Part 6 (1/2)

FOOTNOTE:

[7] In some Protozoa a number of similar cells temporarily unite to form a colony, but each cell may still be regarded as an individual animal.

CHAPTER XVI

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 ma.s.ses, 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 cus.h.i.+on-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 ma.s.s, nor long thread-like fibres such as compose the ma.s.ses of pond algae (pond sc.u.m). 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 ma.s.s. 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 pa.s.sing out through the osculum. Crush a bit of dried sponge, or boil a bit of soft sponge in caustic potash and mount on a gla.s.s 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 as.e.xually. 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 ma.s.s 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, h.o.r.n.y 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 ca.n.a.ls 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 sh.o.r.e 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.