CORAL-MAKERS AND JELLY-FISH
A VERY beautiful kind of sea-anemone (common at Felixstowe) is the
Daisy or Sagartia troglodytes, (Fig. 6, a), which has a very long body
attached to a rock or stone far below the sandy floor of the pool,
on the level of which it expands its thin, long, ray-like tentacles,
coloured dark brown and white, and sometimes orange-yellow. As soon as
you touch it it disappears into the sand, and is very difficult to dig
out. The most beautifully coloured of all sea-anemones are the little
Corynactids (half an inch across), which you may find dotted about
like jewels, each composed of emerald, ruby, topaz, and creamy pink
and lilac, on the under surface of slabs of rock at very low tide in
the Channel Islands. One of the most puzzling facts in natural history
is that these lovely little things live in the dark. No eye, even
of fish or crab, has ever seen what you see when you turn over that
stone. It is a simple demonstration of the truth of the poet Gray's
statement, that many a gem of purest ray serene is concealed in the
dark, unfathomed depths of ocean! A splendid anemone is the Weymouth
Dianthus (see the frontispiece of this volume), so named because it is
dredged up in Weymouth Bay. It is often six inches long, and has its
very numerous, small tentacles arranged in lobes, or tufts, around the
mouth. It is either of a uniform bright salmon-yellow colour or pure
white. When kept in an aquarium it fixes itself by its disk on the
glass wall, and often, as it slowly moves, allows pieces of the disk to
become torn off and remain sticking to the glass. These detached pieces
develop tentacles and a mouth, and grow to be small and ultimately
full-sized Weymouth anemones.
If the disk were spread out and gave rise to little anemones without
tearing--so that they remained in continuity with the parent--we
should get a composite or compound animal, made up of many anemones,
all connected at the base. This actually happens in a whole group
of polyps resembling the sea-anemones. They grow into "stocks,"
"tree-like" or "encrusting" masses, consisting of hundreds and even
thousands of individuals, each with its mouth and tentacles, but with
their inner cavities and bases united. These are the "coral polyps," or
"coral-insects" of old writers, of so many varied kinds. One further
feature of great importance in a "coral" is the production of a hard
deposit of calcite, or limestone, which is thrown down by the surface
of the adhesive disk, and is also formed in deep, radiating "pockets,"
pushed in to the soft animal from the disk. The hard deposit of calcite
is continuous throughout the "stock," or "tree," and when the soft
sea-anemone-like animals die, the hard, white matter is left, and is
called "coral." Very commonly this white coral shows star-like cups
on its surface, which correspond to the lower ends or disks of the
soft sea-anemone-like creatures which deposited the hard coral. In a
less common group (represented commonly on our coast by the so-called
"Dead men's fingers" found growing on the overhanging edges of low-tide
rocks) the hard coral material does not form cups for the minute
sea-anemones which secrete it, but takes the form of a supporting
central or axial rod (sea-pens), or branched tree (sea-bushes), upon
which the fleshy mass of polyps are tightly set. This is the case with
the precious red and pink coral of the Mediterranean (which is now
being "undersold" actually in the Mediterranean markets by a similar
red coral from Japan, usually offered as the genuine article, which it
is not!).
On the British coast you do not, as a rule, find coral-forming polyps.
A small kind, consisting of two or three yellow and orange-red
anemone-polyps united and producing a small group of hard calcite
cups (Caryophyllia and Balanophyllia) is not uncommon at Plymouth at
a few fathoms depth. But you have to go to the Norwegian fiords or
else far out to sea where you have 300 fathoms of sea-water in order
to get really luxuriant white corals--the beautiful Lophohelia (Fig.
3, p. 9), which I used to dredge in the Nord Fiord near Stavanger, as
branching, shrub-like masses of a foot cube in area, each white marble
cup standing out from the stem, an inch long and two-thirds of an
inch across, and the stems giving support to a whole host of clinging
growths (among them Rhabdopleura!) and sheltering wonderful deep-water
worms and starfish.
But these, beautiful as they are, are nothing, so far as mass and
dominating vigour of growth are concerned, in comparison with the
reef-building corals of the warm seas of the tropics. There these
lime-secreting conglomerated sea-anemones separate annually hundreds of
tons of solid calcite per square mile of sea bottom from the sea-water,
and build up reefs, islands, and huge cliffs of coral rock. They
get the calcite--as do calcareous seaweeds and shell-making clams,
oysters, whelks, and microscopic chalk-makers--from the sea--the water
of the sea which always has it ready in solution for their use. And the
sea gets it from the rivers and streams which wear away and dissolve
the old limestone deposits now raised into mountain chains, as well as
by itself dissolving again in due course what living creatures have so
carefully separated from it. Sea water or fresh water with a little
carbonic acid gas dissolved in it dissolves limestone and chalk--it
becomes what we call "hard." Neutralize the dissolved carbonic acid
(as is done in the well-known Clark's process for softening water),
and down falls the dissolved calcite as a fine white sediment. These
alternating processes of solution and "precipitation" are always going
on in the waters of the earth and sea.
The name "jelly-fish" has reference to the colourless, transparent,
soft, and jelly-like substance of the bodies of the animals to which it
is applied. There are a number of marine animals, besides the common
jelly-fish, belonging to different classes, which are glass-like in
transparency and colourless--so as to be nearly or quite invisible in
clear water, and some, too, occur in fresh waters (larvæ of gnats,
notably of the plume-horned gnat Corethra). The transparency of these
animals serves them in two different ways--some are enabled by it to
escape from predatory enemies; others, on the contrary, are enabled
to approach their own prey without being observed. The latter was
obviously the case with the little fresh-water jelly-fish which
appeared in great abundance some years ago in the lily tank in Regent's
Park. The water was full of small water-fleas (minute crustacea),
and the little jelly-fish, if removed from the tank and placed in a
tall glass jar filled with the tank water, spent its whole time in
swimming upwards to the surface by the alternate contraction and
expansion of its disk-like body, and then dropping gently through the
full length of the jar to the bottom, when it would again mount. On
the downward journey--owing to its transparency--it would encounter
unsuspecting, jerkily-moving water-fleas, unwarned by any shadow
cast by the impending glass-like monster of half an inch in breadth
slowly approaching from above; and as soon as they touched it they
were paralysed (by microscopic poison-threads like those of the
sea-anemones), and were grasped and swallowed by the mobile transparent
proboscis (like that of an elephant, though certainly smaller, and
having the mouth opening at its end, instead of a nostril), which hangs
from the centre of the disk-like jelly-fish.[2]
[2] See "Science from an Easy Chair" (First Series, 1910), p. 60, for a
further account and figure of the freshwater jelly-fish.
There are some glass-like transparent creatures, including some small
fishes, which live at 500 fathoms depth and a good deal deeper on the
sea bottom. We know that the sun's light does not penetrate below
200 fathoms, so that one is led to ask--What is the good of being
transparent if you live at the bottom of the sea, at a greater depth
than this? There is also a very beautiful prawn, which I dredged in
Norway in 200 fathoms, which looks like a solid piece of clearest,
colourless glass. And then there are some very beautiful little stalked
creatures (called Clavellina), fixed to the under-side of rocks in the
tidal zone, which are absolutely like drops of solid glass an inch
long. One cannot easily imagine how colourless transparency can be of
"life-saving value" to these varied inhabitants of the dark places of
the sea bottom--any more than we can assign any life-saving value to
the brilliant, gem-like colouring of some of the sea-anemones which
live in the dark on the under-surface of rocks.
The most probable view of the matter is that neither the colourless
transparency of the one set nor the brilliant colouring of the other
has any value; it just happens to be so, and is not harmful. So, for
instance, some crystals are colourless, some blue or green or yellow
or red, without any advantage to them! On the other hand, we know that
a large number of the animals which live in the dark unfathomed depths
themselves produce light, that is to say, are phosphorescent, and it
seems probable that at great depths, though there is no sunlight,
the sea bottom is illuminated--we can only vaguely guess to what
degree--by the strange living lanterns--fish, crustaceans, worms,
and even microscopic creatures--which move about in quest of their
food, carrying their own searchlight with them. Another suggestion is
that the eyes of these inhabitants of the dark may be more sensitive
than our own, and even be affected by rays invisible to us. This,
however, is not probable, since whilst there are among them some with
enormous eyes, we find that at the greatest depths (2 to 4 miles)
even the fishes have no eyes at all, and at a depth of a mile there
are many shrimp-like creatures in which the eyes have been completely
transformed into peculiar "feelers," or otherwise aborted. So that we
cannot suppose there is a possibility of developing the eye of the
dwellers in deep-sea darkness to a degree of sensitiveness greatly
beyond that of terrestrial animals. A limit of obscurity is reached
at which it is of no use having an eye at all, and eyes cease to have
life-saving value, and accordingly are not maintained by natural
selection.
The transparency and colourlessness of marine animals which float near
the surface is, on the other hand, obviously useful, and to this group
our jelly-fishes belong. Not only do they escape observation by their
transparency and general absence of colour, but some actually have a
blue transparent colouring which blends with the blue colour of the
sea. Such are the gas-holding, bladder-like sac as large as your fist
called the "Portuguese man-of-war," and the little sailing Velella,
both of which float, and even protrude above the surface, so as to
catch the wind. Others are only semi-transparent, and others are marked
with strong red, brown, or yellow streaks. Many of the smallest kinds
of jelly-fish have eyes which are bright red in colour.
[Illustration: FIG. 7.--A common British Jelly-fish.
Aurelia aurita, usually as large as a breakfast-plate and often larger.]
The animals to which the name "jelly-fishes" is now more or less
strictly applied are (as that fine zoologist Aristotle knew) in their
structure closely similar to the sea-anemones, but even simpler. They
are called the Medusæ by naturalists. Their disk-like bodies are
largely formed by a jelly-like material, on the surface of which are
stretched delicate transparent skin, nerves, and delicate muscles,
whilst in the middle of the disk, on the surface which faces downwards
as the creature floats, is the mouth, leading into a relatively small
pouched cavity excavated in the jelly, from which a delicate system of
canals is given off, and radiates in the jelly of the disk. There is,
as in the sea-anemones, only one continuous cavity. The edge of the
disk is beset with fine, sensitive tentacles, sometimes many feet in
length, and the lips of the mouth are often drawn out into a sort of
depending trunk, or into four large tapering lobes or lips of jelly,
which, with the longer tentacles, are used for seizing prey. The
commonest jelly-fish on our coast--so common as to be "the" jelly-fish
_par excellence_--is often to be seen left on the sands by the receding
tide or slowly swimming in quiet, clear water at the mouth of a river
in enormous numbers. It is known as "Aurelia" (Fig. 7). It is as big
as a cheese-plate, and the four pouches connected with the stomach
are coloured pink or purple, and appear in the middle of the circular
plate of jelly, like a small Maltese cross. The reproductive particles
(germ-cells and sperm-cells) are produced in that coloured region, and
escape by the mouth. There is a fringe of fine, very short tentacles
round the edge of the disk, and they, as well as the great lobes of
the mouth, are provided with innumerable coiled-up stinging hairs
or "thread-cells," similar to those of the sea-anemones, which led
Aristotle to call both groups "sea-nettles." Eight stalked eyes are set
at equal intervals around the disk.
Usually accompanying the floating crowd of the common and abundant
Aurelia are a few specimens of a very unpleasant kind of Medusa of a
turbid appearance, often called "slime balls" by fishermen, from six
inches to a foot in diameter. It is known to naturalists by the name
"Cyanæa capillata." The tentacles on the edge of the disk of this kind
of jelly-fish are very long and elastic, stretching to several feet,
even yards, in length, and are provided with very powerful stinging
hairs. The tentacles not infrequently become coiled around the body of
a bather; the stinging hairs are shot out of the little sacs in which
they are rolled up, and the result may be very painful to the person
stung in this way and even dangerous. There are two other common large
jelly-fish on the English coast, one called "Chrysaora" (Fig. 8), with
a wheel-like pattern of brown pigment on the disk, and the other with
the mouth lobes very large and bound together like a column.
[Illustration: FIG. 8.--A common British Jelly-fish.
Chrysaora hysoscella, usually twice as big as the figure.]
The common Aurelia is remarkable for the fact that the young which
hatch from its eggs attach themselves to stones and rocks on the sea
bottom, and grow into little white tube-like polyps, about half an
inch long, quite unlike their parent, with a crown of small tentacles
surrounding the mouth, whilst they are fixed by the opposite end of
the body. Then a very curious thing happens. The little polyp becomes
nipped at intervals across its length, so that it looks like a pile
of saucers--a dozen or more. And then the top saucer swims away as a
minute jelly-fish, the next follows, and so on, so that, in the course
of an hour or two, the whole pile separates into a number of freely
swimming young, each of which gradually grows into a full-sized
Aurelia. I have only once had the chance of witnessing this beautiful
sight, and that was many years ago in a tank at the Zoological Gardens
(they have no such tanks now), where the polyp-like young (called
"Hydra tuba") spontaneously put in an appearance, and proceeded to
break up into piles of little disks, which separated and swam off as
one watched them. The French poet, Catulle Mendés, imagined a world
where the flowers flew about freely and the butterflies were fixed to
stalks. His fancy is to some degree realized by the swimming away of
the young jelly-fish from their stalks. There are a host of very minute
jelly-fish, measuring when full grown only half an inch or less in
diameter. They originate as buds from small branching polyps, one kind
of which is common on oyster-shells, and is called "the herring-bone
coralline." The dried skins of these coralline polyps (which are horny)
are often to be picked up with masses of seaweed on the seashore after
a storm. The little jelly-fish are the ripe individuals of the polyps,
and produce eggs and sperm which grow to be polyp-trees. These, again,
after growing and branching as polyps, give rise to little jelly-fish
here and there on the tree, which in most kinds (though not in all)
break off and swim away freely.