SHRIMPS, CRABS, AND BARNACLES
WE have no word in English to indicate the varied crab-and-shrimp-like
creatures of salt and fresh waters in the same way as "insect"
designates the six-legged, usually winged, terrestrial creatures
of many kinds--beetles, bees, bugs, two-winged flies, dragonflies,
day-flies, and butterflies. They are all "insects." Naturalists call
the aquatic shrimp-and-crab creatures "crustaceans." Perhaps "crab"
might be used in a large sense to include them all, together with the
true crabs, as the Germans use their word, "krebs." The shore-crab is
the most familiar of all crustaceans, in the living, moving condition.
Boiled lobsters, prawns, and shrimps are more generally familiar
members of the class, but the "undressed" living crab is better known
to every one who has been on the seashore than the live lobster,
prawn, and shrimp. Londoners have been heard to express interest in
the curious blue variety of lobster caught on the coast, not being
aware that the hot bath which he takes before he, too, is "dressed,"
causes his blue armour to change its colour to a brilliant scarlet.
Occasionally a regular ordinary lobster is caught in which this change
has occurred during life in the sea--and there are some enormous
deep-sea prawns of a pound in weight which when living have a splendid
crimson colour. A large series of "crustaceans," carefully prepared so
as to show their natural colours in life, is exhibited in the Natural
History Museum in Cromwell Road.
A curious kind of prawn (by name Althea rubra), of fair size, is found
under "the low-tide rocks" in the Channel Islands, which not only is
of a deep crimson colour, but snaps his fingers at you--or rather
one of his fingers--or claws--when you try to catch him, making a
loud crack audible at ten yards distance. The common big prawn, if
you see him in a large vessel of sea-water with the light shining
through him, appears very brilliantly marked with coloured bands and
spots--reddish-brown, blue, and yellow--which are displayed on a
transparent, almost colourless surface. Of course, boiling turns him
pale red. A common smaller species of prawn when boiled is often sold
as "pink shrimps," and lately a deep-sea prawn--a third species--has
come from the Norwegian coast into the London market. There are many
kinds which are not abundant enough to become "marketable." Prawns
are at once distinguished from the true "brown shrimp" by having the
front end of the body drawn out into a sharp-toothed spine, which
is absent in the shrimp. Besides the prawns (Palæmon and Pandalus),
the shrimp (Crangon), and the common lobster (Homarus), you may see
in the London fish shops the large spiny lobster (Palinurus) called
"langouste" by the French, and apparently preferred by them as a table
delicacy to the common lobster, although it has no claws. It used to
be called "craw-fish" or "sea craw-fish" in London; why, I am unable
to say. The name was certainly bad, as it leads to confusion with the
cray-fish, the fresh-water lobster of British and all European rivers
(there are many other kinds of fresh-water lobsters in other parts of
the world, as well as fresh-water prawns and crabs), whose English
name is a curious corruption of the French one, "écrevisse" (cray-vees,
cray-fish). Another lobster of our markets is the little one known as
the "Dublin prawn," which is common enough on the Scotch and Norwegian
coasts, as well as that of Ireland. Naturalists distinguish it as
Nephrops Norvegicus. The great edible crab completes the list of
British marketable crustaceans, but in Paris I have eaten, as well as
at Barcelona, a very large Mediterranean prawn, three times as big as
our biggest Isle of Wight prawns, but by no means so good. It is called
"Barcelona prawn" and "Langostino" ("Penæus" by naturalists). In Madrid
I have seen in the fish shops and eaten yet another crustacean--a very
curious one--namely, a long-stalked rock-barnacle of the kind known to
naturalists as Pollicipes.
That the barnacles--ship's barnacles (Fig. 10) and with them the
little sea-acorns (Fig. 11), those terribly hard and sharp little
white "pimples" which cover the rocks nearly everywhere just below
high-tide mark, and have so cruelly lacerated the hands and shins
of all of us who swim and have had to return to a rocky shore in a
lively sea--should be included with crabs, lobsters, and shrimps as
"crustaceans" must appear astonishing to every one who hears it for the
first time. The extraordinarily ignorant, yet in their own estimation
learned, fishermen of the Scottish coast will tell you with solemn
assurance that the ubiquitous encrusting sea-acorns are the young
of the limpet, whilst the creature living inside the shell of the
long-stalked ship's barnacles has for ages been discoursed of by the
learned as one of the marvels of the sea--nothing more or less than a
young bird--the young, in fact, of a goose--the barnacle goose which,
since it was thus proved to be a fish in origin, was allowed to be
eaten by good Catholics on fast days! Two hundred years or more ago
this story was discredited by serious naturalists, but the barnacles
and sea-acorns were thought (even by the great Cuvier) to be of the
nature of oysters, mussels, and clams (Molluscs), because of their
possessing white hard shells in the form of "valves" and plates, which
can open and shut like those of mussels. Their true history and nature
were shown about eighty years ago by a great discoverer of new things
concerning marine creatures, Dr. Vaughan Thompson, who was Army Medical
Inspector at Cork, and studied these and other animals found in the
waters of Queenstown Harbour.
The crab class, or Crustacea, have, like the insects, centipedes,
spiders, and scorpions, a body built up of successive rings
or segments. The earth-worms (as every one knows) and marine
bristle-bearing worms also show this feature in the simplest and
most obvious way. The vertebrates, with their series of vertebræ or
backbone-pieces and the body muscles attached ring-wise to them, show
the same condition. The marine worms have a soft skin and a pair of
soft paddle-like legs upon each ring of the body, often to the number
of a hundred such pairs. But the crab class and the classes called
insects, centipedes, arachnids, and millipedes are remarkable for the
hard, firm skin, or "cuticle," which is formed on the surface of their
bodies and of their legs, which, as in the marine worms, are present--a
pair to each body-ring or segment--often along the whole length of
the body as in centipedes. This hard cuticle is impregnated with lime
in the bigger members of the crab class, such as the lobster. It is
not equally thick and hard all over the surface of the lobster, but
is separated by narrow bands of thin, soft cuticle into a number of
harder pieces, thus rendered capable of being bent or "flexed" on one
another. Thus the body is jointed into a series of rings, and the legs
are also divided each into several joints (as many as seven), which
gives them flexibility and so usefulness of various kinds. The various
joints are "worked" by powerful muscles, which are fixed internally to
the cuticle and pass from one hard ring or segment, whether of body or
of leg, to a neighbouring ring.
Every one knows the structure of a lobster's tail and of its legs,
which can be readily examined in illustration of my statement, and
the same structure can be seen in the leg of a beetle or a fly.
Naturalists term all this series of creatures with hard-jointed
cuticle, to which the muscles are attached, including the crab class,
the insects, centipedes, spiders, and scorpions, "jointed-leg owners,"
or Arthropods. It is easy to appreciate this characteristic difference
which separates the Arthropods from other animals. The sea-worms differ
from them, in that they have soft cuticle, but stiffen and render their
paddle-like legs firm by squeezing the liquid of the body into them in
the same sort of way as the sea-anemones distend their tentacles with
liquid, though in that case the liquid is sea-water taken in by the
mouth. The Molluscs also distend their muscular lobe, or "foot" as it
is called, by pressing the blood from the rest of the body into it, and
so making it swell and become stiff, so that the muscles can work it;
when not distended in that way it is flaccid. The Vertebrates (bony
animals) and the star-fishes have again another and peculiar mechanism.
Their muscles are attached to hard internal pieces, sometimes
cartilaginous but often calcareous or bony, which are spoken of as "the
internal skeleton." There are thus three distinct kinds of mechanism in
animals for giving the necessary resisting surfaces, hinged or jointed
to one another, and made to "play" one on the other by the alternate
contraction and relaxation of the muscles attached to them.
The Arthropods differ among themselves in the number of body-rings,
the enlargement or dwindling of certain rings, and the fusion of a
larger or smaller number of the rings to form a composite head, or a
jointless mid-body or hind-body. The successive legs are primarily and
essentially like to one another, and each body-ring, with its pair of
legs, is but a repetition of its fellows. At the same time, in the
different classes included as "Arthropoda" a good deal of difference
has been attained in the structure of the legs, and they have in each
class a different form and character in successive regions of the
body, distinctive of the class, and are sometimes, but not always,
absent from many of the hinder rings. All these Arthropods agree in
having a leg on each side immediately behind the mouth--belonging to a
body-ring, which is fused with others to form the head--very specially
shortened, of great strength and firmness, and shaped so as to be
pulled by a powerful muscle attached to it, against its fellow of the
opposite side, which is similarly pulled. These two stumpy legs form
thus a powerful pair of nippers called "the mandibles." They are jaws,
although they were in the ancestors of the Arthropods merely legs.
These jaw-legs, or leg-jaws, are characteristic of all the crab class,
as well as of the other Arthropods, but no bristle-worm or other animal
has them. The jaws of marine worms are of a totally different nature.
So are the jaws of snails, whelks, and cuttle-fish. Many of the crab
class have not one only, but several, pairs of legs following the
mouth converted into jaws. Thus, if you examine a big shore-crab, or,
better, an edible crab, and a lobster, and a large prawn, you will find
that they all have five pairs of legs converted into short foliaceous
jaws (hence called "foot-jaws"), and overlying the first very strong
pair, or mandibles.
Following these "foot-jaws" you find in a crab or a lobster the great
nipping claws and the four large walking legs--the same in proportion
and shape in crab, lobster, and prawn, much bigger than the foot-jaws.
But the curious thing is that if you set them out and carefully compare
them (for they are not simple jointed limbs, but each has two or even
three diverging stems carried on a basal joint), you will find a
strange and fascinating "likeness in unlikeness," or an agreement of
the parts of which they are built, and yet a difference between all of
them.
The rings of the body to which the jaw-legs and legs are attached are
fused into one unjointed piece. The spine in front of the mouth and
the support of the eyes and the feelers or "antennæ" are fused with
that piece. It forms on the back a great shield--often called "the
head"--which overhangs and is bent down over the sides of this region,
so as to protect the gills, which you can see by cutting away the
overhanging flap.
Following on the jaw-legs or foot-jaws and walking-legs, in the three
crustaceans we are looking at, comes the jointed tail or hind-body,
consisting of seven pieces. The first five rings of the tail have small
=Y=-shaped legs, a pair to each ring. They are called "swimmerets,"
whilst the sixth has legs of the same shape, but very large and flat.
In the middle between these large flat legs is the last ring, which
has no legs, but is perforated by the opening of the intestine. You
will see if you compare the crab and the lobster (or the prawn, which
is very much like the lobster), that the crab has the so-called head
(really head and mid-body combined) drawn out from side to side, so as
to make it much wider than it is long. And, moreover, the jointed tail
or hind-body seems at first sight to be absent in the crab. But if you
turn the crab (a dead one) on his back, you will find that he has a
complete tail, on the whole like that of the lobster, but pointed and
bent forwards, and closely packed under the fused head and mid-body in
a groove, from which you can raise it and turn it back.
[Illustration: FIG. 9.--The larval or young form of Crustacea known as
"the Nauplius." This is the "Nauplius" of a kind of Prawn. The three
pairs of branched limbs are well seen. Much magnified.]
We have not yet done with the various forms assumed by the legs of our
three crustaceans--for, actually in front of the mouth, there are two
pairs of peculiarly altered legs. Originally in crab-ancestors, and at
the present day in the very minute young stage of growth called "the
Nauplius" (Fig. 9), the mouth was not behind these two front pairs.
It has sunk back as it were, gradually moved so as to leave the legs
in front of it. As we now see them in the crab, lobster, and prawn,
the two pairs of legs in front of the mouth are jointed filamentous
things--the feelers or antennæ--very long in prawns and lobsters, short
in crabs. In the ancestors of crabs, lobsters, and prawns these feelers
were undoubtedly swimming legs. In the "nauplius" stage (Fig. 9) of
some prawns, and in many minute crustaceans often called "water-fleas,"
we find these feelers not acting as mere sensory organs of touch, but
relatively strong and large, with powerful muscles, striking the water
and making the little creatures bound or jump through it in jerks.
It has been discovered that in the growth from the egg of many
crustaceans the young hatches out as a "nauplius" with only three
pairs of legs. The front two pairs later gradually grow to be the
feelers, the third pair become eventually the mandibles or first pair
of jaw-legs. These legs all present themselves at first as active,
powerful swimming "oars," beset with peculiar feathery hairs and
not in the shape which they later acquire. The kite-shaped nauplius
baby-phase, smaller than a small flea, with its three pairs of
violently jerking legs, is a very important little beast. It is the
existence of this young stage in the growth of barnacles and sea-acorns
which has demonstrated that they are crustaceans, that is to say,
belong to the crab class. The fixed shell-like barnacles and sea-acorns
hatch from their eggs each as a perfect little "nauplius," like that
drawn in Fig. 9. They swim about with jerking movements caused by
the strokes of the two front legs and of the pair which will become
the mandibles. Their limbs have the special form and are beset with
the feather-like hairs, and the whole creature has the kite-like
shape--characteristic of the nauplius young of other Crustacea. They
are indeed indistinguishable from those young. Whilst it was the Army
doctor, Vaughan Thompson, who discovered that barnacles are strangely
altered "shrimps," it was Darwin who made one of the most interesting
discoveries about them--a discovery of which he was always, and
rightly, very proud--as I will explain in the next chapter.