SPONGES
SUGGESTIONS.—In many parts of North America, fresh-water sponges may,
by careful searching, be found growing on rocks and logs in clear
water. They are brown, creamy, or greenish in colour, and resemble
more a cushion-like plant than an animal. They have a characteristic
gritty feel. They soon die after removal to an aquarium.
[Illustration:
FIG. 21.—FRESH-WATER SPONGE.
]
A number of common small bath sponges may be bought and kept for use
in studying the skeleton of an ocean sponge. These sponges should not
have large holes in the bottom; if so, too much of the sponge has been
cut away. A piece of marine sponge preserved in alcohol or formalin
may be used for showing the sponge with its flesh in place.
Microscopic slides may be used for showing the spicules.
[Illustration:
FIG. 22.—SECTION of fresh-water sponge (enlarged).
]
The =small fresh-water sponge= (Fig. 21) lacks the more or less vaselike
form typical of sponges. It is a rounded mass growing upon a rock or a
log. As indicated by the Arrows, where does _water enter_ the sponge?
This may be tested by putting colouring matter in the water near the
living sponge. Where does the _water come out_? (Fig. 22.) Does it pass
through _ciliated chambers_ in its course? Is the _surface_ of the
sponge rough or smooth? Do any of the skeletal _spicules_ show on the
surface? (Fig. 21.) Does the sponge thin out near its edge?
[Illustration:
FIG. 23.—EGGS and SPICULES of fresh-water sponge (enlarged).
]
The _egg_ of this sponge is shown in Fig. 23. It escapes from the parent
sponge through the _osculum_, or large outlet. As in most sponges, the
first stage after the egg is ciliated and free-swimming.
=Marine Sponges.=—The _grantia_ (Fig. 24) is one of the simplest of
marine sponges. What is the _shape_ of grantia? What is its length and
diameter? How does the free end differ from the fixed end? Are the
_spicules_ projecting from its body few or many?
[Illustration:
FIG. 24.—Grantia.
]
Where is the _osculum_, or large outlet? With what is this surrounded?
The osculum opens from a central cavity called the cloaca. The canals
from the pores lead to the _cloaca_.
_Buds_ are sometimes seen growing out from the sponge near its base.
These are young sponges formed asexually. Later they become detached
from the parent sponge.
[Illustration:
FIG. 25.—Plan of a sponge.
]
=Commercial “Sponge.”=—What part of the complete animal remains in the
bath sponge? _Slow growing sponges_ grow more at the top and form tall,
simple, tubular or vaselike animals. _Fast growing sponges_ grow on all
sides at once and form a complicated system of canals, pores, and
oscula. Which of these habits of growth do you think belonged to the
bath sponge? Is there a large hole in the base of your specimen? If so,
this is because the cloaca was reached in trimming the lower part where
it was attached to a rock. Test the _elasticity_ of the sponge when dry
and when wet by squeezing it. Is it softer when wet or dry? Is it more
elastic when wet or dry? How many _oscula_ does your specimen have? How
many _inhalent pores_ to a square inch? Using a probe (a wire with knob
at end, or small hat pin), try to trace the _canals_ from the pores to
the cavities inside.
[Illustration:
FIG. 26.—Bath Sponge.
]
Do the _fibres_ of the sponge appear to interlace, or join, according to
any system? Do you see any fringe-like growths on the surface which show
that new tubes are beginning to form? Was the sponge growing faster at
the top, on the sides, or near the bottom?
[Illustration:
FIG. 27.—Bath Sponge.
]
[Illustration:
FIG. 28.—Bath Sponge.
]
Burn a bit of the sponge; from the odor, what would you judge of its
composition? Is the inner cavity more conspicuous in a simple sponge or
in a compound sponge like the bath sponge? Is the bath sponge branched
or lobed? Compare a number of specimens (Figs. 26, 27, 28) and decide
whether the common sponge has a typical shape. What features do their
forms possess in common?
[Illustration:
FIG 29.—Skeleton of a glass sponge.
]
Sponges are divided into _three classes_, according as their skeletons
are flinty (silicious), limy (calcareous), or horny.
Some of the _silicious sponges_ have skeletons that resemble spun glass
in their delicacy. Flint is chemically nearly the same as glass. The
skeleton shown in Fig. 29 is that of a glass sponge which lives near the
Philippine Islands.
The _horny sponges_ do not have spicules in their skeletons, as the
flinty and limy sponges have, but the skeleton is composed of
interweaving fibres of _spongin_, a durable substance of the same
chemical nature as silk (Figs. 30 and 31).
The _limy sponges_ have skeletons made of numerous spicules of lime. The
three-rayed spicule is the commonest form.
[Illustration:
FIG. 30.—A horny sponge.
]
[Illustration:
FIG. 31.—Section of horny sponge.
]
The commercial sponge, seen _as it grows in the ocean_, appears as a
roundish mass with a smooth, dark exterior, and having about the
consistency of beef liver. Several large openings (oscula), from which
the water flows, are visible on the upper surface. Smaller holes
(inhalent pores—many of them so small as to be indistinguishable) are on
the sides. If the sponge is disturbed, the smaller holes, and perhaps
the larger ones, will close.
The outer layer of cells serves as a sort of skin. Since so much of the
sponge is in contact with water, most of the cells do their own
breathing, or absorption of oxygen and giving off of carbon dioxide.
_Nutriment_ is passed on from the surface cells to nourish the rest of
the body.
=Reproduction.=—Egg cells and sperm-cells are produced by certain cells
along the canals. The egg cell, after it is fertilized by the sperm
cell, begins to divide and form new cells, some of which possess cilia.
The embryo sponge passes out at an osculum. By the vibration of the
cilia, it swims about for a while. It afterwards settles down with the
one end attached to the ocean floor and remains fixed for the rest of
its life. The other end develops oscula. Some of the cilia continue to
vibrate and create currents which bring food and oxygen.
The _cilia_ in many species are found only in cavities called ciliated
chambers. (Figs. 22, 32.) There are no _distinct organs_ in the sponge
and there is very little _specialization_ of cells. The ciliated cells
and the reproductive cells are the only specialized cells. The sponges
were for a long time considered as colonies of separate one-celled
animals classed as protozoans. They are, without doubt, many-celled
animals. If a living sponge is cut into pieces, each piece will grow and
form a complete sponge.
[Illustration:
FIG. 32.—Microscopic plan of ciliated chamber. Each cell lining the
chamber has a nucleus, a whip-lash, and a collar around base of
whip-lash. _Question_: State two uses of whip-lash.
]
=That the sponge is not a colony of one-celled animals=, each like an
amœba, but is a many-celled animal, will be realized by examining Fig.
32, which shows a bit of sponge highly magnified. A sponge may be
conceived as having developed from a one-celled animal as follows:
Several one-celled animals happened to live side by side; each possessed
a threadlike flagellum (E, Fig. 32) or whip-lash for striking the water.
By lashing the water, they caused a stronger current (Fig. 25) than
protozoans living singly could cause. Thus they obtained more food and
multiplied more rapidly than those living alone. The habit of working
together left its impress on the cells and was transmitted by
inheritance.
Cell joined to cell formed a ring; ring joined to ring formed a tube
which was still more effective than a ring in lashing the water into a
current and bringing fresh food (particles of dead plants and animals)
and oxygen.
=Few animals eat sponges=; possibly because spicules, or fibres, are
found throughout the flesh, or because the taste and the odour are
unpleasant enough to protect them. Small animals sometimes crawl into
sponges to hide. One sponge grows upon shells inhabited by hermit crabs.
Moving of the shell from place to place is an advantage to the sponge,
while the sponge conceals and thus protects the crab.
=Special Report=: _Sponge “Fisheries.”_ (Localities; how sponges are
taken, cleaned, dried, shipped, and sold.)