for a
living"--of the ways in which structure and habit are adapted to the
conditions of life in the Crustacea of the shore and of shallow water.
Though it might seem that the heavily armoured Lobsters and the larger
Crabs would be sufficiently protected against most enemies when once
they have attained their full size, yet they are preyed upon by the
Octopus, which seizes them with its suckers and pierces their armour
with its powerful beak, injecting a poison that paralyzes its victims.
Some years ago a "plague" of Octopus very seriously affected the Lobster
fishery in the English Channel. To escape from enemies such as these,
the Lobsters and many Crabs have the habit of lurking in crevices of the
rocks, while in case of sudden alarm the Lobster may escape from danger
by swimming, or rather darting, with great swiftness, tail foremost,
through the water by powerful strokes of the abdomen and tail-fan. In
the more lightly armed Prawns and other Crustacea of the tribe Natantia,
which are characteristically swimmers, the power of rapid motion is
probably the chief means of protection against enemies. There is reason
to believe that the Lobsters have been derived from prawn-like swimming
forms which have sacrificed some of their agility in developing their
heavy armour-plating, retaining, however, the power of sudden and rapid
motion in emergency. This power, again, has been lost by the typical
Crabs (Brachyura), in which the abdomen is reduced in size and without a
tail-fan, so as to be useless for swimming. While most of the Crabs,
however, are somewhat slow of movement, trusting to their armour and
their powerful pincers for defence, the Swimming Crabs (Portunidae--Plate
XIII.) have reacquired the power of swimming by means of the
paddle-shaped legs of the last pair. Some of the tropical species of
Portunidae are probably the most expert swimmers among the Crustacea,
and are described as shooting through the water like fish.
[Illustration: FIG. 36--A COMMON HERMIT CRAB (_Eupagurus bernhardus_)
REMOVED FROM THE SHELL]
The Lobster's habit of seeking shelter in rock-crevices or under stones
is one which is shared by a very large number of shore Crustacea. From
some primitive kind of Lobster which discovered the advantages of a
portable shelter have been derived the Hermit Crabs. In rock-pools one
may often see whelk or periwinkle shells tumbling about with an
activity quite foreign to the nature of their original molluscan
inhabitants, and closer examination will show that each contains a
Hermit Crab, which retreats into the shell when disturbed. If extracted
from the shell, the Crab (Fig. 36) can be seen to be most beautifully
adapted to its peculiar mode of life. The abdomen is soft and spirally
twisted to fit into the interior of the spiral shell, and the uropods,
instead of forming a tail-fan, are modified into holding organs, with
roughened, file-like surfaces which can be pressed outwards against the
walls of the shell, and wedge the body so firmly that an attempt to drag
the animal forcibly from its retreat often results in tearing it in
half. The front part of the body, which is exposed when the animal is
walking, retains its shelly armour. One of the pincer-claws, most
commonly the right, is much larger than the other, and serves to block
the opening of the shell when the body is withdrawn into it. The next
two pairs of legs are long and slender, and are used for walking; but
the last two pairs are short, with a roughened surface at the end, and
serve to steady the body in the mouth of the shell. The swimmerets on
the right side of the body, which is pressed against the central pillar
of the shell, have disappeared, but those of the left side remain.
As the Hermit grows, it is necessary for him to remove from time to time
into a larger dwelling. It has been stated that he will sometimes
dispossess the rightful owner of a whelk-shell for this purpose,
dragging him out piecemeal and eating him; but other observers deny that
this ever happens, and in most cases, at all events, the Hermit is
content to wait until he finds an empty shell of suitable size. After
turning this over and exploring the interior with his claws, to satisfy
himself that it is unoccupied, he deftly whips the unprotected hinder
part of his body into the new habitation, keeping hold of the old one
meanwhile, so that he can return to it if the other proves unsuitable.
The Hermits are very pugnacious, and fight with one another for the
possession of desirable shells, the victor dragging his opponent out and
establishing himself in his place. Besides appropriating the shell of a
dead Mollusc, many Hermits seem to go into partnership with living
animals of various kinds, and some of these associations will be noticed
in a later chapter. A number of species adopt other dwellings than
molluscan shells, and some tropical Hermits, for instance, are found
living in the cavities of water-logged stems of bamboo (Fig. 37); while
others, relinquishing the advantages of a portable shelter, live in
holes in corals or in the canals of living sponges. Although in some of
these cases the body is straight, it usually shows traces of its
original adaptation to a spiral shell in having no swimmerets on the
left side.
[Illustration: FIG. 37--_Pylocheles miersii_, A SYMMETRICAL HERMIT CRAB.
(After Alcock.)
The upper figure gives an end view of the animal lodged in a tube of
water-logged mangrove or bamboo, its large claws closing the opening.
The lower figure shows the animal removed from its shelter.]
The only Hermits which have a full series of swimmerets are the
primitive Pylochelidae (Fig. 37), which come very near to what we imagine
the ancestral form of the group to have been like, and can hardly be
separated from the mud-burrowing, lobster-like Thalassinidea. A few
Hermits have given up altogether the use of any protective covering. One
of these is the Coconut Crab (_Birgus_), to be mentioned when we come to
deal with the Crustacea of the land. Another is the Stone Crab
(_Lithodes_--Plate VIII.) of our own seas, and its kindred, which have
redeveloped shelly plates on the back of the abdomen, but carry it
doubled up under the body like the true Crabs. These also preserve some
traces of the original twisting of the abdomen, and have swimmerets only
on one side.
Some Crustacea construct habitations for themselves. On turning over a
flat stone between tide-marks, one often finds a little mass of bits of
weed and rubbish attached to it, and if this be torn open a
greenish-brown, shrimp-like animal, about three-quarters of an inch
long, is seen slithering away on its side. This is an Amphipod
(_Amphithoe rubricata_) which builds the shelter for itself, sticking
the fragments together with threads of a cementing material produced by
glands on the surface of its body and legs. Other Amphipods construct
more neatly finished tubular dwellings of mud, or even of small stones,
which are attached to sea-weeds and the like; and some make portable
shelters of the same kind, which they carry about with them like the
caddis-worms of fresh-water streams.
Some of the true Crabs also employ portable shields for purposes of
defence or of concealment. The species of _Dorippe_ which are found in
tropical seas have the last two pairs of legs short, elevated on the
back so that they cannot be used for walking, and ending in a kind of
grasping claw. By means of these claws the Crab holds over its back some
object, generally one valve of a molluscan shell, sometimes even a
mangrove-leaf, to supplement the protection afforded by its carapace.
The "Sponge Crabs" (_Dromiidae_), of which one species, _Dromia
vulgaris_ (Plate IX.), occurs on the southern coasts of Britain, have
also the last two pairs of legs elevated on the back and used in a
similar way; but in this case the covering is usually a mass of living
sponge, one of the Sea-squirts (Tunicata), or some similar organism.
[Illustration: _PLATE XIII_
A SWIMMING CRAB, _Portunus depurator_. BRITISH. (REDUCED)
A SPIDER-CRAB, _Maia squinado_, DRESSED IN FRAGMENTS OF WEED. BRITISH
(REDUCED)]
Even more remarkable are the "masking" habits of the Spider Crabs
(Oxyrhyncha). In these the carapace is almost always covered with
sea-weeds, zoophytes, and other organisms which afford a very effective
disguise. For example, specimens of the British species of _Hyas_ (_H.
araneus_ and _H. coarctatus_) and _Maia_ (_M. squinado_--Plate XIII.),
which are very common on our coasts, readily escape the notice of the
collector, as they lurk in the rock-pools. They are slow-moving animals,
and the carapace and limbs are usually quite hidden by dense tufts of
growing sea-weed, sponges, and other organisms. By observing the Crabs
in an aquarium, it has been found that they actually dress themselves,
plucking pieces of weed and the like and placing them on the carapace,
where they are held in position by numerous hooked hairs. The
transplanted fragments continue to live and grow until the Crab appears
like a miniature moving forest. Still more strange is the fact that the
Crabs appear to be able in some degree to adapt the nature of their
covering to their surroundings. It has been found that specimens dressed
in sea-weeds, when placed in an aquarium among sponges, picked off
the weeds from their bodies and limbs, and planted fragments of sponge
in their place. Not only does this habit afford the Crabs protective
concealment, but it may also in some cases serve as a source of
food-supply. The late Dr. David Robertson, of Cumbrae, one of the most
observant of marine naturalists, saw the Crab _Stenorhynchus_ (or
_Macropodia_) _longirostris_ picking food-particles from among the
vegetation on its body, and conveying them to its mouth.
Many Crustacea of different orders seek concealment and protection by
burying themselves in sand. A pool left by the tide on a sandy beach may
at first sight appear empty of all life, but if it be watched for a
little while a greyish, shadowy form may often be seen to dart across
it, to settle on the bottom with a little puff of sand, and to
disappear. Even a close scrutiny of the spot will hardly discover
anything, but with a hand-net one may succeed in scooping up, before it
can dart away again, a specimen of the Common Shrimp (_Crangon
vulgaris_--see Fig. 78, p. 244), whose translucent body is finely
mottled with greyish-brown so as to match exactly the sand among which
it rests.
If a spadeful of sand from between tide-marks be stirred up in a bucket
of sea-water and allowed to settle for a few seconds, and the water then
poured off through a fine muslin net, a wonderful assemblage of minute
Crustacea may often be obtained. Numerous species of Ostracods,
Copepods, and Amphipods, and some Isopods, can be collected in this way,
and some of these, at least, show peculiarities of structure which
appear to be adapted to a sand-burrowing habit. Perhaps the most
remarkable Crustacea living in such situations, however, are the
Cumacea. In these, as already mentioned, the gills, which are attached
to the first pair of thoracic limbs, lie one on each side of the thorax
in a cavity enclosed by the carapace. These cavities are continued
forwards to the front of the head, where they unite in a single opening
from which a transparent tube (or a pair of tubes) can be protruded. It
appears probable that this very peculiar arrangement of the respiratory
system is adapted to enable the animals to breathe while buried in sand
or mud. The water is probably drawn in behind through the narrow slit
between the side-plates of the carapace and the bases of the legs, and
is expelled through the tube which is protruded from the front of the
head. In this way the delicate gills are protected from injury and kept
from becoming clogged with sand, while the effete water, loaded with the
products of respiration, is carried off to a safe distance, so that it
does not re-enter the gill chamber.
In the case of such minute forms, however, it is very difficult to
determine the precise details of their mode of life by observation of
the living animals. In the larger Decapods, which can be watched in
their natural haunts, or more closely in aquaria, many interesting
adaptations to burrowing in sand have been discovered. Many Crabs
belonging to the tribe Brachyrhyncha often take refuge in sand or
gravel, burying themselves till only the eyes remain exposed. The
Swimming Crabs (Portunidae--Plate XIII.) of our own coasts have been
found to use the paddle-shaped last pair of legs for digging as well as
for swimming. In the sand, the Crab keeps its large claws, or chelipeds,
folded close up to the front edge of the carapace, which is cut into
sharp, saw-like teeth. Between these teeth the water passes, to reach
the entrance to the gill chamber which lies at the base of each
cheliped, and in this way an efficient strainer is provided, which in
coarse sand at least prevents the clogging of the respiratory passages.
The out-going current of water from the gills passes through channels
that open on either side of the mouth-frame.
[Illustration: _PLATE XIV_
_Corystes cassivelaunus._ MALE (ON LEFT) AND FEMALE (ON RIGHT).
BRITISH (REDUCED)
_Albunea symnista_, ONE OF THE HIPPIDEA. INDIAN SEAS. (REDUCED)]
A more complex adaptation of structure to the habit of sand-burrowing is
found in the Masked Crab (_Corystes cassivelaunus_--Plate XIV.). This
Crab is common on the British coast, living in moderately deep water
wherever the bottom is sandy, and it has received its English name from
the fact that the furrows on the back of the carapace give it a
grotesque resemblance to a human face. It is noteworthy, among other
things, for the marked difference between the sexes, the male having
very long, slender chelipeds, while those of the female are quite
short. The most remarkable features of its organization, however, have
to do with its habit of burrowing in sand. The antennae, which in most
Crabs are extremely short, are in this species as long as the body, and
each bears a double fringe of stiff hairs disposed along the upper and
under sides of the antenna, but curved inwards, so that when the two
antennae are brought together parallel with each other, the hairs
interlock and form a long tube. At its base this tube communicates with
a space in front of the mouth, into which open the channels from the
gill chamber at the front corners of the mouth-frame. The Crab burrows
in fine sand, and the process is thus described by Professor Garstang:
"The Crab sits upright on the surface of the sand; the elongated,
talon-like claws of the four hindmost pairs of legs dig deeply into the
sand; the body of the Crab is thus forcibly pulled downwards by the grip
of the legs, and the displaced sand is forced upwards on the ventral
side of the body by the successive diggings and scoopings of the legs;
the slender chelate arms of the first thoracic pair assist in the
process of excavation by thrusting outwards the sand which accumulates
round the buccal region of the descending Crab." In this way the Crab
descends deeper and deeper, until nothing is visible above the surface
of the sand but the tips of the antennae. The antennal tube keeps open a
channel leading from the buried Crab to the water above. Since this
tube communicates at its base with the passages through which the water
passes _out_ from the gill chamber in most Crabs, it was assumed by the
older observers that the antennal tube served to carry the outflowing
water to the surface of the sand. It has recently been shown, however,
by Professor Garstang that when the Masked Crab is buried in sand the
normal respiratory current is reversed, water being drawn _down_ the
antennal tube, into the gill chambers, and passing out through the
openings at the base of the chelipeds which, when the Crab is not
buried, serve for its entrance.
Most, if not all, of the Crabs belonging to the tribe Oxystomata are
sand-burrowers, and the structure of the mouth parts characteristic of
the tribe appears to have been acquired as an adaptation to this habit.
As already mentioned, the mouth-frame in these Crabs is triangular
instead of square, being produced forwards between the eyes, and the
third maxillipeds, which cover it, are also elongated. In this way the
exhalent channels carrying the water from the gill chambers open on the
front margin of the head, and are exposed even when the Crab is buried.
In the different families of this tribe the inhalent openings by which
the water enters the gill cavities are protected in various ways, and so
arranged that respiration can go on without danger of the gills becoming
clogged by sediment.
The members of the tribe Hippidea (sometimes called "Mole Crabs"), among
the Anomura, have habits somewhat similar to those of the Crabs just
described. They are common on sandy beaches in the warmer parts of the
globe, and they burrow with great rapidity by means of the curved,
flattened end-segments of the legs. The carapace is generally smooth and
oval, and the body is compact, the short abdomen being folded up as in
the Crabs.
In _Albunea_ (Plate XIV.), which belongs to this tribe, a long "antennal
tube," which looks very like that of _Corystes_, is believed to have a
similar function in connection with respiration when the animal is
buried. In this case, however, the tube is formed, not by the antennae,
as in _Corystes_, but by the antennules, so that it affords a striking
example of the independent evolution of similar structures from quite
different origins.
_Hippa emerita_, which is found on the coasts of North and South
America, has the mouth parts imperfectly formed, and not adapted for
biting; and it is stated by Professor S. I. Smith that the animal feeds
in the way that an earthworm does, swallowing the sand through which it
burrows, and extracting the nutriment which it may contain. This habit,
however, is not followed by other members of the tribe, for Mr.
Borradaile found that a species of _Remipes_ in the Maldive Islands
could "easily be caught by a bait of Crab at the end of a line,
pouncing on it with its sharp maxillipeds, and allowing itself to be
flicked out of the sand if the rod be sharply lifted."
In the cases mentioned above, the Crustacea do not bury themselves much
below the surface of the sand, and do not form definite burrows; but
there are many Crustacea which live in open tunnels dug deep into the
sand. Some of these belong to the category of amphibious forms, to be
mentioned presently; but there are others which live in deeper water,
and of which the habits are less open to observation.
[Illustration: FIG. 38--_Callianassa stebbingi_ (FEMALE), A SAND-BURROWING
THALASSINID FROM THE SOUTH COAST OF ENGLAND. NATURAL SIZE]
Nearly all the Thalassinidea (Fig. 38) live in burrows, often of
considerable depth, in sand or mud. Although now classed with the
Anomura, these animals are lobster-like in form, loosely built,
generally with short carapace and long, soft abdomen. They have usually
very small eyes, which appear as if they were not of much use for
vision; and some of the hinder pairs of legs are short, and carried
folded against the sides of the body, probably for use when the animal
is moving up or down in its burrow.
Most of the Stomatopoda resemble the Thalassinidea in their mode of
life, and show some curious similarities to them in structure, although
by no means closely related. They are described as lying in wait for
prey at the mouth of their burrows, darting out on passing fish or other
animals, which they seize with their great saw-toothed claws, and
retreating with great rapidity to the bottom of the burrow.
Most of the Crustacea mentioned live below tide-marks, and at all events
are rarely seen when the sand in which they burrow is left bare by the
tide; but there are others, especially on tropical shores, which seem to
have their chief period of activity when the sand or mud banks on which
they live are exposed to the air. Chief among these amphibious forms in
the warmer seas are the Crabs of the genera _Ocypode_ and _Gelasimus_
and some of their allies.
[Illustration: _PLATE XV_
_Ocypode cursor._ WEST AFRICA. (REDUCED)
_Gelasimus tangeri._ MALE ABOVE, FEMALE BELOW. WEST AFRICA. (REDUCED)]
Some of the species of _Ocypode_ (Plate XV.) dig their burrows between
tide-marks, where they are swamped by the advancing tide, and must be
excavated afresh when the water retreats. Other species, however, live
above high-water mark, and are practically terrestrial animals, only
entering the water occasionally, and, indeed, unable to survive
prolonged immersion. The work of excavating the burrows has been watched
in several species. The Crab comes out of the burrow sideways, carrying
a load of sand between two of the walking legs on the rear side. By a
sudden movement the sand is jerked away to some distance, where it
accumulates in a little heap, and the Crab dives into the burrow for
another load. Most of the Crabs belonging to this genus possess a
curious "stridulating organ" on one of the large claws, by means of
which they can produce a buzzing or hissing sound. On the inner surface
of the "hand" there is a raised patch, which, when examined with a lens,
is seen to be made up of a series of fine ridges, like the teeth of a
file. When the limb is bent in towards the body, this patch can be
rubbed up and down against a sharp-edged ridge or scraper on the third
segment of the limb, and in this way the sound is produced. What the use
of the sound may be is not quite clear, but there is probability in Dr.
Alcock's suggestion that it serves to warn intruders that the burrow is
already occupied. These Crabs run very swiftly, and one species was seen
by Professor S. I. Smith to catch Sand-hoppers (Amphipods of the family
Talitridae) by springing on them suddenly, "very much as a cat catches
mice," but it also fed on dead fish and the like.
Of somewhat similar habits are the numerous species of the genus
_Gelasimus_ ("Fiddler Crabs"--Plate XV.), which abound on sand and mud
flats of tropical shores. These little Crabs are remarkable for the
great dissimilarity between the sexes in the form of the chelipeds. In
the female both chelipeds are small and feeble, but in the males one of
them, either the right or the left, is enormously enlarged, sometimes
exceeding in length and breadth the body of the Crab which carries it.
What the precise use of this enormous claw may be does not seem to be
quite certainly known. It is said to be used as a weapon by the males in
fighting with one another, but it seems too clumsy to be very efficient
for this purpose. It is often brilliantly coloured, and has been
supposed to be a sexual adornment.
In _Ocypode_ and _Gelasimus_ the respiratory apparatus is modified for
the purpose of breathing air. The gills are similar to those of purely
aquatic Crabs, and no doubt serve for respiration when the animal is in
the water; but the gill chambers are much more spacious than usual, and
the lining membrane is richly supplied with bloodvessels. Air is
admitted to the gill chambers by an opening, protected by a brush of
hairs, between the second and third pairs of walking legs on each side.
It is believed that in this way the gill chamber is fitted to be used as
a lung when the animals are out of the water. Similar arrangements in
some of the more exclusively terrestrial Crustacea will be mentioned in
a later chapter.
There are many Shore Crabs, however, which lead a more or less
amphibious existence without showing any marked modifications of
structure as compared with their more purely aquatic relatives. On our
own coasts, the Common Shore Crab (_Carcinus maenas_--Plate IX.) commonly
spends several hours each day exposed to the air, and in an aquarium it
will voluntarily leave the water if the opportunity be afforded it. On
tropical coasts the species of _Grapsus_ and allied genera are often
seen clambering with great agility about exposed rocks.
Analogous habits to those of the sand-burrowing, amphibious Crabs
described above are shown on a small scale by the Amphipods of the
family Talitridae, known as "Sand-hoppers" or "Beach-fleas." Everyone who
has walked over the firm sand near high-water mark on our own shores
must have noticed the myriads of actively hopping little creatures
disturbed at every step. The commonest species of Sand-hopper on the
British coasts is _Talitrus saltator_ (Fig. 39), but _Orchestia
gammarellus_ is also common. Both species occur together on sandy
beaches or among decaying sea-weeds, and are among the most important
scavengers of the seashore, picking clean the bones of fish or other
animals cast up by the tide. In this country the Sand-hoppers do not, as
a rule, venture far above high-water mark; but in warmer climates
species of Talitridae live in the damp forests at great distances from
the sea, and deserve to be ranked among the terrestrial Crustacea.
[Illustration: FIG. 39--THE COMMON SAND-HOPPER (_Talitrus saltator_),
MALE, FROM THE SIDE. x 3. (After Sars.)]
It has been mentioned above that the Common Shrimp is protected, not
only by its habit of lying half buried in sand, but also by its close
resemblance in colour to the sand among which it lives. There are many
others among the shore Crustacea which show what seems to be a
"protective resemblance" in colour and form to their surroundings. It is
necessary to be cautious in interpreting these resemblances as
necessarily protective, since the fish and other enemies which prey on
these Crustacea see them with eyes very different from ours, and
probably, in many cases, are guided to their prey by the sense of smell
rather than by sight. The "masking" habit of the Spider Crabs, already
described, strongly suggests, however, that concealment from sight is an
important protection to some shore Crustacea, and helps to make it
probable that the same end is reached in other cases by modifications of
form and colour.
There can be no doubt, at all events, that many Crustacea are very
inconspicuous to human eyes when they remain motionless in their natural
surroundings. Thus, for example, the Caprellidae, or "Skeleton Shrimps"
(see Fig. 22, p. 54), are hard to detect without very close search, as
they cling to the feathery branches of the hydroid zoophytes among which
they are usually found. They are strangely modified Amphipods, in which
the body is slender and thread-like, and generally of a
semi-transparent, whitish or yellowish colour, like the zoophytes on
which they live. They clamber about among the branches with a movement
like that of a "looper" caterpillar, and often remain clinging by means
of the hooked claws of the hinder pairs of legs, with the fore part of
the body gently waving about.
The little Crabs of the family Leucosiidae (Oxystomata), of which the
British representatives are several species of the genus _Ebalia_, are
often extremely like pebbles of the gravel among which they live. In
many tropical species the carapace is pitted and eroded, so as to
resemble a worn fragment of coral shingle. One of the most striking
cases among the Crabs, however, is that of _Huenia proteus_ (Fig. 40),
one of the Spider Crabs (Oxyrhyncha), which is found in the Indian and
Pacific Oceans. In this little Crab the carapace is flat, and is
extraordinarily variable in form. In most of the males it is triangular
in outline, but in most of the females and in some males it is broadened
by leaf-like expansions of the side edges. Borradaile has pointed out
that these broad individuals are usually found among the sea-weed
_Halimeda_, and that they closely resemble the fronds of this weed in
form and in their greenish colour.
[Illustration: FIG. 40--A, A PIECE OF A TROPICAL SEA-WEED (_Halimeda_);
B, A CRAB (_Huenia proteus_) WHICH LIVES AMONG THE FRONDS OF _Halimeda_,
AND CLOSELY RESEMBLES THEM IN FORM AND COLOUR. REDUCED. (After
Borradaile.)]
A number of Crustacea are known to possess a chameleon-like power of
changing their colour. The mechanism by which this change is effected is
similar to that found in other animals, such as fish and frogs, which
have the same power. The pigment which gives its colour to the animal is
lodged in microscopic star-shaped bodies known as _chromatophores_,
lying for the most part just below the skin. Each chromatophore consists
of a central body from which a number of branching filaments radiate.
The pigment may contract into the centre of the chromatophore, forming
a minute and hardly visible speck, or it may spread out into the
branching filaments, forming a distinct spot of colour. Each
chromatophore may in some cases contain several colours of pigment, and
these may expand or contract independently of each other, so that a
whole series of changes may be produced by a single chromatophore. In
the larger Crabs and Lobsters the visible colour of the animals depends
on pigment in the shelly exoskeleton, which is thick enough to hide the
chromatophores in the living tissues underneath, and no very rapid or
considerable changes are apparent; but in the smaller forms, in which
the exoskeleton is thin and translucent enough to allow the underlying
colours to appear through it, the changes in the chromatophores may
produce striking effects. Thus, Fritz Mueller describes a species of
Fiddler Crab of the genus _Gelasimus_, in which the hinder part of the
carapace was brilliantly white, but five minutes after the Crab was
captured it had changed to a dull grey. Many other cases of colour
change have been described, but most remarkable and the most fully
studied is that of the Prawn, _Hippolyte varians_, which is very common
on our own coasts, and has recently been the subject of a very elaborate
series of researches by Professors Keeble and Gamble. The specimens of
this Prawn show "a bewildering variety of colour and of colour-pattern";
they may be uniformly coloured in various shades of brown, green, or
red, or they may be "blotched," "barred," or "lined," with colour. These
different varieties are generally found among sea-weeds, which they
resemble in colour and pattern, the "lined" forms, for instance,
frequenting finely branched and feathery weed. Like many other
protectively coloured animals, they are of sedentary habits, clinging to
the weed, and seldom moving by day. If a specimen be removed from its
habitat and placed in an aquarium with different kinds of sea-weed, it
will take refuge among that which it most closely resembles. It appears
that this resemblance in colour-pattern is acquired during the growth of
the Prawn, and that a young specimen kept among finely branched sea-weed
will acquire the "lined" pattern, while others, living among coarser
weed, become "barred," "blotched," or "monochrome." Even in the adult
Prawns the colour (though not the pattern) becomes changed in a day or
two if they are placed among weed of a different colour--from green to
brown, or the like. Within certain limits still more rapid changes of
colour take place. If kept in the dark, or if placed on a white
background (for example, in a porcelain dish) in the light, the Prawn
quickly becomes nearly colourless, by contraction of the chromatophores,
a transparent bluish tint alone remaining, due to a substance which
diffuses from the chromatophores into the fluids of the body. In natural
conditions this phase is assumed at night; and the interesting
observation has been made that Prawns kept in the dark continue for
three or four days to show a periodic expansion and contraction of the
chromatophores, corresponding to the alternation of day and night. It
seems that the rhythm of light and darkness has become impressed on the
chromatophore system of the animal, and the movement of the pigments is
regulated by something analogous to memory.
[Illustration: FIG. 41--THE COMMON PORCELAIN CRAB (_Porcellana
longicornis_), SLIGHTLY ENLARGED, AND ONE OF THE THIRD MAXILLIPEDS
DETACHED AND FURTHER ENLARGED TO SHOW THE FRINGE OF LONG HAIRS]
It has already been mentioned, in dealing with the Lobster, that certain
Crustacea have the power of voluntarily throwing off some of their limbs
(autotomy). In many cases, as in the Lobster, this power is mainly of
use in enabling the animal to discard an injured limb; but there are
some Crustacea which seem to adopt it as a means of escaping from the
attack of an enemy. On our own coasts the shore collector will often
find, on turning over a large stone, one or more specimens of the little
Porcelain Crabs (_Porcellana platycheles_, or _P. longicornis_--Fig. 41)
clinging to its under-side. If these Crabs be seized by one of the large
claws, they frequently leave the claw in the captor's hand and scuttle
off without it; and it cannot be doubted that, as in the case of lizards
and other animals which have a similar power of self-mutilation, this
habit often enables them to escape from their natural enemies.
Although the Crustacea as a whole are predominantly active animals, many
examples have already been mentioned of species which are more or less
sluggish and sedentary in their habits. The extreme degree of passivity
is reached by the Barnacles (Cirripedia), which differ from all other
Crustacea (except some parasites) in being fixed to one spot, and quite
without the power of locomotion in the adult state. Most of the
Barnacles met with on the shore or in shallow water belong to the
division of the Sessile Barnacles or Acorn-shells (Operculata). Every
visitor to the seashore has noticed the little conical shells which
cover exposed rocks as if with a coat of rough-cast. On the British
coasts the commonest species is _Balanus balanoides_ (Plate III.),
though other species closely resembling it are also common. They are to
be found almost up to high-water mark in situations where they are left
uncovered for many hours every day; but the valves which close the
opening of the shell fit so tightly that a little sea-water is enclosed,
and the animal is protected from drying up even when exposed to the heat
of the sun. If a stone or a chip of rock, with a few of these animals on
it, be placed in a jar of sea-water, their peculiar mode of obtaining
food can easily be watched. The valves will presently be seen to open a
little, and the curled cirri will be protruded, opened out like the
fingers of a hand, and withdrawn again with a sort of grasping motion.
These movements are continued without stopping while the animal is under
water. If the cirri be examined with a pocket-lens or under a
microscope, it will be seen that they are fringed with stiff bristles,
so that, when they are opened out, the whole forms a kind of
"casting-net." As it is swept through the water, this net entangles
minute floating particles of animal or vegetable matter, and carries
them into the shell, so that they can be seized by the jaws and
swallowed. The cirri, as we have already seen, are really the modified
thoracic limbs, so that, in Huxley's words, "A Barnacle may be said to
be a Crustacean fixed by its head, and kicking the food into its mouth
with its legs."
A mode of obtaining food by "net-fishing," not unlike that employed by
the Barnacles, is found in certain Crustacea belonging to a widely
different group--the little "Porcelain Crabs" (Fig. 41) mentioned above.
Mr. Gosse observed that the Broad-clawed Porcelain Crab (_Porcellana
platycheles_) employed its third pair of maxillipeds, which are thickly
fringed with long feathered hairs, in making alternate casting movements
"exactly in the manner of the fringed hand of a Barnacle, of which both
the organ and the action strongly reminded me."