CHAPTER III

SPONGES

SUGGESTIONS.--In many parts of the United States, fresh-water sponges may, by careful searching, be found growing on rocks and logs in clear water. They are brown, creamy, or greenish in color, and resemble more a cushion-like plant than an animal. They have a characteristic gritty feel. They soon die after removal to an aquarium.

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. 21.--FRESH-WATER SPONGE.]

[Illustration: FIG. 22.--SECTION of fresh-water sponge (enlarged).]

The small =fresh-water sponge= (Fig. 21) lacks the more or less vase-like form typical of sponges. It is a rounded mass growing upon a rock or log. As indicated by the arrows, where does _water enter_ the sponge? This may be tested by putting coloring 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 vase-like 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.

Do the _fibers_ 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?

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. 26.--Bath Sponge.]

[Illustration: FIG. 27.--Bath Sponge.]

[Illustration: FIG. 28.--Bath 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.

[Illustration: FIG. 29.--Skeleton of a glass sponge.]

The _horny sponges_ do not have spicules in their skeletons, as the flinty and limy sponges have, but the skeleton is composed of interweaving fibers of _spongin_, a durable substance of the same chemical nature as silk (Figs. 30 and 31).

[Illustration: FIG. 30.--A horny sponge.]

[Illustration: FIG. 31.--Section of horny sponge.]

The _limy sponges_ have skeletons made of numerous spicules of lime. The three-rayed spicule is the commonest form.

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 dioxid. _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.

=That the sponge is not a colony of one-celled animals=, each like an ameba, 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 thread-like 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.

[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.]

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.

=No animals eat sponges=; possibly because spicules, or fibers, are found throughout the flesh, or because the taste and odor is unpleasant enough to protect them. Small animals sometimes crawl into them to hide. One species grows upon shells inhabited by hermit crabs. Moving of the shell from place to place is an advantage to them, while they conceal the crab and thus protect it.

=Special Report:= _Sponge “Fisheries.”_ (Localities; how sponges are taken, cleaned, dried, shipped, and sold.)