SPONGES.
To many persons the statement that we are going for a ramble among the
rocks in quest of sponges will merely suggest the idea of wreckage, and
they will suppose that we have had information that a vessel, part of
whose cargo was Turkey sponges, has gone to grief on the rocks near,
and that sponges are to be had for the trouble of picking them up. And
should they venture to accompany you on so promising an expedition,
they would certainly consider you demented as, having reached the rocks
that are only uncovered at very low tides, you proceeded to point out
the green and orange and brown and whity-yellow expanses that coat
the vertical faces of the rocks. All these things to them bear no
resemblance to the only sponges they know--the ones they use daily for
purposes of ablution. You can show them something approaching nearer to
their ideal, if you hunt among the thick stems of the shrubby weeds on
the rock. There, encrusting a branch, is a yellowish-brown form with
rough surface and large pores very much like those they know all about.
And attached to various weeds are others of the shape, size, and colour
of melon seeds, with porous surface and open end.
[Illustration: SPONGES.
Hymeniacidon albescens; Pachymatisma johnstoni; Halichondria panicea;
Grantia coriacea; Halichondria incrustans; Leuconia nivea; Leuconia
gossei.]
Your friend, though disappointed, maybe, that he is not to share in
the salvage of some splendid bath sponges from the supposed wreck,
cannot help feeling some interest in the extensive layers of colour
on the rocks, some of it raised into conical hillocks, and suggesting
a fairy plain thickly studded with volcanoes. You tell him that these
are really aquatic volcanoes so-to-speak, and that if you could get a
portion off the rock, you could exhibit the phenomenon to him at
work in a shallow dish of sea-water. Thereupon he, thinking to be of
service to you tears off a slice of the pale green, hillocky sponge
(_Halichondria panicea_) and breaks it up hopelessly. However, we will
turn his clumsiness to account and take a view of the interior thus
violently exposed. We see that these crater-like openings are the
outlets to tubular spaces running through the sponge, and from these
passages smaller branches go off at right angles, whilst these and the
larger openings are surrounded by tissues that are very like bread in
consistence; and that is really only a way of explaining that they are
spongy. Now the whole of the substance of these sponges, as you may
see by microscopical examination, is composed of myriads of minute
flint spicules, finer than the most delicate fragments of “spun glass,”
and of beautiful forms. Some are simple rods, straight and curved;
others forked at one end; some like a gribble; others what is known as
quadriradiate in form.
Now in some species these spicules are not arranged in any order; they
are merely jumbled together, and their remarkable forms make it easy
for them to become entangled. When so entangled they form the skeleton
of the sponge. Each sponge is a co-operative colony containing many
thousands of members, and these are represented to our view, through
my pocket lens, in the mass only, as a thin clear jelly investing the
spicule-tangles, or rather the spicules are imbedded in the _sarcode_
as this living matter is termed. If we were to chip off a thin flake of
rock with its investing sponge intact, and place the whole in a glass
vessel full of water, we could observe the movements which manifest
its vitality. A little finely powdered indigo or other colouring
matter should be dropped into the water near the specimen. On closely
observing it would be seen that many of these minute granules were
flowing towards the sponge, then that they entirely disappeared through
the very fine openings in the surface. A little later these particles
will reappear, not where they went in, but in a denser stream issuing
from one of the craters, which are scientifically designated _oscula_
to distinguish them from the minute pores.
[Illustration: SECTION THROUGH CRUMB-OF-BREAD SPONGE.]
If we dissect the sponge under a microscope, we shall find that from
one of these _oscula_ a broad passage runs through the centre of the
mass, and from the walls of this the minute pores run off to the
outer surface. This central cavity is invested by a living membrane
which, when examined through a higher power of the microscope, is seen
to consist of myriads of organisms closely packed together side by
side, and each resembling a glass vase, spherical below, with a wide
neck, and from its centre there issues a long antenna-like process.
This is called the _flagellum_ (Latin, a whip), because its office
is to lash the water. These flask-like organs, with their flagella,
present a wonderful likeness to some free infusoria known as collared
monads, and over this likeness and all that it may or may not imply
to the systematic naturalist much ink has been shed, and the sounds
of controversial strife it engendered, though now faint, are still
audible. Into _that_ question we do not go.
The combined lashing of these little whips in unison sets a strong
current of water flowing through the central passages and out at the
_oscula_. To feed this stream, water flows in automatically through all
the little pores, and brings with it the infusoria and other minute
particles of life with which the sea is swarming. These come in contact
with the lips of the flasks in the interior over which the living jelly
of the sponge is steadily flowing. The infusoria flow with it and are
carried away by the current to a little clear space (_vacuole_) in the
lower part of the flask, where it is digested, and the refuse portions
are thrust out to go in the general stream and be carried out through
the oscula. Each of these cells may be taken therefore as a separate
individual, enjoying home rule, yet taking part in general efforts
for the whole sponge-community, for we find that by some strangely
communicated understanding, all these cells cease lashing the water for
a time as though resting (or digesting their food), and the craters
cease to pour forth their streams. But then after a time activity is
resumed, the craters belch forth again, and we know thereby that the
flagella are in active operation down below, not merely capturing and
digesting food, but also absorbing oxygen from the inflowing streams,
whereby vital energy is maintained.
After the cells have become full grown, they split transversely or
longitudinally, and so increase their number, which means that the size
of the colony increases. But some of these divided portions develop
into eggs, which after fertilization are swept out into the ocean by
the outflowing current, and settling upon some rock become glued down
and grow, gradually, by division and subdivision, producing a new
colony. Such is a highly condensed account of the general phenomena
of sponge life. There are variations upon it in the life-history of
well-nigh every species; but this will suffice to give my reader
a general idea of what sponges are. For the rest, he must go down
among the rocks, and search out the various species of many forms,
and endeavour to add to the general sums of knowledge by some fresh
observations respecting British Sponges.
However startling the statement may sound, there is no lack either
of specimens or species on the British coasts. Some of the most
conventionally sponge-like of these must be sought by the dredge in
deep waters, but our own hunting ground, the rocks that mark the
shoreward-bounds of the laminarian zone, if carefully inspected at low
spring tides, will afford more specimens in half an hour than we can
exhaust the interest of in a week. That this is no mere figure of
speech you will agree when I add that Dr. Bowerbank published a work
in three volumes dealing only with British Sponges, and to these a
supplementary posthumous volume, edited by Dr. Norman, has since been
added.
Where the rocks rise high above the shore with their upper portions
tilted towards the cliffs, we shall find several species incrusting the
vertical or overhanging surfaces of these rocks, such as _Halichondria
incrustans_, whose buff-coloured bread-like surface is diversified with
slightly raised oscula. Its principal spicules are knobbed at one end,
in which respect it differs from the similar _Halichondria panicea_
which is peculiar in having only one type of spicules--a rounded
rod, slightly curved or quite straight, but pointed at each end.
Ellis called this species the Crumb-of-bread sponge, a name which is
reflected in the scientific cognomen _panicea_. It is one of the most
plentiful of the encrusting species, and may be readily known by the
greenish-yellow or distinctly green colour of its extensive patches.
Not far from the Crumb-of-bread will in all probability be found
the similar Sanguine sponge (_Halichondria sanguinea_), of a bright
red colour. The conical elevations of the _oscula_ in these species
distinguish them readily from the plump, though narrow bands of
_Microciona carnosa_, a plentiful species that creeps extensively
between the other kinds, its pale red branches being very unequal
in width, and alternately contracting and swelling out, joining and
separating. This will be found figured in the lower left-hand corner of
our illustration on page 29.
[Illustration: GRANTIA COMPRESSA.]
[Illustration: GRANTIA CILIATA.]
A very noticeable species on account of its neat compact shape will be
found attached to various red seaweeds, with which its whitish colour
contrasts well. It is a small oval, usually from a quarter to an inch
in length, very flat, but yet hollow, with a large vent at the free and
larger end. This is the _Grantia compressa_. Careful search among the
indescribable medley of “unconsidered trifles” that crust the rocks
beneath the shelter of the Fucus-growth, will reward us with a little
spherical sponge with tubular oscula at the summit formed of spicules,
and its general surface bristling with long spicules. This is the
_Grantia ciliata_, looking like a little gooseberry.
There are many other forms, for which I must refer my readers to Dr.
Bowerbank’s work, where also will be found descriptions and figures
of many deep-water species, such as the more conventional sponge-like
_Chalina oculata_, in branching masses nine or ten inches high.
There is, however, one other we must mention; the so-called
Boring sponge (_Cliona celata_), which attacks various shells and
stones. It is quite a common occurrence for the rambler along the shore
to pick up the shell of some mollusk, and find it so tunnelled, the
borings branching in every direction, that what would otherwise be as
strong as stone is now as weak as poor strawboard, and will yield to
very slight pressure or strain. On breaking such a shell across we get
both cross and longitudinal sections of these tunnels and chambers,
and find some of them to be lined with a dark brown filmy tissue,
the remains of some past inhabitant; others contain portions of this
_Cliona_ sponge, living or dead; others again contain little bivalve
shells that just fit the aperture, whilst yet another set exhibit clean
walls that may not have had any animal inmate. Much controversy has
raged over the question whether these excavations have been made by
the sponge, or by some boring worm, and there have not been wanting
as advocates of either view men whose authority on sponge matters is
unquestioned. Where such doctors differ how shall humble observers
venture to give a verdict? For my part, I cannot give my support to
the contention that the sponge has bored the clean holes, hollows,
and tubes that I have seen in the large numbers of attacked shells I
have broken; neither am I prepared with an opinion as to the creature
that did make them. I believe that on this matter, as on many others
connected with natural history, we have much still to learn, and
every student of Nature should have his eyes and his mind ever open
to receive hints from Nature herself as to her methods. One of these
days, some lonely wanderer by the margin of the wave will show us how
simply this boring is accomplished, and we shall all wonder that we
never thought of the possibility before. But whatever views or lack of
views we may have upon the question, “who made the burrows?” there is
no doubt that the sponge does exist in some of them, and its spicules
embedded in the yellow sarcode are well worthy of minute observation.