_BRITISH SPONGES_
It seems to be the popular opinion that sponges are essentially natives
of the warmer seas, and it will probably be a surprise to many young
amateur naturalists to learn that there are about three hundred species
of this sub-kingdom of the animal world to be found on our own shores.
It must not be thought, however, that they are all comparable with the
well-known toilet sponges in regard to either size or general form and
structure, for some of them are very small objects, no larger than about
one-twentieth of an inch in diameter, and some form mere incrustations
of various dimensions on the surfaces of rocks and weeds, often of such
general appearance that they would hardly be regarded as animal
structures by those who have not studied the peculiarities of the group.
Sponges are known collectively as the _Porifera_ or _Polystomata_, and
constitute a separate sub-kingdom of animals of such distinct features
that they are not readily confused with the creatures of any other
group. Their principal characteristic is expressed by both the group
names just given, the former of which signifies ‘hole-bearing,’ and the
latter ‘many openings’; for in all the members of the sub-kingdom there
are a number of holes or pores providing a means of communication
between the body cavity or cavities and the surrounding water. Most of
these holes are very small, but there is always at least one opening of
a larger size at the anterior end.
It will be seen from what we have just stated that sponges exhibit a
distinctly higher organisation than the _protozoa_ described in the last
chapter, inasmuch as they possess a permanent body-cavity that
communicates with the exterior; but in addition to this there are many
points of differentiation of structure that denote a superior position
in the scale of life.
In order to ascertain the general features of a sponge we cannot do
better than select one of the simplest forms from our own shores. If we
place the live animal in a glass vessel of sea water, and examine it
with a suitable magnifying power, we observe a number of minute pores
scattered over its whole surface; and a much larger opening at the free
end. The animal is motionless, and exhibits no signs of life except that
it may contract slightly when touched. The water surrounding the sponge
also appears to be perfectly still, but if we introduce some fine
insoluble powder, such as precipitated chalk, or a drop of a soluble
dye, the motion of the suspended or soluble material will show that the
water is passing into the sponge through all the small pores, and that
it is ejected through the larger opening.
On touching the sponge we observe that it is of a soft, gelatinous
consistence throughout, or if, as is often the case, the body is
supported by a skeleton of greater or less firmness, a gentle
application of the finger will still show that this framework is
surrounded by material of a jelly-like nature. This gelatinous
substance is the animal itself, and a microscopic examination will show
that its body-wall is made up of two distinct layers, the inner
consisting of cells, many of which possess a cilium or whip-like
filament that protrudes from a kind of collar, its free extremity
extending into the body-cavity.
[Illustration: FIG. 65.--SECTION OF A SIMPLE SPONGE]
These minute cilia are the means by which the water currents just
described are set up. By a constant lashing movement they urge the fluid
contained in the body-cavity towards the larger hole, thus causing the
water to flow in through the numerous small pores. This circulation of
sea water through the body-cavity of the sponge is the means by which
the animal is supplied with air and food. Air is, of course, absorbed
from the water by the soft material of the external layer of the body,
but the constant flow of fresh water through the body-cavity enables
this process of respiration to go on with equal freedom in the interior.
The mode of feeding of the sponge is very similar to that of the
_protozoa_. Organic particles that are carried into the body-cavity, on
coming in contact with the cells of the internal layer, are absorbed
into their protoplasm by which they are digested. Thus the sponge may be
compared to a mass of protozoon cells, all united into a common colony
by a more or less perfect coalescing of the cell-substance, some of the
units being modified in structure for the performance of definite
functions. The air and food absorbed by any one cell may pass readily
into the surrounding cells, and thus each one may be said to work for
the common weal.
[Illustration: FIG. 66.--DIAGRAMMATIC SECTION OF A PORTION OF A
COMPLEX SPONGE]
The description just given applies only to the simplest of the sponges,
and we have now to learn that in the higher members of the group the
structure is much more complicated. In these the surface-pores are the
extremities of very narrow tubes which perforate both layers of the
body-wall and then communicate with wider tubes or spaces within, some
of which are lined with the ciliated cells above described. These
spaces, which are sometimes nearly globular in form, and often arranged
in groups with a common cavity, communicate with wider tubes which join
together until, finally, they terminate in a large opening seen on the
exterior of the sponge. Hence it will be seen that the water entering
the minute pores of the surface has to circulate through a complicated
system of channels and spaces, some of which are lined with the ciliated
cells that urge the current onwards before it is expelled through the
large hole. Further, imagine a number of such structures as we have
described growing side by side, their masses coalescing into one whole,
their inner tubes and spaces united into one complex system by numerous
inter-communications, and having several large holes for the exit of the
circulating water, and you then have some idea of the general nature of
many of the more complex sponges to be found on our shores (see fig.
66).
[Illustration: FIG. 67.--HORNY NETWORK OF A SPONGE, MAGNIFIED]
But even this is not all, for as yet we have been regarding the sponges
as consisting of animal matter only, whereas nearly all of them possess
some kind of internal skeleton for the support of the soft, gelatinous
animal substance. The skeleton consists of matter secreted by certain
cells from material in the water and food, and is either horny,
calcareous, or siliceous. The horny skeleton is formed of a network of
fibres of a somewhat silky character, and often, as in the case of the
toilet sponges, highly elastic; but it is sometimes so brittle that the
sponge mass is easily broken when bent. The fibres of this framework
support not only the outer wall of the sponge, but also the walls of all
the internal tubes and spaces, which are often of so soft a nature that
they would collapse without its aid.
The other forms of skeletons consist of minute bodies of carbonate of
lime or of silica, respectively, which assume certain definite shapes,
resembling stars, anchors, hooks, pins, spindles, &c., and are known as
_spicules_. Such spicules are usually present in those sponges that have
horny skeletons, but in others they form the entire skeleton.
Sponges sometimes increase by division, a part being separated from the
parent mass and then developing into a complete colony; and they may be
reproduced artificially to almost any extent by this method, each piece
cut off, however small, producing a new sponge. They also increase by a
process of ‘budding,’ the buds produced sometimes remaining attached to
the original colony, thus increasing its size, but on other occasions
becoming detached for the formation of new colonies on a different site.
In addition to these methods of reproduction there are special cells in
a sponge that possess the function of producing eggs which are ejected
through the larger holes. The eggs are usually developed in the autumn,
and, after being ejected, swim about freely for a time, after which they
become fixed to rocks or weeds, and produce sponges in the following
year. The eggs may often be seen towards the end of the summer by
cutting through a sponge, or by carefully pulling it asunder. They are
little rounded or oval bodies, of a yellowish or brownish colour,
distinctly visible to the naked eye, occupying cavities in the interior.
Sponges are classified according to the composition of the skeleton and
the forms of the spicules, the chief divisions being:--
1. The CALCAREOUS Sponges (_Calcarea_). Skeleton consisting of
spicules of carbonate of lime in the form of needles and
three-or four-rayed stars.
2. The SIX-RAYED SPONGES (_Hexactinellida_). Skeleton of six-rayed
glassy spicules.
3. COMMON SPONGES (_Demospongia_). Skeleton horny, flinty, or
entirely absent.
The first of these divisions contains about a dozen known British
species, which are to be found on the rockiest shores, attached to
stones, weeds, or shells, generally hidden in very secluded holes or
crevices, or sheltered from the light by the pendulous weeds. They
should be searched for at the lowest spring tide, particular attention
being given to the under surfaces of large stones, narrow, dark
crevices, and the roofs of small, sheltered caves. They may be readily
recognised as sponges by the numerous pores on the surface, though these
are often hardly visible without a lens, and the calcareous nature of
the skeleton may be proved by dropping a specimen into dilute
hydrochloric acid, when the carbonate of lime will speedily dissolve,
the action being accompanied by the evolution of bubbles of carbonic
acid gas.
If calcareous sponges are to be preserved for future reference, they may
be placed in diluted spirit, in which case the animal matter, as well
as the mineral substance, will be preserved with but little alteration
in the natural appearance and structure. A specimen which has been
decalcified by means of acid, as above described, may also be preserved
in the same manner; and small portions of this will serve for the
microscopic study of the animal portion of the sponge. If the skeleton
only is required, the sponge is simply allowed to dry, when the soft
animal substance, on losing its contained water, will leave hardly any
residue; or, better, allow the calcareous sponge to macerate in water
for some days for the animal substance to decompose, and then, after a
few minutes in running water, set it aside to dry.
[Illustration: FIG. 68.--_Grantia compressa_]
[Illustration: FIG. 69.--SPICULES OF _Grantia_, MAGNIFIED]
Small portions of the skeleton, examined under the microscope, will show
the nature of the calcareous spicules of which it is composed. These
consist of minute needles and stars, the latter having generally either
three or four rays.
We give figures of three of the calcareous sponges of our shores, the
first of which (_Grantia compressa_) resembles little oval, flattened
bags, which hang pendulous from rocks and weeds, sometimes solitary, but
often in clusters. The smaller openings are thickly scattered over the
flat sides of the bag, and the larger ones, through which the water is
expelled, around the margin. When the sponge is out of the water and
inactive, the two opposite sides of the bag are practically in contact,
but, when active, the cavity is filled with water by means of the
whip-cells that line it, and the sides of the sponge are then more or
less convex.
[Illustration: FIG. 70.--_Sycon ciliatum_]
The ciliated sycon (_Sycon ciliatum_), fig. 70, though of a very
different appearance externally, is similar in structure to _Grantia_.
It is also found in similar situations, and is not uncommon on many
parts of the South Coast, from Weymouth westwards. The other example,
_Leucosolenia botryoides_, shown in fig. 71, is a branching calcareous
sponge, consisting of a number of tubes, all united to form one common
cavity which is lined throughout with whip-cells. It is usually found
attached to weeds.
[Illustration: FIG. 71.--_Leucosolenia botryoides_, WITH PORTION
MAGNIFIED]
Nearly all our British sponges belong to the group _Demospongia_--common
sponges; but the members of this group present a great variety of form
and structure. Most of them have a skeleton consisting of siliceous
spicules, but some have a horny skeleton, somewhat after the nature of
that of the toilet sponges; and others, again, have fleshy bodies
entirely, or almost entirely, unsupported by harder structures. They are
sometimes known collectively as the _Silicia_, for the greater number of
them have skeletons consisting exclusively of siliceous matter, while
the so-called horny sponges usually have spicules of silica
intermingled with the horny substance, and even those which are
described as having no skeleton at all sometimes contain scattered
spicules of silex.
[Illustration: FIG. 72.--_Chalina oculata_]
As the spicules of sponges are in themselves beautiful objects, and are
important to the naturalist, inasmuch as they form a basis for the
classification of sponges, it is well to know by what means they may be
separated from the animal for microscopic examination. The separation is
based on the fact that nitric acid (aqua-fortis) will destroy organic
matter while it has not the slightest action on silica. In some of our
common horny sponges the fibres are so transparent that, when teased out
and placed under the microscope, the siliceous spicules may be seen
embedded within them, but the spicules, both in these and the fleshy
sponges, may be separated completely from the animal matter by putting a
fragment of the sponge in a test-tube, covering it with nitric acid, and
boiling it for a short time. The tube should then be filled up with
water and allowed to stand undisturbed for a time, after which the
liquid is poured off gently from the sediment. If the sediment is then
put under the microscope on a slip of glass, it will be seen to consist
of grains of sand, of which there is always a considerable amount in the
pores and cavities of a sponge, and the siliceous spicules.
Among the common objects of the sea shore is the horny skeleton of the
sponge _Chalina oculata_, which is frequently washed on the beach by
the waves, especially after storms. This sponge is not likely to be seen
between the tide-marks except at the lowest spring tide, when it may be
found suspended in a sheltered crevice or cave. The skeleton consists of
a fine network of horny fibres, in the centre of which lie the spicules,
imbedded in the horny material. The spicules are short and straight,
tapering at both ends.
[Illustration: FIG. 73.--_Halichondria panicea_]
The Bread-crumb sponge (_Halichondria panicea_) is even more common, for
it is to be found on every rocky coast, encrusting weeds and rocks,
often considerably above low-water mark. It is of a yellowish or pale
greenish colour, and forms an incrustation varying in thickness from
one-twentieth of an inch to half an inch or more; and, like most
sponges, should be looked for in narrow crevices, under heavy growths of
weeds, or in other situations where it is protected from the light.
Sometimes its free surface is unbroken, except, of course, by the minute
pores, and, here and there, the larger openings that serve for the
outgoing currents; but when it is found encrusting a rock in patches of
considerable size, the larger holes all occupy the summit of a little
cone resembling a miniature volcano with its crater. This sponge is
easily removed from the rock with the aid of a blunt broad-bladed knife,
and retains its natural appearance to perfection if preserved in
methylated spirit. Its horny skeleton is of a very compact nature, and
the spicules are minute siliceous needles pointed at both ends.
Rambling on the sea beach we frequently meet with old oyster and other
shells perforated by a number of circular holes about the size of a
pin’s head or less, and chalk and limestone rocks also are seen
similarly bored. On breaking into or grinding down the substance we find
that the openings are the ends of channels that form a network of canals
and chambers, some of which are so near the surface that they are
covered by an exceedingly thin layer of the calcareous substance. These
canals and chambers form the home of the Boring Sponge (_Cliona_),
which, although a very soft-bodied animal, has itself excavated them.
[Illustration: FIG. 74.--SPICULES OF _Halichondria_, MAGNIFIED]
[Illustration: FIG. 75.--AN OYSTER SHELL BORED BY _Cliona_]
The manner in which the _Cliona_ excavates such a complicated system of
passages in so hard a material has naturally raised a considerable
amount of curiosity, and those who have studied the matter are divided
in opinion as to whether the work is done by chemical or by mechanical
action.
Some of those who advocate the chemical theory suppose that an acid
fluid is secreted by the sponge, and that the carbonate of lime forming
the shell or stone is thereby dissolved; but such advocates have, as
yet, failed to detect the presence of any acid substance in the body of
the animal. Others ascribe the action to the solvent power of carbonic
acid gas. This gas certainly has the power of dissolving carbonate of
lime, as may be proved by a very simple experiment: Pour a little lime
water into a glass, and blow into it through a glass tube. The lime
water speedily becomes milky in appearance, the lime having been
converted into particles of chalk or carbonate of lime by union with the
carbonic acid gas from the lungs. Continue to blow into the liquid for
some time, and the carbonate of lime will slowly disappear, being
gradually dissolved by the excess of the gas--the gas over and above
that required for the formation of the carbonate. Thus, it has been
said, the carbonic acid gas evolved as a product of the respiration of
the sponge is the agent by which the channels are excavated. Whatever be
the acid to which this power is ascribed, whether it be the carbonic
acid or a special acid fluid secreted for the purpose, there is still
this difficulty in the way of accepting the theory, namely, that an
acid, though it has the power of dissolving the mineral matter of a
shell--the carbonate of lime--has no action on the laminæ of animal
substance that form part of the structure. If we put the shell of a
mollusc in hydrochloric or _dilute_ nitric acid, we obtain, after the
complete solution of the carbonate of lime, a substantial residue of
animal matter which the acid does not touch, but in the case of _Cliona_
both animal and mineral substances yield to its power.
[Illustration: FIG. 76.--SPICULES OF _Cliona_]
Those who favour the mechanical theory assert that the material is worn
away by siliceous particles developed by the sponge, and kept in
constant motion as long as the animal lives; and the theory is supported
by the statement that, in addition to the spicules of silica, which are
pin-shaped, and occupy the interior of the animal, there are little
siliceous granules scattered on the surface of the sponge which are kept
in constant motion resembling that of cilia; and the minute particles of
carbonate of lime that form a dusty deposit within the galleries are
supposed to be the product of the rasping or drilling action of these
granules.
The pin-shaped spicules of _Cliona_ may be obtained for microscopic
examination by breaking any old oyster shell that has formed its home,
and brushing out the dust from the galleries; or, a part of the shell
may be dissolved in acid, and the sediment examined for spicules on a
slip of glass.