BRANCH PORIFERA: THE SPONGES
THE FRESH-WATER SPONGE (_Spongilla_ sp.)
TECHNICAL NOTE.--Fresh-water sponges may perhaps not be readily
found in the neighborhood of the school, but they occur over most
of the United States, and careful searching will usually result in
the finding of specimens. They are compact, solid-looking masses,
sometimes lobed, resting on and attached to rocks, logs, timbers,
etc., in clear water in creeks, ponds, or bayous. They are creamy,
yellowish-brown or even greenish in color and resemble some
cushion-like plant far more than any of the familiar animal forms.
They can be distinguished from plants, however, by the fact that
there are no leaves in the mass, nor long thread-like fibres such
as compose the masses of pond algæ (pond scum). When touched with
the fingers a gritty feeling is noticeable, due to the presence of
many small stiff spicules. Sponges should be removed entire from
the substance they are attached to, and may be taken alive to the
laboratory. They die soon, however, and should be put into alcohol
before decay begins.
Note the form of the sponge mass. Is it lobed or branched? Examine the
surface for openings. These are of two sizes; the larger are _osteoles_
or _exhalant openings_, while the smaller and more numerous are _pores_
or _inhalant openings_. The sponge-flesh is called _sarcode_. Examine a
bit of sarcode under the microscope; note the _spicules_. Have these
spicules a regular arrangement? Of what are they composed?
Draw the entire sponge, showing shape and openings; draw some of the
spicules.
Embedded in the body-substance, especially near the base, note (if
present) numerous small, yellowish, sub-spherical or disk-like
bodies, the _gemmules_. These are reproductive bodies. Each gemmule is
a sort of internal bud. It is composed of an interior group of
protoplasmic cells, enclosed by a crust thickly covered with spicules.
In winter the sponge dies down and the gemmules are set free in the
water. In spring the protoplasmic contents issue through an aperture
in the crust, called the _micropyle_ or _foraminal opening_, and
develop and grow into a new sponge.
For a good account of the fresh-water sponge, see Pott's "Fresh-water
Sponges."
A CALCAREOUS OCEAN-SPONGE (_Grantia_ sp.) (fig. 7, D, E, F.)
TECHNICAL NOTE.--For inland schools, specimens preserved in
alcohol or formalin must be used. They may be obtained from
dealers in naturalists' supplies (see p. 453). Specimens of some
species of this genus can be obtained at almost any point on the
Atlantic or Pacific coasts of this country.
Examine the external structure of a specimen. Note the elongate,
sub-cylindrical form, the attached base, the free end. Note the large
exhalant opening, _osteole_ or _osculum_, at the free end; the
numerous small inhalant openings elsewhere on the surface (best seen
in dried specimens). Note the _spicules_ covering the surface of the
body, and the longer ones surrounding the osculum. Cut the sponge in
two longitudinally and note the simple cylindrical body-cavity, the
_gastric cavity_ or _cloaca_. Note the thickness of the body-wall;
note the tubes running through the body-wall from cloaca to external
surface. Through these tubes water laden with food enters the gastric
cavity, where the food is digested, the water and undigested particles
passing out through the osculum. Crush a bit of dried sponge, or boil
a bit of soft sponge in caustic potash and mount on a glass slide.
Examine under a microscope and note the abundance of spicules and the
variety in their form. Two kinds may always be found, and sometimes
three. These spicules are composed of carbonate of lime and can be
dissolved by pouring on to them a drop of hydrochloric acid.
Some of the sponges may have buds growing out from them near the base.
These buds are young sponges developed asexually. If allowed to
develop fully the buds would have detached themselves from the parent
and each would have become a new sponge.
Make drawings showing the form of a whole sponge; the appearance of
the inner face of the sponge bisected longitudinally; the shape of the
spicules.
A COMMERCIAL SPONGE
TECHNICAL NOTE.--For the study of the skeleton of an ocean-sponge
with more complex body buy several common small bath-sponges without
large holes running entirely through them. The teacher should have
also a few specimens of small marine sponges preserved in alcohol or
formalin. Such specimens should be part of the laboratory equipment
(see account of laboratory equipment, p. 450), and can be readily
and cheaply obtained from dealers in naturalists' supplies.
The bath-sponge or slate-sponge consists simply of the hard parts or
skeleton of a sponge animal. In life all of the skeleton is enclosed
or covered by a soft, tough mass composed of layers of cells. Note the
many openings on the surface of the sponge. Crush a bit of the
skeleton and examine it under the microscope. Note that it is composed
of fine fibres of a tough, horny substance called _spongin_, instead
of tiny distinct calcareous spicules.
OTHER SPONGES
The sponges are fixed, plant-like aquatic animals. The members of a
single family live in fresh water, being found in lakes, rivers, and
canals in all parts of the world. All the other sponges, and there are
several thousand species known, live in the ocean. They are to be
found at all depths, some in shallow water near the shore and others
in deeper water, even to the deepest depths yet explored. They are
found in all seas, though especially abundantly in the Atlantic Ocean
and Mediterranean Sea.
=Form and size.=--The shape of the simplest sponges is that of a tiny
vase or nearly cylindrical cup, hollow and attached at its base. At the
free end there is a large opening. But there is a great deal of variety
in the form and size of different sponges. There is, indeed, much
variation in the shape and general character of different individuals of
the same species. Unlike most other animals, sponges are fixed, and the
character of the surface to which a sponge is attached has much
influence upon its shape. If this surface is rough and uneven the sponge
may follow in its growth the sinuosities of the surface and so become
uneven and distorted in shape. At best, only a few kinds of sponges have
any very even and symmetrical shape. Most of them are very unsymmetrical
and grow more like a low compact bushy plant than like the animals we
are familiar with. The smallest sponges are only 1 mm. (1/25 in.) high,
while the largest may be over a meter (39 in.) in height. In color
living sponges may be red, purple, orange, gray, and sometimes blue.
Most sponges have the whole body of one color.
[Illustration: FIG. 12.--The skeleton of a "glass" sponge (skeleton
composed of siliceous spicules) from Japan. (From specimen.)]
=Skeleton.=--A very few sponges have no skeleton at all. The others
have a skeleton or hard parts composed of interwoven fibres of the
tough, horny substance called spongin, or of hosts of fine needles or
spicules of silica or of carbonate of lime. The siliceous skeletons of
some of the so-called glass-sponges (fig. 12) are very beautiful. The
lime and siliceous sponge spicules exhibit a great variety of outline,
some being anchor-shaped, some cross-shaped, and some resembling tiny
spears or javelins.
=Structure of body.=--The skeleton of a sponge whether composed of
interlacing fibres or of short spicules is always invisible from the
outside when the sponge is alive. It is embedded in, or clothed by,
the soft, fleshy part of the body. The soft part of the sponge is
composed simply of two layers of cells, one constituting the external
surface of the body, and the other lining the interior cavities and
canals of the body. Between these two cell-layers there is a mass of
soft gelatinous substance all through which protoplasm ramifies, and
in which are embedded numerous scattered cells. There are, as seen in
the case of _Spongilla_ and _Grantia_, no systems of organs such as
characterize the higher animals. No heart, lungs, alimentary canal,
nervous system, organs of locomotion, eyes, ears, or other organs of
special sense; the sponge has none of these. It is simply an aggregate
of cells, arranged in two layers, and supported usually by a skeleton
of horny fibres or calcareous or siliceous spicules. Its body is
usually shapeless, unsymmetrical and without front or back, right or
left. It is not to be wondered at that sponges were for a long time
believed to be plants.
=Feeding habits.=--The sponges feed on minute bits of animal or plant
substance and on the microscopic unicellular plants or animals which
float in the water which bathes their bodies. The water entering the
sponge-body through the various openings of the surface is moved
along by the waving or lashing of the flagella of the cells which line
the canals, and these currents of water bear with them the tiny
organisms which are taken up by these same cells and digested. The
incoming currents of water meet in the central cavity or cavities of
the body and pass out through the large opening called the osculum at
the free end of the vase-like body, or if the body is branched,
through the large openings at the tips of these branches.
The same currents of water bring also oxygen for the sponge's breathing
and carry away the carbonic acid gas given out by the body-cells.
As a German naturalist has said, the one necessary condition for the
life of a sponge is the streaming of water through its body. All
sponges have a system of canals for this water-current and all have
means, in the waving flagella or cilia with which these canals are
lined, for producing these currents. When a live sponge is put into a
vessel of water, currents are immediately set up, and they always flow
into the body through the many fine openings and out of the body
through the osculum.
=Development and life-history.=--Although the sponge in its adult
condition is permanently attached by its base to the sea-bottom or to
some rock or shell, when it is first born it is an active free-swimming
creature. The sponges reproduce in two ways, asexually and sexually. The
asexual mode of reproduction of the fresh-water sponge by gemmules has
already been described. The ocean sponges also reproduce asexually
either by forming interior gemmules or external buds. In this latter
method a bud forms on the outer surface of the body which increases in
size and finally grows into a new sponge individual. In some species
this new sponge does not become separated from the body of the mother,
but remains attached to it like a branch to a tree-trunk. By the
continued production of such non-separating individuals, a colony of
sponges is formed which has the general appearance of a branching plant.
In other species the new sponge formed by the development and growth of
a bud falls off and becomes a distinct separate individual.
In the sexual mode of reproduction, male or sperm-cells and female or
egg-cells are developed in the same individual. The sperm-cells are
motile and swim about in the cavities and canals of the sponge-body
until they find egg-cells, which they fertilize. The fertilized eggs
begin to develop and pass through their first stages in the
sponge-body. Finally the embryo sponge, which is usually a tiny oval
or egg-shaped mass of cells, escapes from the body of the parent into
the water. The young sponge has some of its outer cells provided with
cilia, and by means of these it swims about. After a while it comes to
rest on the ocean-floor or on some rock or shell, attaches itself, and
begins to take on the form and character of the parent. It leads
hereafter a fixed sedentary life.
=The sponges of commerce.=--The sponge-skeletons which are the
"sponges" that we use all belong to a few species, not more than half
a dozen. Most of the commercial sponges come from the Mediterranean
Sea, though some come from the Bahama Islands, some from the Red Sea,
and a few from the coasts of Greece, Asia Minor, and Africa. The
commercial sponges do not live in very deep water; they are usually
found not deeper than 200 feet. The living sponges are collected by
divers, or are dragged up by men in boats using long-poled hooks, or
dredges. "When secured they are exposed to the air for a limited time,
either in the boats or on shore, and then thrown in heaps into the
water again in pens or tanks built for the purpose. Decay thus takes
place with great rapidity, and when fully decayed they are fished up
again, and the animal matter beaten, squeezed, or washed out, leaving
the cleaned skeleton ready for the market. In this condition after
being dried and sorted, they are sold to the dealers, who have them
trimmed, re-sorted and put up in bales or on strings ready for
exportation. There are many modifications of these processes in
different places, but in a general way these are the essential-steps
through which the sponge passes before it is considered suitable for
domestic purposes. Bleaching-powders or acids are sometimes used to
lighten the color, but these unless very delicately handled injure the
durability of the fibres."
=Classification.=--The sponges are classified according to the character
of the skeleton. In one group are put all those sponges which have a
skeleton of calcareous spicules, and this group is called the Calcarea.
All other sponges are grouped as Non-Calcarea, the members of this group
either having no skeleton at all, or having a skeleton composed of
siliceous spicules or of spongin fibres. According to the absence or
presence of a skeleton and the character of the skeleton when it exists
the Non-Calcarea are subdivided into smaller groups.