_MARINE ARTHROPODS_
The sub-kingdom _Arthropoda_ contains a vast assemblage of animals, all
of which, as the name implies, possess jointed appendages. Their bodies
are covered with a skin that is hardened by a horny substance
(_chitin_), and frequently, also, by the deposit of carbonate of lime.
The body of Arthropods is made up of a chain of segments, all of which
are built up on one common pattern, and each one is surrounded by a ring
of the hardened skin or exo-skeleton that gives attachment to a pair of
appendages. Commonly, however, two or more of the segments become fused
together, being covered by a continuous plate or shield, in which the
boundaries of the rings are almost or completely obliterated; but in
such cases the appendages they bear always remain distinct, so that the
true number of segments is always apparent. The skin between those
segments that are not so fused together remains soft and flexible, thus
allowing the body to be freely bent.
The appendages exhibit a great variety of structure, and are as varied
in their functions. Some are used as feelers, and others as jaws for
seizing or masticating food. Some are developed into powerful seizing
organs for purposes of defence or attack, some into paddles for
swimming, while others are legs adapted for walking.
All these appendages are made up of segments, each of which, like those
of the body itself, is surrounded by a ring of hardened skin, and
connected with its neighbours by a flexible integument that allows
perfect freedom of movement; while within are the muscles, often very
powerful, by which the appendage is moved.
In the arthropods we have a sub-kingdom of highly organised animals,
with distinct, and often very complicated, systems of organs for
digestion, circulation, and respiration; and the nervous system consists
of a well-developed chain of ganglia, connected by nerve cords, and from
which nerve fibres are distributed to the various parts of the body. It
should be noted, however, that some members of the group have
degenerated into parasites, and in these, as with all such degraded
creatures, many of the organs have retrogressed to such an extent that
they are quite functionless, or have even disappeared entirely. These
parasitic forms, when very young, are really highly organised creatures,
not unlike the young of their industrious and more noble relatives; but,
as the natural result of their degraded mode of living, in which they
find no use for their organs of locomotion, digestion, circulation and
respiration, these eventually disappear, with the result that the organs
of reproduction predominate to such an extent that they often fill the
greater part of the cavity of the body.
It should be noted, too, that the sense organs of arthropods are well
developed, most of them being supplied with complex eyes, hearing
organs, and highly sensitive feelers.
This sub-kingdom consists of four classes--the _Crustacea_, including
lobsters, crabs, shrimps, prawns, &c.; _Arachnoidea_, containing
spiders, mites, and scorpions; _Myriopoda_--centipedes and millepedes;
and _Insecta_.
[Illustration: FIG. 188.--THE NERVE-CHAIN OF AN ARTHROPOD (LOBSTER)
_o_, optic nerve; _c_, cerebral ganglion; _i_, large ganglion
behind the œsophagus; _th_, ganglia of the thorax; _ab_, ganglia
of the abdomen]
The first of these classes consists mainly of marine animals, and will
therefore occupy much of our attention, but the members of the other
three are mostly terrestrial and aërial creatures that do not fall
within the scope of this work, except in the case of a few species that
are more or less decidedly marine in their tendencies. The aquatic
members are generally provided with well-formed gills by means of which
they are enabled to extract the dissolved oxygen from the water in which
they live, while those of terrestrial and aërial habits breathe by means
of a system of tracheæ or air-tubes that are open to the air and supply
branches to all parts of the body.
The _Crustaceans_ are mostly gill-breathers, though some of the aquatic
species have no special organs for respiration, but obtain the oxygen
necessary for respiration by absorption through their thin, soft skin,
while the terrestrial species breathe by means of tracheæ, as we have
just observed.
Most of them are covered with a calcified skin, as in the case of crabs
and lobsters; but many are protected with a chitinous or horny covering
such as we observe in shrimps and prawns. In either instance the
hardened integument constitutes what is known as the _exo-skeleton_.
None of the crustaceans have an internal skeleton of any kind, though
some of the inner parts are supported by extensions of the hard skin
that penetrate into the body.
It will be readily understood from the nature of the exo-skeleton of the
crustacean, and especially of the more or less rigid calcareous covering
of the crab and the lobster, that a uniform growth of the body is
absolutely impossible, and, in fact, that an increase in size cannot
take place without an occasional casting of the hard coat of mail. Hence
we find most crustaceans throwing off their coverings at intervals, and
growing by fits and starts during the periods between the ‘moultings’
and the hardening of the newly exposed skin.
When a crab or a lobster is about to undergo the process of moulting, it
retires to a secluded niche in the rock, where it is not so easily found
by its numerous enemies--a necessary precaution, since the creature in
its soft or unarmoured condition is eagerly devoured by fishes and other
marine animals--and there awaits the first stage of the ordeal.
Presently the skin splits; and, after a time, the crustacean succeeds in
extricating itself from its shell, which is cast off in a perfect
condition, every joint being entire, even to the coverings of the
antennæ, the stalked eyes, and other delicate appendages. And not only
this, for the portions of the shell that penetrate inward into the body
are also discarded, as well as the linings of the stomach and the gills;
and these cast-off coats of crabs and lobsters--especially the
former--may often be found in the most perfect condition on the sea
shore, being washed up without injury on the sandy beach, or found in
the very niche in which the creature changed its attire.
If one examines the powerful pincers of a crab or lobster, a thin plate
of considerable size will be seen to extend within from the movable
‘jaw’ to give attachment to the muscles by which it is moved, and it
seems impossible that this can be removed with the cast skin without
considerable injury to the new claw that is already formed, though as
yet in a soft condition, within the old and hard one. But it has been
observed that this plate actually cuts through the new claw, and that
the claw thus divided almost immediately closes up and unites again.
The moulting process being over, the crustacean’s body extends itself
within the new, yielding skin; and, the latter becoming gradually hard
by the deposition of carbonate of lime, the creature is able, after a
period of rest, to roam at large again, without much fear of injury,
until the time for the next moulting has arrived.
Those who have made but a slight acquaintance with the common
crustaceans of our shores must have noted the frequency with which
imperfect specimens occur--specimens with missing appendages, or with a
well-formed limb on one side of the body opposed to a puny and almost
useless fellow on the opposite side. As to the loss of appendages, this
matter will be readily understood by those who have watched crustaceans,
and especially crabs and lobsters, in their native element, so often do
these pugnacious creatures become engaged in furious broils with their
neighbours. And, when we are at work at the collection of various
species on the sea shore, how often do we find that a creature escapes
from our grip by leaving us in possession of a severed limb, while the
owner retreats rapidly among the stones and weeds apparently none the
worse for its trifling loss! This is, in fact, a very common method of
securing its escape from an enemy; and it appears that many crustaceans
have the power of thus rendering a seized limb so brittle that it may be
snapped off with the greatest of ease.
We have spoken of the loss thus sustained as a trifling one; and so it
is, for crustaceans have the faculty of reproducing lost appendages; and
though the loss may be one of considerable inconvenience at first, a new
limb eventually appears in the place of each one so willingly discarded.
When such mutilations occur, it will be observed that the severed limb
invariably breaks away at the end of the first or basal joint--a point
where the bloodvessels are so narrow and contractile that but little
loss of blood takes place when the rupture is made--and it has been said
that the animal would soon bleed to death if the fracture were to take
place at any other point. As it is, the wound soon heals, but no trace
of a new limb is to be seen, at least without dissection, until the time
of the next moult. The part is developing, however, beneath the cover of
the basal joint; and when the moulting period arrives, the new limb,
still very small, is exposed to view. It then rapidly enlarges, though
not to anything like its proper size, and its surrounding skin becomes
hardened by the deposit of the calcareous secretion simultaneously with
that of the rest of the body. Further enlargements of the new appendage
take place at subsequent moults, with the final result that it is but
slightly inferior to its fellow either in size or in power.
The eye of a crustacean is a very complicated structure, commonly
described as a compound eye. It consists of a large number of conical,
radiating, crystalline rods, collected together into a mass that
presents a convex outer surface. This surface is covered with a
transparent layer of chitin which naturally presents a more or less
distinct netted appearance, the bases of the rods being in contact with
its inner surface, and visible through it. Each rod is surrounded by a
layer of pigment that prevents light from passing from one to another,
and the optic nerve passing into the base of the compound structure
sends a sensitive filament into each one.
[Illustration: FIG. 189.--SECTION THROUGH THE COMPOUND EYE OF AN
ARTHROPOD]
In many crustaceans this compound eye is situated on the end of a
movable stalk that generally allows it to be protruded or drawn under
cover as occasion requires, but in others the organ does not project
beyond the general surface of the body. Thus we hear of the animals of
this class being divided into the _stalk-eyed_ and the _sessile-eyed_
groups; the former being represented by crabs, lobsters, shrimps, &c.;
and the latter by sandhoppers and sandborers.
Crustaceans undergo metamorphoses while very young, the body being
altered considerably in form at several successive moults. Some, in
their earliest stage, consist of a little oval body that shows no signs
of a division into segments. It swims about by means of three pairs of
appendages, and has only one eye. Others start life with four pairs of
limbs, attached to the front portion of the body, a segmented abdomen,
as yet perfectly limbless, and a pair of compound eyes. Then as the
successive moultings take place, new segments and new appendages are
developed, until, at last, the form of the adult is assumed. The
accompanying illustration shows four stages in the development of the
Common Shore Crab.
[Illustration: FIG. 190.--FOUR STAGES IN THE DEVELOPMENT OF THE COMMON
SHORE CRAB]
The lowest division of the crustaceans contains the _Cirripedia_ or
Curl-footed crustaceans, which includes the Barnacles that are so
frequently seen attached to the bottom of ships and of floating timber,
and the Acorn Barnacles, the conical shells of which often completely
cover large masses of rock on our shores.
[Illustration: FIG. 191. THE BARNACLE]
For some time naturalists could not agree as to the proper place of
these animals in the scale of life, but the matter was finally settled
when some minute creatures only about a twelfth of an inch in length,
and closely resembling the early stages of certain crustaceans, were
seen to undergo metamorphoses, and finally develop into acorn barnacles.
Their position in the animal kingdom was thus determined by their early
stages; but these, instead of changing into a segmented and highly
organised creature like the typical crustacean, lose some of their
appendages, cease to be free-moving animals, and attach themselves to
floating bodies by which they are carried about. Thus they are enabled
to find the food they can no longer seek without such aid. In their
young state they possess not only the means of freely moving in search
of their food, but have organs of vision to aid them in the capture of
their prey. As they grow, however, the foremost appendages are
transformed into a sucking-disc, and the eyes, no longer necessary,
disappear. It will thus be seen that the degenerated adult--the product
of a _retrograde development_--is attached by what was originally the
front of its body, while the abdomen is undeveloped, and the thorax,
with its appendages, forms the summit of the free extremity.
[Illustration: FIG. 192.--FOUR STAGES IN THE DEVELOPMENT OF THE ACORN
BARNACLE
A, newly hatched larva; B, larva after second moult; C, side view
of same; D, stage immediately preceding loss of activity;
_a_, stomach; _b_, base of future attachment. All magnified]
Some of the Cirripedes attach themselves to the bodies of whales and
other marine animals. The majority of these are
pseudo-parasites--creatures that live on the bodies of other animals,
but do not derive their food at the expense of their hosts; others,
however, are true parasites, subsisting on the nourishing juices they
extract from the animals to which they are attached.
[Illustration: FIG. 193.--A CLUSTER OF ACORN SHELLS]
The Acorn Barnacles, so numerous on our shores, are good types of the
_Cirripedia_, and they are so easily kept alive in the indoor aquarium
that their interesting movements may be well observed. A cluster of
these animals may be obtained by chipping off a piece of the rock on
which they grow; or, instead of this, a few minutes’ searching on a
rocky coast at low tide will certainly provide us with a stone of
suitable size, or the shell of a mollusc, on which the creatures have
found a home.
[Illustration: FIG. 194.--SHELL OF ACORN BARNACLE (_Balanus_)]
Place them in the indoor aquarium, or in any shallow vessel containing
just sufficient sea-water to cover them, and carry out your
observations with the aid of a hand lens. They will soon open the inner
cone of their many-valved shell, and slowly protrude six pairs of
gracefully curved and delicately-feathered appendages which, as
previously stated, are attached to the thoracic portion of the body.
Then, with a much more rapid movement, the appendages will be withdrawn,
and the shell closed. These alternate movements are continued
incessantly, and are the means by which the animals provide themselves
with both food and air. The reader should also obtain some specimens of
the larger species for the examination of the shell, the structure of
which is interesting and, of course, peculiar to this order.
[Illustration: FIG. 195.--THE ACORN BARNACLE (_Balanus porcatus_) WITH
APPENDAGES PROTRUDED]
In general structure and habits Barnacles are very similar to the acorn
barnacles, except that the body is supported on a tough stalk, which, as
we have already stated, is the modified anterior portion of the animal.
These animals also may be easily kept alive and examined in the indoor
aquarium. They are not creatures of the sea shore, but may often be
obtained on masses of timber that have been washed ashore, or from the
bottoms of ships that have been placed in the dry dock for repairs.
Another order of the crustaceans--the _Copepoda_, or oar-footed
group--is so called on account of the bristled feet that are employed
after the manner of oars when the creatures are swimming.
These Copepods are small animals, so small indeed that the compound
microscope is generally necessary merely for the examination of their
external characters. Many species inhabit fresh water, and the study of
the group is more commonly pursued by the investigator of fresh-water
pond life than by the sea-side naturalist. However, marine species are
abundant, and may be captured in the open water or in rock pools by
means of a muslin net. As with the last order, some degenerate from the
comparatively complicated free-swimming and eyed larval state to blind
and limbless parasites that feed on the bodies of fishes and are known
as fish lice.
The body of the typical copepod is distinctly segmented, and the head
and thorax are both enclosed in a hardened buckler. It has two pairs of
antennæ, two pairs of foot jaws by which it captures its prey, and four
or five pairs of bristled feet for swimming. The jointed abdomen has
also a tuft of bristles at its extremity. The annexed illustration
represents some marine species, and will serve to show the general
features of the order.
[Illustration: FIG. 196.--A GROUP OF MARINE COPEPODS, MAGNIFIED]
The sea-side naturalist, intent on the collection of small life, may
possibly meet with representatives of two other orders of
crustaceans--the _Ostracoda_ or shelled crustaceans, the bodies of which
are enclosed in a bivalve, hinged shell; and the _Branchiopoda_, so
called because the branchiæ or gills are attached to the feet.
[Illustration: FIG. 197.--A GROUP OF OSTRACODE SHELLS]
The Ostracodes have two or three pairs of feet which subserve
locomotion, but are not adapted for swimming; and two pairs of antennæ,
one of which assists in locomotion. The mouth is provided with organs of
mastication, the branchiæ are attached to the hind jaws, and the
animals have but one eye. Some of these crustaceans inhabit deep water
only, while others live in sand between the tide-marks; but several
species, belonging chiefly to the genus _Cythere_, abound in rock pools,
where they may be readily obtained by scraping the confervæ and
corallines with a small muslin net.
The branchiopods are free swimmers, and are protected by a buckler-like
envelope. Most of them are inhabitants of fresh water, and are popularly
known as water fleas. We have figured one marine species, belonging to
the genus _Evadne_, which has a colourless body, and a single
conspicuous black eye, and is interesting as being the food of the
herring.
[Illustration: FIG. 198.--_Evadne_]
The four orders of crustaceans that have been briefly described belong
to the division _Entomostraca_, which signifies ‘shelled insects.’ This
term is not a happy one when judged from the standpoint of our present
knowledge of animal life, but it must be remembered that, at the time it
was applied (1785), spiders and crustaceans were all included in the
same class as the insects; and this is hardly surprising when we observe
the close relationship of these animals, as shown in their segmented
bodies and jointed appendages; for, as we have already shown, the lowly
organised parasitic crustaceans which, in the adult state, lose most of
their appendages and cease to be distinctly segmented, are more or less
insect-like in their larval and free-swimming stage.
All the other crustaceans are included under the term _Malacostraca_, or
soft shelled, since, although many of them are protected by an
exo-skeleton that is hardened by the deposit of carbonate of lime, yet,
generally speaking, their coverings are softer than those of the
molluscs; and therefore the term _Malacostraca_ was originally applied
by Aristotle in order to distinguish them from the animals that are
covered by harder and thicker shells.
This division of the crustaceans contains wood lice, sandhoppers,
lobsters, shrimps, crabs, &c., and consists of two main groups--the
Sessile-eyed (_Edriophthalmata_) and the Stalk-eyed (_Podophthalmata_)
crustaceans.
We shall now consider the Sessile-eyed group, dealing first with the
order _Isopoda_ or equal legged, and then the _Amphipoda_, which have
appendages adapted both for walking and swimming.
The general nature of an Isopod may be readily understood by the
examination of the common woodlouse that abounds in gardens and damp
places almost everywhere, and the reader will probably remember having
seen similar creatures crawling over the rocks on the sea shore.
The body is generally depressed or flattened, but convex above, and is
composed of seven segments, each segment bearing a pair of legs which
terminate in a pointed claw, while the posterior appendages are modified
into flat, leaf-like organs of respiration.
[Illustration: FIG. 199.--MARINE ISOPOD
1. _Sphæroma serratum._ 2. _Limnoria lignorum._ 3. _Ligia
oceanica._ 4. _Nesæa bidentata._ 5. _Oniscoda maculosa_]
When engaged in ‘shrimping’ one frequently meets with shrimps or prawns
that are disfigured by a tumourous swelling on the side of the body, and
if the swelling be opened a little parasite will be dislodged. This
parasite is an Isopod (_Bopyrus_), the appendages of which are
imperfectly developed. The female is very much larger than the male,
and, as is usual with parasitic creatures, the greater part of the
body-cavity is occupied by the well-developed organs of reproduction.
There are several other parasitic isopods, some of which live on the
bodies of fishes, and are popularly known as fish-lice, but these are
not so likely to come in the way of the sea-side naturalist as the more
typical forms that crawl about on the rocks and among the weeds of the
coast. A few of the latter are shown in the accompanying illustration,
including the Sea Pill-ball (_Nesæa bidentata_), common on the rocky
coasts of the south-west, and distinguished by the two sharp
projections on the last segment; the Serrated Pill-ball (_Sphæroma
serratum_), very common on most rocky shores, and characterised by the
fine sawlike teeth on the outer edge of the outer plates of the ‘tail’;
the Great Sea-slater (_Ligia oceanica_), also an abundant species; the
Spotted Hog Louse (_Oniscoda maculosa_) that lives among the tufted sea
weeds; and the Boring Pill-ball (_Limnoria lignorum_) that bores into
the woodwork of piers and jetties, often doing considerable damage.
[Illustration: FIG. 200.--MARINE AMPHIPODS
1. The spined sea screw (_Dexamine spinosa_). 2. _Westwoodia
cœcula._ 3. _Tetromatus typicus._ 4. The sandhopper (_Orchestia
littorea_). 5. _Montagua monoculoides._ 6. _Iphimedia obesa._ All
enlarged]
The above and other isopods feed on various animal and vegetable
substances, some species being quite omnivorous in habit. Most of them
are eagerly devoured by birds and fishes.
The Amphipods, six species of which are shown in the above illustration,
include the Sandhoppers or Beach Fleas, so numerous on our coasts that
it is almost impossible to go any distance without making their
acquaintance. They are invaluable as scavengers, as they rapidly devour
decaying sea weeds, and will speedily reduce the body of any animal
washed on the beach to a clean skeleton. Although they are all small
creatures, they make up in numbers for any deficiency in size; and
though devoured in enormous quantities by the various shore birds, they
multiply so prodigiously that they are never lacking wherever there is
decomposing organic matter to be consumed.
The bodies of these animals are usually flattened from side to side,
very distinctly segmented, and have a well-developed abdomen. The head
is furnished with two pairs of antennæ and a pair of sessile eyes,
though some species possess only one pair of antennæ, while others have
four eyes. The limbs of the thorax are used either for walking or for
swimming, and give attachment to the gills. The abdomen has generally
six pairs of appendages, the foremost three pairs of which are usually
small, and employed in swimming, while the others are stronger and
directed backwards, and are often adapted for jumping.
It is very interesting to observe the habits of the Sandhoppers and
other Amphipods both on the sandy beach and in the water, and the
student will find that certain species burrow into the sand with
considerable agility, and live principally at the extreme high-water
mark, where they feed on the organic matter washed in by the breakers at
each high tide, while others dwell almost exclusively in the water,
among weeds and stones, and should be searched for at low water. The
latter may be kept alive for some time in the aquarium providing they
are the only occupants, but a little experience will show that these and
all other Amphipods are readily devoured by many marine creatures, and
consequently they are of real value to the aquarium keeper as food for
other animals.
We now come to the Stalk-eyed Crustaceans (_Podophthalmata_), which
contain those members of the class most generally known, such as crabs,
lobsters, shrimps, and prawns. In these the eyes are mounted on movable
pedicels, the head and thorax are generally covered by a large shield
called the carapace, and the appendages are adapted partly for seizing
and masticating, and partly for locomotion.
The group includes two orders--the _Stomapoda_ or Mouth-footed
crustaceans, so called because some of the limbs are crowded round the
region of the mouth; and the _Decapoda_, or Ten-footed crustaceans.
The Stomapods, though very abundant in tropical seas, are not often met
with on our own shores. However, since a few interesting species are
inhabitants of our seas we will briefly describe the distinguishing
characteristics of the group.
We have just mentioned the fact that the head and thorax of a decapod is
usually covered by a large shield--the carapace. Now, the general
character of this carapace may be seen at once in either the shrimp or
the lobster. In these animals the segments that form the head and the
thorax are all fused together, and are completely covered by the
protective buckler of hardened skin; but in the Stomapoda the carapace
is much smaller in proportion, and a few of the segments of the thorax,
instead of being fused into the general mass of the _cephalo-thorax_,
are quite distinct from it. The abdomen, also, is large and strongly
formed in these animals. Five pairs of the thoracic limbs are directed
forwards, and are adapted both for catching food and for climbing, while
others are used in walking. The limbs of the abdomen generally number
six pairs, of which the first five bear feathery gills.
[Illustration: FIG. 201.--THE MANTIS SHRIMP (_Squilla Mantis_)]
Two species of Mantis Shrimps, one of which is represented in fig. 201,
have been found off the south and south-west coasts, but these are not
likely to be seen on the shore, since they inhabit deep water. Allied to
these, and sometimes included with the Stomapods, are the Opossum
shrimps, so called because the females of some species carry their eggs
in a kind of pouch, thus reminding us of the marsupial quadrupeds of the
same name. They are of very slender build compared with the mantis
shrimps, and differ from them in that the carapace completely covers the
thorax; but though this is the case, the fusion of the thoracic segments
is not complete, since the posterior ones have still a certain amount of
freedom of movement. Some species of opossum shrimps are abundant in the
rock pools of our coasts, particularly in the south-west, but their
bodies being often so transparent as to be almost invisible, they are
consequently easily overlooked. Their general appearance may be
gathered from our illustration of _Mysis chamæleon_, which is probably
the most common species inhabiting our coast.
The highest crustaceans--the Decapods--are divided into two
sub-orders--the _Macrura_, or Great-tailed, including lobsters, shrimps,
&c.; and the _Brachyura_ (Short-tailed), containing the crabs; but the
number of British species is so large that it is impossible to give, in
our limited space, a detailed description of all the commonest even. All
we can do is to note a few of the more interesting features of certain
species, to introduce such illustrations as will enable the young
naturalist to identify a number of the commoner ones, and to give the
general characteristics of the main divisions so that the student may be
able to classify his specimens intelligently.
[Illustration: FIG. 202.--THE OPOSSUM SHRIMP (_Mysis chamæleon_)]
In the _Macrura_, as with other divisions of the crustaceans, we meet
with very interesting modifications of the appendages, adapted to quite
a variety of uses; and if the reader is unacquainted with these
adaptations of structure to habit he cannot do better than secure a
lobster or crayfish for study. It will be observed that the body may be
divided into two main portions--the _cephalothorax_, consisting of head
and thorax combined, and the _abdomen_. The former is composed of
fourteen segments, so thoroughly fused together that they are denoted
only by the fourteen pairs of appendages to which they give attachment,
while the calcified skin forms one continuous shield surrounding the
whole. The abdomen, on the other hand, consists of six distinct
segments, each of which is surrounded by its own ring of the hardened
integument, and is connected with its neighbours by means of a portion
of uncalcified skin that renders the whole very flexible. A groove in
the front portion of the great shield (_carapace_) marks the division
between the head and the thorax, the former composed of six, and the
latter of eight united segments.
[Illustration: FIG. 203.--PARTS OF LOBSTER’S SHELL, SEPARATED, AND
VIEWED FROM ABOVE]
The calcareous covering of each segment consists of an upper portion,
called the _tergum_, and a lower, named the _sternum_, united at the
sides; the sternal portion of the cephalothorax, which gives attachment
to the walking limbs, is a most complicated and beautifully formed
structure.
[Illustration: FIG. 204.--A SEGMENT OF THE ABDOMEN OF A LOBSTER
_t_, tergum; _s_, sternum, bearing a pair of swimmerets; _h_,
bloodvessel; _d_, digestive tube; _n_, nerve chain]
The six pairs of appendages belonging to the head are easily made out
with a little care. The first are the jointed _eye-stalks_ that bear the
compound eyes previously described; and these are followed by two pairs
of _antennæ_, or feelers, the first being shorter and double, while the
second are very long. The former contain the organs of hearing. Then, in
front of the mouth, and completely hiding it, are a pair of strong
_mandibles_ or jaws that move horizontally, and the two pairs of
_maxillæ_ that are also employed in reducing the food.
Following these, but belonging to the thorax, are three pairs of
appendages that are known as foot-jaws; for, although they assist the
preceding organs in breaking up the food, they bear a resemblance in
some respects to the longer limbs behind them. Of the latter there are
five pairs (hence the term _decapoda_), the first being a very powerful
pair of seizers or pincers, and the remaining four, which are well
adapted for walking, terminating in either double or single claws.
All the appendages above mentioned are not only attached to the body by
movable joints, but are themselves made up of jointed parts, sometimes a
considerable number, each of which, like the segments of the body
itself, is surrounded by a ring of hardened skin, and connected with
those above and below it by a portion of soft and flexible skin.
[Illustration: FIG. 205.--APPENDAGES OF A LOBSTER
1. Second maxilla. 2. Third foot-jaw. 3. Third walking leg.
4. Fifth walking leg]
Lastly, beneath the abdomen, are paired limbs called _swimmerets_, which
are used as paddles, and probably assist the animal more or less in its
progress through the water; but the principal organ of locomotion in the
_macrura_ is undoubtedly the powerful muscular abdomen, aided by the
broad and fanlike tail formed by the appendages of the last segment. To
demonstrate this fact, put a live lobster, or even a shrimp, in a still
rock pool, and threaten it from before, when it will rapidly retreat
backwards by a series of powerful jerks, produced by suddenly doubling
its abdomen forwards beneath its body.
In addition to the external characters above mentioned, there are many
interesting features connected with the internal structure of the
lobster that may be studied on making easy dissections. Thus, the gills,
which are attached to the bases of the thoracic limbs, may be exposed by
cutting away the side of the carapace, and at the same time we may
discover the bailing organ by means of which a current of water is kept
flowing forwards through the gill-cavity to keep up the necessary supply
of oxygen for respiration. The removal of the upper portion of the
carapace will expose the heart and some of the principal bloodvessels,
and also the stomach with its powerful and complicated ‘gastric mill,’
formed by the hardening of portions of the wall of the latter organ for
the purpose of crushing and masticating the food. Then, if these organs
be carefully removed from above, together with the others we have not
space to describe, and the powerful muscles that fill up the segments of
the abdomen, the chain of ganglia and their connecting nerve cords that
form the central part of the nervous system may be seen extending along
the central portion of the body.
[Illustration: FIG. 206.--LONGITUDINAL SECTION OF THE LOBSTER
_a_, antenna; _r_, rostrum or beak; _o_, eye; _m_, mouth; _s_,
stomach; _in_, intestine; _l_, liver; _gl_, gills; _h_, heart;
_g_, genital organ; _ar_, artery; _n_, nerve ganglia]
Several species of lobsters inhabit our seas, but they are generally to
be found beyond the tide-marks, and are, therefore, not often caught by
sea-side collectors without the aid of some kind of trap or the
assistance of fishermen. The common lobster (_Homarus vulgaris_),
however, is often left behind by the receding tide on our rocky coasts,
and may be seen and caught if one knows where to look and how to
capture.
On cautiously approaching a deep rock pool one may often see a lobster
rapidly retreat in its usual backward fashion, and snugly house itself
in a narrow chink from which it is impossible to remove it. And, when
once surprised, it is not likely to show itself again as long as the
intruder is in view.
If one remains perfectly still for a time, a pair of waving antennæ may
be seen gradually protruding from the safe retreat; but, as soon as the
stalked eyes have advanced sufficiently to detect the figure of a
stranger, the lobster silently withdraws itself till quite out of sight.
Lobsters, usually of rather small size, may often be seen quite out of
the water at low tide, in the narrow chinks of the rock, or under large
stones, but it is no easy matter, as a rule, to get them out. It is of
little use poking a stick into the entrance of their hiding-places,
though occasionally they will grasp the stick so tenaciously with their
forceps that they may be pulled within reach. You _may_ be able to haul
them out by their long antennæ, but if you can find a second way into
their home such that you can disturb them from behind you are pretty
sure of your victim.
[Illustration: FIG. 207.--THE SPINY LOBSTER (_Palinurus vulgaris_)]
It will be unnecessary to describe other species of lobsters
individually, but we have introduced figures of a few for
identification. The Norway Lobster (_Nephrops norvegicus_) is often
landed in large numbers by the fishermen of the east and south-east
coasts and sold at a shilling or so a dozen under the name of Norway
Prawns. They are pretty and interesting creatures, and may be easily
kept alive in the indoor aquarium, where they may be fed on any kind of
fish.
[Illustration: FIG. 208.--THE NORWAY LOBSTER (_Nephrops norvegicus_)]
Fig. 209 represents the two allied creatures that may sometimes be dug
out of the sandy beach, or from the mud in the estuary of a river. The
one on the left is the mud-borer (_Gebia stellata_), which is of a dull
yellowish colour, marked more or less distinctly by pinkish starlike
spots--a feature that has suggested the specific name. The beak in front
of the carapace is very prominent and spiny, and the long abdomen is
narrower in front than in the middle. This creature hides in the holes
that have been excavated by boring molluscs, and seems also to extend
the cavities it inhabits by its own labours.
The other is very similar in general form, but has no spiny beak and the
abdomen is much broader in the middle than at the base. It is also to be
distinguished by the very unequal size of its front legs, one of which
is much more developed than the other.
It is known as the mud-burrower (_Callianassa subterranea_), and is said
to burrow very deeply into mud-banks, scooping out its retreat
principally by means of the second and third pairs of legs. Although
found at times between the tide-marks, its principal habitat is probably
in the mud that is covered by deep water, for it is not uncommonly to be
found in the stomachs of fishes that habitually feed in such localities.
[Illustration: FIG. 209.--THE MUD-BORER (_Gebia stellata_) (1) AND THE
MUD-BURROWER (_Callianassa subterranea_) (2)]
Lobsters of all kinds, and, indeed, the marine crustaceans generally,
are essentially the scavengers of the sea, for they are carrion-feeders,
greedily devouring flesh in all stages of decomposition. Hence the value
of their work on the sea shore is very considerable.
An examination of shrimps and prawns will at once show their close
relationship with lobsters. The general build of their bodies is
practically the same, and their appendages, though often different in
form from the corresponding limbs of the lobster, will be seen to
resemble them closely in arrangement and structure. The exo-skeletons
of these creatures are, however, generally hardened by a horny substance
(_chitin_) instead of a stony deposit of carbonate of lime.
The shrimps and prawns sold for food in our markets are very similar in
appearance when alive, the leading distinguishing feature being,
perhaps, the presence of a sharp, serrated beak projecting forward from
the front portion of the carapace of the latter.
The reader is probably acquainted with the fact that the shrimps and
prawns used as food have quite a different appearance when alive and in
their native element to that displayed by the corresponding wares in the
fishmonger’s shop--a fact that applies equally well to the edible crabs
and lobsters. Most crustaceans change to a bright red colour when
boiled, and, as stated in a previous chapter, the same result is
produced by the action of strong spirit.
[Illustration: FIG. 210.--THE COMMON SHRIMP (_Crangon vulgaris_)]
The Common Shrimp (_Crangon vulgaris_) is an exception, however, for it
may be distinguished when boiled by its dull greyish brown colour. When
alive this species is of a very pale greenish or greyish tint, lightly
spotted with brown; and its habits are so interesting that it will well
repay one to watch it either in the aquarium or in a rock pool. It
frequents sandy coasts, and can hide itself very quickly by burying its
body in the sand, using for this purpose both its legs and its antennæ.
The Prawn frequents rocky coasts, where it may often be obtained in
large numbers by sweeping with a suitable net under the cover of weeds
and stones. Its body is of an exceedingly pale greenish colour, and so
transparent that it is quite inconspicuous when in the water. Prawns are
turned to a rose-red colour by boiling, and they are captured in large
numbers when young and sold as ‘red shrimps.’
<div class="figcenter" style="width: 500px;">
<a name="fig211" id="fig211"></a>
<img src="images/i_308.jpg" width="500" height="282" alt="" />
FIG. 211.--THE PRAWN (_Palæmon serratus_)
</div>
In addition to the common species mentioned there are quite a number of
shrimps and prawns to be found in our seas, but some of them inhabit
deep water and are rarely to be found between the tide-marks. All,
however, are eagerly devoured by fishes, and, on that account, are often
to be obtained in good condition by examining the contents of the
stomachs of freshly caught fishes. In fact, this mode of search for the
smaller species of deep-sea life is not to be despised, for it is a
means by which we can obtain specimens that are not often secured by the
methods coming within the ordinary range of the amateur’s work.
It will be remembered that we spoke of the Decapods as consisting of two
main groups--the Great-tailed (_Macrura_) and the Short-tailed
(_Brachyura_). Frequently, however, we find the order divided into three
sub-orders as follows:--
1. _Macrura_ (Great-tailed), 2. _Anomura_ (Peculiar-tailed),
3. _Brachyura_ (Short-tailed);
the first containing lobsters, shrimps, &c.; the third the typical
crabs, such as the shore crab and the edible crab; while in the second
are placed those species of crabs which have been regarded as
intermediate in character. Thus, in the _Anomura_ we find decapods in
which the abdomen, though not so well developed as in the _Macrura_, is
either permanently extended or is capable of being extended and used for
swimming as occasion requires. The hindmost legs, also, are not well
developed and adapted for walking, but are employed only as organs of
prehension; and, as is the case with the first sub-order, there are
often two pairs of well-developed antennæ.
In this sub-order of ‘queer tails’ we find the Soldier or Hermit Crabs,
and those flat-bodied crabs that live almost exclusively on the surface
of stones, and are hence known as Stone Crabs; but as opinion now seems
inclined against the formation of a special suborder for these
creatures, we shall briefly deal with them as a first section of the
_Brachyura_.
The Stone Crabs are extremely interesting creatures, and the observation
of their habits, both in and out of the water, is particularly
entertaining and instructive. One species--the Broad-Clawed Porcelain
Crab (_Porcellana platycheles_), shown on Plate VI.--is very abundant on
all our rocky coasts, and may be found in immense numbers near low-water
mark.
Turn over some of the large encrusted stones that strew the beach among
the rocks, and you are almost sure to find numbers of these little crabs
clinging to the freshly exposed surface. A few of them may remain
perfectly still, and exhibit no sign of surprise on their untimely
exposure to the light; and these, on account of their small size, the
closeness with which they apply their flattened bodies to the encrusted
stone, and more than all to the protective colouring of their dingy
bodies, which so closely resembles that of the surface to which they
cling, may well be overlooked by the inexperienced collector. But the
majority of them will immediately scamper away in their own peculiar
fashion towards the edge of the stone, and rapidly make their way to
what is now the under side. As they progress with a hasty, sliding
movement they never for one moment loosen their firm hold on the rough
surface of the stone, but keep both body and limbs in close contact with
it, clinging hard by means of their pointed claws as well as by the
numerous hairs and bristles with which their appendages are liberally
fringed.
Attempt to pull one from its hold, or even take other than the gentlest
means to arrest its progress, and you will probably find that it
suddenly parts company with one of its broad claws in its endeavour to
escape; and, unless some special precautions be taken to remove these
crabs, it is possible that quite half the specimens taken will have been
damaged in this way during their struggles to escape. If, however, you
gently thrust the point of a penknife beneath the body, and then apply
the thumb above, you may lift them from a stone without injury. Another
plan is to press a frond of smooth sea weed as closely as possible to
the surface of the stone in the front of the crabs, and then allow them
to crawl on to it, or cause them to do so if necessary. The piece of
weed, with crab or crabs attached, may then be bagged for future
examination.
On turning over the Broad-clawed Crab its under surface will be seen to
be perfectly smooth, with an appearance closely resembling that of white
porcelain. Its foot-jaws, also, are proportionately large, and closely
fringed with hairs; and the last pair of legs, which are very slender in
build, are folded closely beneath the body. Further, the abdomen is
wide, composed of six distinct movable segments, and terminating in a
tail-fin composed of five fringed plates.
Drop the crab into water, and it will immediately extend its abdomen,
which it will flap sharply under its body somewhat after the manner of
lobsters and shrimps, and thus swim backward by a series of jerks as it
sinks to the bottom. On reaching the bottom it instantly grasps the
solid material, applies itself closely to the surface, and glides away
into the nearest chink it can find.
As one observes the nature and movements of these interesting little
crabs one cannot fail to see how beautifully their form and structure
are adapted to their habits. They are peculiarly constructed for abode
in narrow chinks and crannies, and for feeding on the small forms of
life that inhabit such sheltered places. Their legs move in the plane of
their flattened bodies, and as they glide among the confervæ and other
low forms of life that encrust the stones of the beach they feel their
way by, and are possibly also guided by the sense of smell located in,
their long outer antennæ, while the close fringes of their claws and
foot-jaws form admirable sweep-nets by means of which the little animals
that form their food are swept towards the mouth.
We have other species of stone crabs, one or two of which resemble the
last species, and belong to the same genus, but the others are very
different in general appearance. The Northern Stone Crab (_Lithodes_),
found principally on and off the coasts of Scotland and Ireland, has a
spiny covering with a long beak. Another species--_Dromia vulgaris_--is
somewhat similar in habit, though it can hardly be termed a stone crab,
since it inhabits deep water, and apparently lives among the sponges,
sea firs, and weeds that cover the bottom.
[Illustration: FIG. 212.--_Dromia vulgaris_]
[Illustration: FIG. 213.--THE HERMIT CRAB IN A WHELK SHELL]
The remainder of the Peculiar-tailed Decapods belong to the Soldier or
Hermit Crabs, and constitute the genus _Pagurus_.
Every one who has searched a few rock pools will have seen the familiar
Hermits, and will probably have been interested in their varied antics.
First you observe the shell of a mollusc--a Trochus, Periwinkle, or a
Whelk--travelling at an abnormal rate for a member of its class. You
approach closely to make an inquiry into the matter, when the motion
suddenly ceases, and the shell instantly drops into position with its
mouth close to the surface below. If left undisturbed for only a short
time, the rapid and somewhat jerky motion is resumed, only to cease as
suddenly as before as soon as the inhabitant is again threatened.
On examining the shell we find that it is the home of a species of crab,
and that the animal within it is completely hidden with the exception of
its head, stalked eyes and long, slender antennæ, one very large claw,
and a few walking legs.
To remove the creature from its home is no easy matter as a rule. To
pull it out by means of its legs or its antennæ would probably be to
sever some portion of its body; but if you thrust the creature, shell
and all, among the spreading tentacles of a large anemone, it will at
once grasp the peril of the situation; and, if the shell has already
been secured by the clinging petals of this dangerous marine flower, the
hermit will speedily quit its home and endeavour to rush from the many
snares in order to secure its freedom. Or, it not infrequently happens
that the occupied shell is one that has withstood many a storm, but not
without the loss of the apex of its cone. In this case the insertion of
a very flexible fibre into the opening thus made will cause the hermit
to leave its home in the possession of the enemy.
Having, by some means or other, managed to drive the crab from its
shell, we place it in a shallow rock pool, or in a vessel of sea water,
and observe the chief features of its structure.
The first thing that strikes one is the absence of a calcified skin on
the extended abdomen, which is so soft that, remembering with what
eagerness fishes will attack and devour crabs of all kinds, we can at
once understand the necessity of such a home as the creature selects.
Again, we observe the presence of appendages at the tip of the abdomen
by means of which the crab is enabled to hold itself securely in the
shell. Also, when we note the general form of the armoured portion of
the body, and the position of the soft-skinned abdomen, we can see how
well adapted the whole is to fit snugly into the spiral shell of a whelk
or winkle.
We also observe that one of the pincers is much larger than the other,
and the value of such an arrangement may be estimated when we see the
animal at home. The smaller claw, together with the other appendages
used for walking or prehension, can be retracted within the shell, but
the large claw, which constitutes a formidable weapon of attack and
defence, is not only in such a position as to be ready for immediate
use; but, lying as it does in front of the body, with other portions
hidden more or less behind it, it serves the purpose of a shield when
the animal retires.
If we place a homeless hermit crab in a rock pool, the behaviour of the
creature immediately suggests a feeling of uneasiness--a sense of
danger--for it moves about in a very erratic fashion that is quite
different from the straightforward and deliberate action of the same
animal when properly protected; and very amusing results may be obtained
by making it the subject of a few harmless experiments. For instance,
drop down before it an empty whelk-shell that is much too large to
properly accommodate its body. It will immediately approach the
untenanted house, search and probe it well with its antennæ and other
appendages, and then, finding it uninhabited, and having no apartment of
more suitable size at hand, will abruptly gives its body a turn and
hastily thrust itself backwards into it.
If at the time of this experiment the advancing tide disturbs the water
of the pool, the result is somewhat ludicrous, for the shell, too
cumbersome to be controlled by the creature within, is, regardless of
its attempts to maintain a normal position, turned over and over as each
wave advances and retreats.
Again, supposing the shell supplied to be too small for the intended
occupant, it will, after the usual examination of the interior, thrust
its soft abdomen as far in as possible, and make the best of the
unsatisfactory circumstances until a more suitable home can be found.
And if, at this distressing period, we drop before it a shell of just
the right size--the one from which the creature was originally expelled
for instance, it is astonishing how quickly the change of houses will be
accomplished. After a brief examination of the shell with the object of
determining whether all is right within, during which the crab continues
to avail itself of the imperfect accommodation afforded by the previous
shell, it rapidly extracts its body from the one and thrusts itself
backwards into the other. Its normal habits are at once resumed, all its
movements being now suggestive of confidence and contentment.
We have already referred (p. 153) to the fact that a large anemone
(_Sagartia parasitica_) is commonly found attached to a whelk shell,
which at the same time forms the home of the hermit crab, and (p. 44)
that a marine worm (_Nereis_) is also a common associate of the hermit,
taking up its abode in the interior of the same shell; and we also
briefly discussed the mutual advantage of such an arrangement to the
parties concerned. These triple combinations are not so frequently met
with on the shore between the tide-marks, but are dredged in
considerable numbers by the trawler; and the reader will find it repay
him to secure one in order that he may be able to watch the interesting
habits of the associates. The movements of the hermit crab are always
pleasing, particularly the manner in which it seizes and manipulates its
food; and still more so is the occasional appearance of the head of the
worm, always in exactly the same place, for the purpose of deliberately
stealing the food from the very jaws of the crab.
Hermit crabs are easily kept in captivity, and may be fed on any kind of
animal food, but care should be taken not to allow an excess of food to
remain in the water and render it putrid by decomposition. As long as
the crabs are active and remain within their shells you may assume that
the conditions are favourable; but when they become sluggish in their
movements, and leave their homes, the sanitary condition of the
aquarium should be regarded with suspicion; for hermit crabs, like many
of the marine tube worms, generally quit their homes when the conditions
are unfavourable, as if they preferred to die outside.
The Common Hermit Crab (_Pagurus Bernhardus_), also known as the Soldier
Crab, on account of its very pugnacious habits, is common almost
everywhere on our coasts, and may be distinguished by the numerous
little tubercles on the claws and on the upper edge of the front legs;
and there are several other species, belonging to the same genus,
distributed more or less locally on the various shores. All are similar
in general structure and habits, the various species being identified
principally by means of their colour, the variations in the form of the
appendages, and the general character--smooth, tubercular, spiny,
&c.--of the exo-skeleton. One species, found in the sandy bays of
Cornwall, burrows rapidly in the sand.
Coming now to the true crabs--the _Brachyura_, or Short-tailed
crustaceans, as sometimes distinguished from the _Anomura_--we find
quite a variety of interesting creatures, many species of which are
always within the reach of the collector at work between the tide-marks.
In all these the abdomen is only slightly developed, and is never used
in swimming, being permanently folded beneath the thorax. This portion
of the body, however, is usually very distinctly segmented, and if it be
lifted from its position it will be found that some of the segments bear
appendages corresponding with the swimmerets of the lobster. It is also
wider in the female than in the male, and crabs of the former sex may
often be found during the summer with the abdomen more or less
depressed, and the space beneath it quite filled with eggs.
The upper surface of the carapace of crabs is often very distinctly
grooved, and it is interesting to note that these features of the
exo-skeleton are not merely of external significance, for they usually
correspond in position with various internal structures, some of them
denoting the areas of the insertions of important muscles, and others
enclosing the regions of certain of the internal organs.
It will be noticed, too, that the carapace, which in lobsters is often
less than half the length of the body, covers the entire body of the
crab, except, perhaps, a very small linear portion between the bases of
the last pair of legs, where the first part of the segmented abdomen is
visible from above.
The true crabs of our seas may be divided into four groups, as follow:
1. _Oxystomata_, or Pointed-mouthed Crabs;
2. _Oxyrhyncha_, or Pointed-beaked Crabs;
3. _Catometopa_, with forehead turned downwards; and
4. _Cyclometopa_, or Round-headed Crabs;
and we shall briefly observe some of the more conspicuous and
interesting species in the order of the tribes as just given.
The first division is not well represented in our seas, the principal
species being the Nut Crabs and the Long-armed Crab, all of which may be
distinguished by the peculiar arrangement of the foot-jaws, which, when
closed, form a triangle with an acute angle turned towards the front.
The Nut Crabs are mostly small; and, since they generally inhabit deep
water, are not commonly seen on the shore; but perfect specimens may
sometimes be found among the contents of fishes’ stomachs. They derive
their name from the nature of the carapace, which is of a rounded form
and very hard and strong.
Pennant’s Long-armed Crab (_Corystes Cassivelaunus_) may commonly be
seen entangled among fishermen’s nets, but is not often seen on the
shore at low tide. Its carapace is very convex above, with three sharp
spines on each side, and the grooves are so arranged as to suggest the
appearance of a face. Our illustration represents the female, but the
‘arms’ of the male are very much longer than those of this sex.
The Sharp-beaked Crabs (_Oxyrhyncha_) include all those long-legged
creatures that are known collectively as the Spider Crabs; and here,
again, we have to do with species that almost exclusively inhabit deep
water. Although this is the case, but little difficulty is experienced,
as a rule, in obtaining specimens. If you are unable to take a trip in a
trawler for the purpose of examining the ‘rubbish’ that is dredged from
deep water, simply obtain permission to search the nets and the boats as
they arrive in port. In the latter case you are almost certain to find
the crabs you require, though it is probable that some of the species
will have been damaged by the hauling and shaking of the nets.
[Illustration: FIG. 214.--THE LONG-ARMED CRAB (_Corystes
Cassivelaunus_)]
These interesting crabs have been spoken of as the monkeys of the sea,
and the comparison will certainly be tolerated by anyone who has watched
the creatures as they climb among the corallines and sea firs in an
aquarium. Among such growths they are quite at home; and although their
movements do not often suggest the extreme agility of the monkey tribe,
yet the ease with which they seize the branches of the submarine forest
with their long ‘arms’ and pull their bodies from one tree-like
structure to another is decidedly monkey-like. Their comparison with the
long-legged spiders is also a happy one as far as their general form and
movements are concerned, but it must be remembered that they have not
the same reputation for cruel, predaceous habits, for they are more
truly the scavengers of the deep, subsisting mainly on the decomposing
bodies of their dead associates. The movements of most spider crabs are
so slow and deliberate that one can hardly imagine them capable of
anything of the nature of violent action; yet, when occasion requires
it, they will sometimes strike at the object of their wrath with a most
vigorous snap of their claws.
[Illustration: FIG. 215.--SPIDER CRABS AT HOME]
In these crabs, too, we find most interesting instances of protective
resemblance to their surroundings. Some of the small, slender-legged
species are not to be recognised without a careful search when they are
at rest among clusters of sea firs, their thin appendages and small
bodies being hardly discernible in the midst of the slender, encrusted
branches, and their peculiar forms are still more concealed by their
colouring, which generally closely resembles that of the growths among
which they live. Further, the carapace of spider crabs is in itself a
garden on which thrive low forms of both animal and vegetable life.
Minute _Algæ_, and occasionally some of moderate size, are rooted to the
shell, often securely held by the aid of the rough hairs and tubercles
that are so characteristic of the exo-skeletons of these creatures; and
patches and tufts of animal colonies that have found a convenient
settlement on the moving bed still further serve to obscure the nature
of the living mass below--a mass that is always in danger of becoming
the prey of the fishes which inhabit deep water. It is probable,
therefore, that this association is one that is beneficial to both
sides as far as the animal life is concerned, the lower species serving
to disguise the true nature of the crab, thus protecting it from its
numerous enemies, while they in return are conveyed, carriage paid, to
the feeding-grounds, where they can freely partake of the fragments that
become diffused in the surrounding water.
Our illustration on p. 288 shows three species of spider crabs, all of
which are common on parts of our shores. The Scorpion Spider Crab
(_Inachus dorsetensis_) derives its specific name from the fact that it
was first found off the coast of Dorset; but it is abundant off many of
our shores, both in the south and north, and may frequently be seen
entangled among the fishermen’s nets. It may be distinguished from other
and similar species by the four spines arranged in a line across the
front portion of the carapace, and the five large, pointed tubercles
behind them. This species is undoubtedly a favourite food of the cod,
for several specimens may often be taken from the stomach of a single
fish.
The next species--The Slender-beaked Spider Crab (_Stenorhynchus
tenuirostris_)--is seldom missing from the dredgings hauled in off the
south-west coast, and is fairly common in other parts. Its legs are
extremely slender, and bear spines on the inner side, and its body,
where free from the incrustations so often covering the carapace of
spider crabs, is of a fresh pink colour.
The other one shown in the same illustration is _Arctopsis lanata_,
sometimes known as Gibb’s Crab, the carapace of which is pointed behind,
bears a large pointed tubercle on each side, and is completely covered
with a thick clothing of stiff hairs. It is also common on many parts of
our coasts, more especially the coasts of Devon and Cornwall.
Closely allied to the last-named, and belonging to the same family, is
the well-known Thornback Crab (_Maia Squinado_), also a very common
crab, of which we give a separate illustration.
The tribe _Catametopa_ does not contain many British species, the
principal being the Pea Crabs; the Floating Crab, which is occasionally
washed on the south-west coast; and the beautiful Angular Crab. In these
the front of the carapace is turned downwards--a feature that has
suggested the name of the tribe.
The pea crabs are all small, and they are parasites, living within the
shells of bivalve molluscs. One species--the Common Pea Crab
(_Pinnotheres pisum_) is frequently found in the Edible Mussel; the
female, which is much larger than the male, being much more commonly
found. Another species--the Pinna Pea Crab (_P. veterum_), infests the
Pinna and Modiolus.
[Illustration: FIG. 216.--THE THORNBACK CRAB (_Maia Squinado_)]
[Illustration: FIG. 217.--THE PEA CRAB (_Pinnotheres pisum_)]
On Plate VI. is a drawing of the Angular Crab (_Gonoplax angulata_)
mentioned above, the striking form and delicate colouring of which can
never be mistaken. We would, however, call particular attention to the
broad and square front of the cephalothorax, with its two sharp spines,
and to the length of the eye-stalks. Unfortunately for the amateur, this
pretty crab is only to be found in deep water, off the coasts of Devon
and Cornwall, so that here, again, the aid of the fisherman is valuable;
but, as observed in the case of other deep-sea dwellers, may also be
looked for in the stomachs of cod and other bottom fishes. The sex
figured is the male, in which, when fully grown, the front legs are much
longer than in the female.
[Illustration: PLATE VI.
CRUSTACEA
1. Gonoplax angulata
2. Xantho florida
3. Portunus puber
4. Polybius Henslowii
5. Porcellana platycheles]
The remaining division of the crabs--the _Cyclometopa_ or Round-fronted
Crabs, contains the larger number of species that may truly be described
as common objects of the shore, for while some of them are well adapted
for swimming, and live in the open water, the majority inhabit the
shore, either between or just beyond the tide-marks, roaming about more
or less freely when in the water, but usually hiding under stones or
weeds, or burrowing into the sand, when left behind by the receding
tide.
[Illustration: FIG. 218.--THE COMMON SHORE CRAB (_Carcinus mænas_)]
The members of this tribe may be known at sight by the form of the
carapace, which is wide and rounded in front, and narrowed behind.
The accompanying illustration represents the commonest of the group--the
Common Shore Crab (_Carcinus mænas_), which is found plentifully on all
our coasts, and even in brackish water far up the estuaries of rivers.
It is a very voracious and pugnacious creature, probably the most active
of all our crabs, and its movements, whether connected with its feeding,
its sports, or its warfare, are always very interesting when observed
through clear water. This crab varies considerably in colour, but is
usually of the greenish tinge shown in the frontispiece.
Another abundant and well-known species is the Edible Crab (_Cancer
pagurus_), which is as familiar an object in town as on the sea coast.
Unlike the common lobster, its natural colour is not considerably
changed by boiling, being only turned from a dull to a brighter red.
The finest specimens of this crab are to be caught beyond low-water
mark, the usual snare being the basket or pot, baited with fish refuse,
but large numbers live among the stones and rocks left exposed at low
tide, and sometimes include specimens of considerable size. They should
be looked for under large stones that are loosely piled together, or in
the narrow chinks of rocks.
It is very interesting to compare the habits of the two common crabs
just mentioned. The former, when molested, will run off in great haste,
but always retreat with its front to the enemy, and its sharp and
powerful pincers far apart and wide open, ready for immediate use in its
own defence if necessary. The latter species, on the other hand, though
strongly built and provided with formidable claws, seldom runs far, and
hardly ever attacks one in the act of pulling it out of its
hiding-place; but, on the contrary, doubles all its ten legs under its
body as if endeavouring to approach, as nearly as possible, the form of
a ball, and will allow itself to be rolled about without showing any
signs of life.
The genus _Xantho_ contains two or three species that are common on the
Cornish and Devon coasts, and which may be known by their depressed and
deeply-grooved carapace and the presence of three or four prominent
tubercles on the latero-anterior margins. The abdomen of the female has
seven joints, while that of the male has only five. One of these
(_Xantho florida_), shown on Plate VI., is a powerfully built crab, as
may be seen when, after being disturbed, it pushes its way among the
loose stones of the beach, often lifting masses many times its own
weight.
On the same plate is also a figure of the pretty Velvet Crab (_Portunus
puber_), also known as the Lady Crab and the Violet Fiddler. The first
of these popular names has its origin in the dense covering of close
hairs that clothe the carapace, and the last refers to the beautiful
violet colouring of parts of the front legs, and, to a lesser extent, of
the remaining legs. This is, perhaps, the most ferocious of all our
shore crabs, and its attacks, when disturbed, are of such a determined
nature that the catching of the larger specimens is quite a lively
sport. Though it can hardly be described as an abundant species, yet it
sometimes occurs locally in such numbers that it may be found under
nearly every stone of any size. In fact, we have searched two or three
localities on the south-west coast where this crab is not only
extremely numerous, but is at the same time almost the only species to
be found; and it seems not unlikely that the pugnacious Lady has been
the means of driving the less formidable species from its favourite
haunts.
When you disturb a Velvet Crab it will immediately raise itself in a
menacing attitude, stretching its brightly coloured pincers as wide
apart as possible, and then it will either retreat backwards, or even
make a firm stand, ready to strike as soon as it is threatened with an
attempted touch. Try to grasp it, and its two powerful weapons of
defence are brought together with lightning-like rapidity giving one a
decidedly smart blow, possibly followed by a grip of great tenacity for
a creature of its size; but, should it miss its aim, its pincers strike
together with a sharp click, only, however, to extend at once in
preparation for the next attempt.
It will be observed that the walking legs of this crab are all
flattened, and that while the first three pairs terminate in sharp,
lance-like claws, the last pair are broad and fringed with hairs, thus
showing their close relationship to the swimming crabs. In fact, the
same genus contains British species which are popularly known as
Swimming Crabs.
One of the swimmers is represented in fig. 4 of Plate VI. It is
generally known as Henslow’s Swimming Crab or the Nipper, the scientific
name being _Polybius Henslowii_. The carapace of this species is quite
smooth, thus enabling the crab to move through the water with less
resistance, and the walking legs, particularly the last pair, are
flattened and fringed for use as paddles. It is said that this crab can
raise itself from the bottom to the surface of moderately deep water by
means of the swimming feet, and that it preys on fishes which it pursues
with some vigour.
Other crabs than those briefly described will reveal themselves to the
sea-side collector, but we have not the space to introduce them here.
Sufficient information has been given, however, to enable the reader to
broadly classify his specimens--a matter of more importance to the young
naturalist than the mere naming of species.
Leaving the crustaceans now, and passing for a moment to the
_Arachnoidea_--the second great division of the arthropods--we shall
briefly describe the Shore Spider (_Pycnogonum littorale_), which is the
only representative of the class likely to be met with by the sea-shore
collector.
It will be seen by our illustration that this creature by no means
resembles a typical spider. The powerful jaws, really modified antennæ,
that are such formidable weapons in the latter, together with other
appendages of the head, are undeveloped in the shore spider, and the
head is prolonged forward to form a rigid beak with the mouth at the
summit, and the head and thorax together form a cephalothorax of four
distinct segments, each of which bears a pair of legs. Further, the
cephalothorax forms almost the whole of the body, for the abdomen,
usually so large in spiders, is here represented by a mere tubercle. The
shore spider is unable to swim, but crawls about among the weeds and
stones of the bottom, clinging firmly by means of the curved claws of
its eight thick legs, and is protected by its dull grey colour which
closely resembles that of the encrusted stones among which it spends the
greater portion of its existence. It may sometimes be found hiding under
stones near low-water mark, but is far more commonly seen among the
‘rubbish’ hauled in by the trawl.
[Illustration: FIG. 219.--THE SHORE SPIDER]
We shall conclude our brief survey of the marine arthropods by a short
account of the insect life of the sea shore, referring to a few of the
more prominent forms and observing some of their habits; but since it is
probable that some of our readers are not well acquainted with the
general characters of this interesting class of animal life, it will be
advisable to precede our remarks by a short summary of their principal
distinguishing features, more particularly those in which they differ
from the other arthropods.
Insects, then, may be defined as those arthropods in which the body is
divided into three distinct parts--the _head_, composed of from four to
six fused segments, and bearing as many pairs of appendages; the
_thorax_, formed of three segments, each of which gives attachment to a
pair of legs; and the _abdomen_, composed of eight segments that bear no
appendages.
The head of an insect is furnished with a pair of compound eyes, very
similar in structure to those of a crustacean, and often, in addition, a
cluster of simple eyes; also a pair of antennæ, usually composed of many
joints. These antennæ are important organs of touch, and are employed,
at least by many forms, as a means of communication between one insect
and another. In them are also located the organs of hearing, and,
possibly, those of other senses.
The mouth varies very considerably in different insects, but is often
supplied with a pair of mandibles or biting jaws, and, below them, a
pair of maxillæ or chewing jaws, both pairs being jointed to the head in
such a manner as to be capable only of horizontal movements. Above and
below these jaws are, respectively, the upper lip or labrum, and the
lower lip or labium, the latter having appended to it a pair of jointed
feelers called the labial palpi, and an additional pair of palpi are
also frequently attached to the maxillæ, and therefore called the
maxillary palpi.
These organs of the mouth of an insect are modified in various ways
according to the functions they are called upon to perform. Thus, in
bees, the upper lip, as well as the mandibles, are adapted for chewing,
while the maxillæ and the labium are grooved in such a manner that when
brought together they form a tube through which fluids may be sucked
into the mouth. Also, in the butterfly and the moth, the maxillæ are not
constructed for chewing, but consist of two channelled rods which, when
approximated, form a long tube or proboscis employed for suction; and in
these insects the labial palps are large for the protection of the
proboscis, which is retracted and closely coiled between them when not
in use. Further, in the bugs, the labium is long and tubular, while the
mandibles and maxillæ are often modified into sharp, stiff bristles that
work within the tube, the whole thus forming a combined piercing and
sucking arrangement.
[Illustration: FIG. 220.--THE LEG OF AN INSECT]
The leg of an insect is built up much in the same manner as that of the
typical crustacean. It consists of a basal hip joint or coxa, a ring
segment or _trochanter_, a thigh (_femur_), a shin (_tibia_), and the
tarsus or foot of several joints which terminates in a claw or claws,
and is often provided with sucking-pads. The wings, when present, are
attached to the second and third segments of the thorax, if two pairs,
but if, as in the case of the house fly, the insect has only one pair of
wings, these are always appended to the second segment.
Insects are developed from eggs, but in their young state they are
segmented larvæ, with strong jaws, antennæ, simple eyes, and usually
three pairs of legs attached to the first three segments next to the
head.
As regards internal structure, we need only mention here that the body
is traversed by numerous branching tubes (_tracheæ_) that open at the
exterior and constitute the respiratory apparatus; that the insect is
provided with a contractile, tubular heart by means of which the blood
is propelled through a system of blood-vessels; that the nervous system
consists of a chain of ganglia, connected by a nerve cord, sending nerve
filaments to all parts of the body; and that the digestive tube is often
a complicated structure, especially in the case of those insects that
feed on herbivorous matter.
[Illustration: FIG. 221.--TRACHEA OF AN INSECT, MAGNIFIED]
The above outline will be sufficient to show that insects are not very
unlike the crustaceans in their general characteristics; and, indeed,
when we examine certain forms, noting the distinct segmentation of the
body, the hardened exo-skeleton of chitinous material, and the
unhardened skin between the segments to admit of freedom of movement, we
see a striking resemblance in external appearance to some of the typical
crustaceans.
Insects are divided into several orders, and some of these are fairly
well represented on the sea coasts, though it must be understood that
but few species are strictly aquatic and marine in their habits.
Fresh-water pools and streams teem with insect life, and quite a large
number of the insects that live in these situations are peculiarly
adapted for a life of submersion, their general form being often such as
to allow of rapid progress through the water, their appendages modified
into admirable swimming organs, and, in many cases, their breathing
apparatus adapted for the direct absorption of oxygen dissolved in the
water.
However, one would hardly expect to find similar forms of life abundant
in the water that washes our shores, the disturbing action of the waves,
even in calm weather, being more than such fragile creatures could
withstand. And this is really the case, for there are but few insects
that may be described as marine in the strictest sense of the word; and
of these the species that have been observed are mostly inhabitants of
warmer seas.
It is noteworthy that all the insects which exhibit marine tendencies
are small, and they seldom, if ever, live permanently below the surface.
But few of them can swim. A few run on the surface of the water,
supporting themselves on the surface film after the manner of
water-gnats, whirligig beetles, &c., without ever being wetted; and
these are said to feed on different kinds of floating matter, and
occasionally to dive below the surface.
A rambler on the sea shore in the summer time will always meet with
plenty of insect life, but the number of species observed may not be
large: and omitting all those which show no decided preference for the
coast, but are found in inland districts as well, we find that by far
the larger proportion live at or near the high-water mark, where they
feed on the refuse washed up by the waves. Some species, however, live
among the stones, or burrow into the sand, between the tide-marks; and
these, as a rule, are not driven inland by each advancing tide, but
allow the sea to wash over them, having at first protected themselves
from disturbance by burrowing or seeking other suitable shelter.
These latter, like many of the insects that inhabit fresh water, are
well adapted to withstand prolonged immersion. Their bodies are not
capable of being wetted, a covering of short hairs effectually
preventing the water from coming into actual contact with the body. The
openings of the breathing tubes (spiracles) are also guarded by closely
set hairs which prevent the water from entering; and, in some cases, the
creatures are provided with special air-sacs in which a supply of air is
stored for use while the insect is shut off from the external
atmosphere.
The lowest order of insects includes the so-called Bugs (_Rhynchota_),
which are parasitic on plants or animals. Quite a number of these are to
be found inhabiting fresh water, but only one is truly marine in its
tendencies. This one is a small insect, only about an eighth of an inch
in length, and named _Æpophilus_ (fig. 222). It has never been seen
except between the tide-marks, and occurs so near low-water level that
it is submerged during the greater part of its existence. But little is
known of this peculiar creature. Even its food has not been ascertained.
As with the other Rhynchota, but little change of form takes place
during growth, the young being very much like the adult in appearance.
It has been observed that the larvæ live crowded together under the
protection of stones.
The reader is probably acquainted with those fresh-water bugs that are
popularly known as ‘boatmen’ on account of the oar-like action of their
long, fringed hind legs; and although none of these may be described as
marine, yet certain species may often be seen in salt and brackish
water, living in company with creatures that are decidedly inhabitants
of the sea.
<div class="figcenter" style="width: 500px;">
<a name="fig222" id="fig222"></a>
<img src="images/i_329.jpg" width="500" height="320" alt="" />
FIG. 222.--SEA SHORE INSECTS
1. _Æpophilus._ 2. _Machilis maritima._ 3. _Isotoma maritima._ 4.
_Cœlopa_
</div>
We frequently meet with a pretty, slender-bodied insect, measuring about
half an inch in length without appendages, creeping over the rocks in
the sunshine, generally very near the crevices in which they hide, and
leaping from place to place when disturbed. These are the Bristle-tails
(_Machilis_), belonging to the order _Thysanura_, the members of which,
like the bugs, scarcely undergo any metamorphoses. This insect (fig.
222) has long antennæ, and also a long, stiff, and elastic bristle
extending backwards from the tip of the abdomen; and this bristle is the
means by which the creature leaps. Occasionally the machilis may be
found resting on the surface of the still water of a rock pool, in which
case its body is not wetted, its weight not being sufficient to break
the surface film of the water; and, in fact, the film is even
sufficiently firm to enable the insect to leap on the surface just as it
would on a solid body.
Allied to the bristle-tails, and usually grouped with them in the same
order, are the little Spring-tails, some species of which may often be
seen huddled together on the surface of the water of a rock pool. They
are so small that, unless closely examined, they may be mistaken for
particles of floating inorganic matter which have been blown into a
sheltered corner of the pool, and this idea may be strengthened by the
fact that these minute creatures _are_ driven by the wind into such
sheltered spots. But when we disturb them their true nature immediately
becomes apparent, for they may then be seen to move about on the surface
of the water, sometimes creeping on the surface film, and clambering on
the adjacent rock or weed, or leaping more or less vigorously, in which
latter case their bodies do not become wetted, the surface film
remaining unbroken by their exertions. And even when the rising tide
drives the spring-tails into crevices where they remain submerged,
perhaps for hours together, their bodies still remain dry, the water
being kept off by numerous short bristles and prominences with which
they are furnished.
When we examine a spring-tail by means of a lens we observe that it has
no traces of wings, but that each of the three segments representing the
thorax bears a pair of short legs, and that the abdomen consists of only
five or six segments. The head is furnished with a pair of jaws, and the
antennæ, which are short and thick, are composed of but few
joints--never more than six in number.
Some spring-tails live among the refuse washed up on the beach, where
they may be seen jumping about in company with the sandhoppers when the
material is disturbed. Such is the case with _Isotoma maritima_, the
illustration of which shows the forked tail that enables the little
animal to jump about so vigorously. But some of the marine spring-tails
are not so true to their name, since they are not provided with this
characteristic jumping organ, and have to content themselves by creeping
about slowly with the aid of their short legs. One of these springless
spring-tails (_Anurida maritima_) is one of the commonest of the group,
and is distributed over almost every part of our coast.
Passing over several orders of insects which do not seem to have any
marine representatives, we come to the _Diptera_ or two-winged insects,
of which the familiar house-fly is a type, and here we have to deal with
those troublesome creatures that literally swarm in the neighbourhood of
the matter washed up to the highest level of the tide during the whole
of the summer months. But although these insects are so very numerous,
we do not find among them a particularly large number of species, their
abundance being due more to the extreme prolificacy of those that
occur.
In this order, which includes all gnat-like creatures, as well as those
insects that are generally known as flies, the first pair of wings are
well developed, while the second pair are rudimentary, and represented
merely by a pair of scales, or by two little pin-like bodies called the
balancers or _halteres_. Some are provided with piercing organs by means
of which they can inflict a small wound and then extract the juices of
their victim, as does the female gnat, but the majority have a proboscis
adapted for suction only. The larvæ of the _Diptera_ are generally
limbless maggots, gifted with a pair of jaws, and they are usually very
voracious feeders, devouring decomposing animal or vegetable matter in
enormous quantities.
If we turn over a fermenting mass of the miscellaneous matter thrown up
on the beach quite beyond the reach of the tides, we may observe a
multitude of little maggots which feed on the moist, odorous portion
that was protected from the direct rays of the sun, together with a
number of dark-coloured pupæ that lie at the very bottom of the heap or
buried in the sand below. These are two stages of the black fly
(_Cœlopa frigida_) that is so attentive to us when we rest on the dry
sand above high-water mark. This fly is very like the common house-fly
in general appearance, though its body is rather smaller. Other species
of the same genus often accompany them, all being very similar in
general appearance and habits, and none of the larvæ seem adapted to a
life in the water. They are always found beyond the reach of the tide,
and are drowned if submerged for any length of time.
Another species belonging to the genus _Actora_ will often be seen in
the same company, and this is readily distinguished by their lighter
greyish colour and its superior size. Also, along the water-line, we
often meet with species of the family _Dolichopodidæ_, so called on
account of the length of their legs, and noted for the beautiful
metallic colours which adorn their bodies. These flies are carnivorous
in habit, deriving their food from living as well as from freshly killed
animals, and their short, fleshy proboscis contains a piercing bristle
by which they can puncture the skins of the animals that provide them
with food. Most of the flies of this group live on trees, walls, fences,
&c., where they pursue and attack their prey, but certain species follow
the line of breakers on the sea shore, as before indicated, and obtain
their food from the various marine animals that are stranded on the
beach. A peculiar feature of the family is the nature of the abdomen of
the males, which is bent under the body and furnished with a number of
appendages.
Another marine dipterous insect is a gnat-like fly closely allied to
_Chironomus_, which we have described in a former work[A] of this series
dealing with fresh-water life; and it will be sufficient to mention here
that _Chironomus_ is commonly known as the window-gnat on account of the
frequency with which it may be seen flying on the windows of our
dwellings; also that the larva, known popularly as the bloodworm, is
truly aquatic in habit, being able to swim by rapidly looping its body
in opposite directions, and being provided with a breathing apparatus
adapted for the absorption of the oxygen gas contained in solution in
water. The larva of the marine species referred to above may sometimes
be seen in rock pools, where it shelters itself among the sediment at
the bottom. It is much like the bloodworm in appearance and structure,
but its body is greenish instead of red.
The last order of insects calling for notice here is the _Coleoptera_ or
sheath-winged insects, popularly known as beetles, and characterised by
the hard and horny nature of the front pair of wings (elytra), which are
modified into sheaths and serve to protect the second pair; the latter
are thin and membranous, usually adapted for flight, and lie folded
beneath the former when not in use.
One large section of beetles is known as the _Geodephaga_ or Ground
Beetles--a group of very predaceous insects that burrow into the soil
and attack almost every living thing that comes in their way, and well
represented by numerous species that may be found in our gardens, and,
in fact, almost everywhere.
A considerable number of these insects show a decided preference for
salt marshes and the sea shore, where they hide under stones, or burrow
into the sand or mud in search of their prey. They are not marine in the
strictest sense of the word, for they are not adapted for a life of
submersion in water, either in the larval or in the perfect condition;
yet they are often found below high-water level, and some species burrow
into the sand of the beach as the tide advances, allowing the water to
cover them for hours together.
One interesting family of the ground beetles (the _Bembidiidæ_) includes
several small species, all of which frequent salt and wet places, such
as salt marshes, the mouths of rivers, and the sea shore. We give
enlarged illustrations of a few of these, the actual size being denoted
at the side of each.
[Footnote A: _Life in Ponds and Streams._]
[Illustration: FIG. 223.--MARINE BEETLES OF THE GENUS _Bembidium_
1. _B. biguttatum._ 2. _B. pallidipenne._ 3. _B. fumigatum._
4. _B. quadriguttatum_]
_Bembidium biguttatum_ may be identified by its brilliant bronze-green
colour, and the two distinct impressions on the elytra which have
suggested the specific name. _B. pallidipenne_ is, as its name implies,
a pale-winged species, the elytra being of a light yellowish colour. _B.
fumigatum_ is so called on account of the smoky tint of the elytra; and
the last species of the same genus figured (_B. quadriguttatum_) may be
known by the four conspicuous spots on the deep violet-coloured outer
wings.
The same family contains an interesting little beetle--_Cillenium
laterale_--only about one-sixth of an inch in length, that lives among
the refuse washed on the beach, where it feeds on the sandhoppers; and
although the latter are so much superior in size, the beetle has no
difficulty in holding and killing its prey, always seizing it on the
ventral side of the body, which is less protected by the hardened skin.
This species, which is of a copper colour, does not confine its ravages
to that portion of the beach which is above high-water mark, but often
allows itself to be covered by the advancing tide, remaining submerged
for a considerable time. Another species--_Aëpus (Æpys) marinus_--is
even more aquatic in its habits, for it searches out its prey among
stones, chiefly at the mouths of rivers, below high-water level, and is
often submerged for hours together. It is even provided with air-sacs to
enable it to withstand such prolonged submersions.
[Illustration: FIG. 224.--MARINE BEETLES
1. _Æpys marinus._ 2. _Micralymma brevipenne_]
There is another section of beetles which has elytra so short that they
cover only a small portion of the abdomen; but although so short, these
elytra completely cover the long membranous wings, which are folded up
beneath them in a wonderfully compact manner. The section referred to is
termed _Brachelytra_, from the feature just mentioned, and includes a
few species that are more or less marine in their habits. One of
them--_Micralymma brevipenne_--lives under stones below high-water
level, and apparently passes through all its stages within reach of the
waves. Another of the _Brachelytra_ (_Bledius_) burrows into the sand or
mud near high-water mark, throwing up the débris as it proceeds. Both
these beetles are carnivorous, and the latter is in turn preyed upon by
a ground beetle of the genus _Dyschirius_, which hunts and devours it
within its own home.
The reader will have observed that the sub-kingdom _Arthropoda_ is not
only a very extensive one in the sense that it contains a vast number of
animal forms, but also that its members exhibit a very great variety of
form and structure; and the beginner will probably find no little
difficulty in locating his specimens in their correct position in the
scale of life. The following table, however, will serve to show the
general classification of the group at a glance, and thus form a basis
for a more detailed study at any future time:--
SUB-KINGDOM ARTHROPODA
CLASSIFICATION
Class =CRUSTACEA=.
Sub-class =ENTOMOSTRACA=.
Order =Astracoda=--Free. Body enclosed in a bivalve shell.
Order =Copepoda=--Free. Five pairs of feet adapted for swimming.
Order =Cirripedia=--Sessile. Enclosed in a shell of many valves.
Order =Branchiopoda=--Free. Gills attached to feet.
Sub-class =MALACOSTRACA=.
Division =EDRIOPHTHALMATA=, or Sessile-eyed Crustaceans.
Order =Isopoda=--Body flattened. Seven pairs of legs--equal.
Order =Amphipoda=--Body flattened laterally. Legs adapted for
both walking and swimming.
Division =PODOPHTHALMATA=, or Stalk-eyed Crustaceans.
Order =Stomapoda=--Anterior appendages directed towards the
mouth.
Order =Schizopoda=--Cleft-footed Crustaceans.
Order =Decapoda=--Ten-footed Crustaceans.
Sub-order =Macrura=--Great-tailed. Lobsters, &c.
Sub-order =Brachyura=--Short-tailed. Crabs.
Class =ARACHNOIDEA=.
Order =Scorpionidæ=--Scorpions.
Order =Araneidæ=--Spiders.
Order =Acarina=--Mites.
Class =MYRIOPODA=.
Order =Chilopoda=--Centipedes.
Order =Chilognatha=--Millepedes.
Class =INSECTA=.
Order =Rhynchota=--Imperfect metamorphoses, suctorial mouth.
Bugs.
Order =Thysanura=--Imperfect metamorphoses. No wings.
Divided tail. Spring-tails.
Order =Euplexoptera=--Abdomen with terminal forceps. Earwigs.
Order =Thysanoptera=--Four equal membranous wings. Thrips.
Order =Orthoptera=--Anterior wings usually shorter and firmer.
Grasshoppers, &c.
Order =Neuroptera=--Two pairs of glassy wings--equal.
Order =Trichoptera=--Wings unequal, clad with hairs or scales.
Caddis flies.
Order =Aphaniptera=--No wings, no compound eyes. Fleas.
Order =Diptera=--Two membranous wings. Flies.
Order =Lepidoptera=--Wings clad with scales. Butterflies and
Moths.
Order =Coleoptera=--Fore wings hard and horny. Beetles.
Order =Hymenoptera=--Four membranous wings. Larvæ, footless
grubs. Ants, Bees, &c.