SAND-HOPPERS
WHEREVER there is a sandy seashore with here and there masses of
dead seaweed and corallines thrown up by the waves, you will find
sand-hoppers feeding on the debris. They are crustaceans, like crabs,
shrimps, and barnacles, but in general aspect resemble enormous
fleas. I hope that this comparison will not enable any reader at
once to picture the less familiar by the more familiar. A good-sized
sand-hopper is about half an inch long, and jumps not by means of a
specially large pair of legs as the flea does, but by the stroke of
the hind body, the jointed rings of which are carried curled downwards
and ready to give a sudden blow. The sand-hopper (Fig. 20, a) has some
of the rings or segments of the mid-body distinct, and not fused with
those of the head or overhung by a great shield as in the lobster,
crab, and shrimp. His walking legs and jaw-legs are also not quite
of the same shape, though similar to those of a lobster, and his two
little black eyes are not mounted on stalks, but are flush with the
surface of the head. There are two quite distinct kinds of sand-hopper
which live in crowds together on our sandy shores. They are not very
different, but still are distinguished by naturalists from one another;
one is called Talitrus (Fig. 20, a), the other Orchestia (Fig. 20, b).
They are very similar in appearance and structure to a fresh-water
creature common in weedy streams, which has no English name (except
the general one of "fresh-water shrimp"), and is called by naturalists
Gammarus.
[Illustration: FIG. 20.--a, Talitrus locusta, b, Orchestia littorea,
the two common kinds of "sand-hopper." Of the natural size. c, A kind
of small lobster which burrows in the sand, Callianassa subterranea.
About two-thirds the natural size, linear.]
In the open sea there are many hundreds of kinds of small crustaceans
resembling the sand-hoppers in their compressed (not flattened) shape
of body and in the details of their legs and the grouping of the joints
of the body. Many of the smallest crustaceans which swarm in the
surface waters of the sea and form part of that floating population,
mostly of small transparent or iridescent and blue creatures, which
we call the "plankton," or "surface-floating" population, and may be
gathered by towing a very fine net behind a boat on a quiet day, can
produce flashes of light which are vivid enough when seen at night.
They contribute, together with jelly-fish and the teeming millions of
minute bladder-like Noctiluca, and other unicellular animalcules, to
produce that wonderful display seen from time to time on our coasts,
and called "the phosphorescence of the sea." These minute crustaceans
produce flashes of light by suddenly squeezing from pits or glands in
the skin a secretion which is chemically acted on (probably oxidized)
by the sea-water, the chemical action setting up light-vibrations, but
not the usual excess of heat-vibrations to which we are accustomed when
light accompanies ordinary "burning" or "combustion."
[Illustration: FIG. 21.--A Phosphorescent Shrimp (Euphausia pellucida).
The lamp-like phosphorescent organs are numbered 1 to 6. There is
another on the outer edge of the stalked eye, making seven in all on
each side of the animal. _g_, points to the hindermost gill, enlarged.]
Other crustaceans of several kinds, of an inch and more in
length--transparent, delicate creatures, resembling small prawns
in appearance--also produce light. Some of them are known by names
referring to this fact, such as Lucifer (light-bearer) and Nyctiphanes
(night-shiner). They possess special lantern-like knobs scattered
about on the body, which have transparent lenses, and resemble small
bull's-eye lanterns. Some have a row of seven lanterns on each side
of the body (Fig. 21), but one kind has as many as 150 dotted about.
These lanterns were only a few years ago thought to be eyes, and their
elaborate microscopic structure was described as that of an eye. Of
course, this was due to the fact that dead preserved specimens were
studied, and not the living animal. Some twenty years ago I witnessed
a most impressive exhibition of these phosphorescent shrimps at the
house of my friend Sir John Murray, of the "Challenger," at Millport,
on the Clyde. He had obtained them (the kind called Nyctiphanes) in
great quantities at a depth of ninety fathoms in the great Scotch
fiord, and amongst other curious facts about them had shown that they
enter Loch Fyne in vast numbers, and are the special nourishment of the
celebrated Loch Fyne herrings. It had been noticed that the intestine
of the plump, well-fed herrings is full of a deep-black substance, and
Sir John Murray showed that this was the black, indigestible pigment
of the eyes of the hundreds of phosphorescent shrimps swallowed by
these favoured fish, which owe their fine quality to their special
opportunity for feeding in the depths of the loch on the exceptionally
abundant and nutritious light-producing crustaceans! At night my friend
showed me a large glass vessel holding four or five gallons, in which
were a hundred or so of the phosphorescent shrimps swimming around. We
turned out the lamps of the room, and all was dark. Then a gentle tap
was given to the jar, and each little crustacean lit up, as though by
order, a row of seven minute lamps on each side of its body, swimming
along meanwhile, and reminding one of a passenger steamer, as seen from
the shore, as it glides along at night with its lights showing through
a row of cabin windows. The shrimps' lights shone steadily for a minute
or so, then ceased, and had to be lit up again by again signalling
their owners by knocking on the glass. These little lamps, with their
bull's-eye lenses, are far more elaborate structures than the glands
which in other cases cause a flash by discharging a luminous secretion
into the water. They are even more elaborate than the internal
permanent phosphorescent structure of the glow-worm (an insect, not a
crustacean), which has no condensing lens.
I have mentioned these phosphorescent organs of small and smallest
crustaceans because not many years ago a French naturalist, my friend
Professor Giard, found that many of the sand-hoppers on the great sandy
shore near Boulogne are phosphorescent. A year or two later I found
them myself on the shore above tide-mark at Ouistreham (Westerham),
near Caen, where they had actually been mistaken for glow-worms! It was
easy at night to pick up a dozen phosphorescent sand-hoppers during
a stroll of five or ten minutes on the sands. Yet I have never seen
them nor heard of their being seen on the English coast, and one of
the results which I hope for in mentioning them here is that some of
my readers will discover them on British sands and let me know. The
remarkable fact about the luminous sand-hoppers is that they have no
apparatus for producing light, and, as a matter of fact, do not produce
it! Their luminosity is a disease, and is due (as was shown by that
much-beloved teacher and discoverer the late Professor Giard) to the
infection of their blood by a bacillus. Hence it is only here and there
that you see the brilliant greenish ball of light on the sand due to a
phosphorescent sand-hopper. And when you pick it up you find that the
poor little thing is quite feeble and unable to hop. Examine its blood
under the microscope and you find it teeming with excessively minute
parasitic rods like those which cause the phosphorescence of dead fish,
of stale bones, and occasionally of butcher's meat. Similar bacilli may
be obtained by cultivation from any sea-water, and in such abundance
that a room can be lit up by a bottleful of the cultivation. Perhaps
all the light-producing bacteria or bacilli are only varieties of one
species--perhaps they are distinct species. Whether a species or a
variety, that which gets into the blood of the sand-hopper and gives
it the luminosity of a glow-worm, inevitably and rapidly causes its
death--a severe price to pay for brief nocturnal effulgence. Some of
the germs can be removed on a needle's point from the dead sand-hopper
and introduced by the most delicate puncture into a healthy sand-hopper
or into a young crab, with the result that they too become illuminated,
the bacillus multiplying within them. Being thus morbidly illuminated
and having astonished the crustacean, not to say the human world, by
their alarming brilliance, they quickly perish: a little history which
may be read as a parable. The sand-hoppers give the disease to one
another. It is, of course, a merely non-significant thing that the
bacillus happens to set up light vibrations. Its chemical activity is
concerned with its nourishment and growth, and in the course of these
processes it not only produces light but poisonous by-products which
kill its host. Some day we may get an "immune" race of sand-hoppers
who will acquire the illuminating bacillus and defy its poison. Then
we shall have a permanent and happy breed of brilliant sand-hoppers
illuminating the dark places of the seashore.
It is conceivable that some of the disease-producing bacilli (bacteria,
cocci, etc.) which multiply in man's blood and tissues should also
produce light vibrations, and if one could be found that would render
the blood luminous, whilst not producing much pain or _malaise_, no
doubt some excuse would be found for its use as a fashionable toilet
novelty. Cases are on record of luminosity of the surface of the body
and its secretions being developed during serious illness by human
beings, especially in acute phthisis; but these ancient records need
confirmation.
Luminous bacilli or bacteria only give out light when free oxygen is
in the water or liquid inhabited by them. A chemical combination of
the oxygen with substances in the bacteria is the necessary condition
of their evolution of light. When frozen, these bacteria cease to be
luminous--the chemical combination cannot take place when the substance
of the bacterium is frozen solid and maintained in that condition; the
liquid condition is a necessary condition for these changes. These
luminous bacteria have been used recently by Sir James Dewar in the
Faraday Laboratory of the Royal Institution (where Sir James has shown
them to me), for the purpose of investigating the action of intense
cold on living matter. Although their luminous response to oxygen
is arrested when they are frozen, yet immediately on allowing the
temperature to rise above freezing-point the response of the living
matter to oxidation recommences, and a luminous glow is seen. Hence
we have in this glow a ready means of answering the question, "Does
extreme cold, of long duration, destroy the simplest living matter?"
Sir James Dewar has exposed a film of these bacteria to the extremest
degree of cold as yet obtained in the laboratory, that at which
hydrogen gas is solidified, and he has kept them in this, or nearly
this, degree of cold for several months. Yet immediately on "thawing"
the luminous glow was visible in the dark, showing that the bacteria
were still alive. Curiously enough, whilst all chemical action in
living matter can be thus arrested by extreme cold, and yet resumed
on rise of temperature and restoration to the liquid condition, so
that the old phrase and the conception of "suspended animation" are
justified--yet there is one widely-distributed form of activity, the
effect of which the bacteria, even when hard frozen, cannot resist,
namely, that of the blue and ultra-blue rays of light. These rays, if
allowed to fall on the hardest frozen bacillus, get at its chemical
structure, shake it to pieces, destroy it. Hence Sir James Dewar argues
that, whilst it would appear that the extreme cold of space would
not kill a minute living germ, and prevent it passing from planet to
planet, or from remotest space to our earth, yet one thing which is
more abundant in space than within the shell of our atmosphere is
absolutely destructive to such minute particles of living matter,
even when hard-frozen, and that is intense light, the ultra-visible
vibrations of smallest wave-length.
[Illustration: A dance on the seashore: a sketch by Edward Forbes
(1852).]