In all these cases the "sponge" is the cleaned skeleton of a creature that lives attached to the sea bottom. The skeleton is formed of spongin—a substance resembling silk (q.v.) in chemical character and composition, and, like silk, belonging to the "sclero proteins" (of which horn is another familiar example), a group of highly insoluble nitrogenous substances, apparently easily formed as waste-products from the life of protoplasm.
Sponges with a skeleton of spongin are known as Horny Sponges (Ceratosa), and include many other species besides the dozen used by man. There are two main groups : those with latticed or netted skeletons (Dictyoceratida) like the market-sponges, and those with tree-like skeletons (Dendroceratida). The tree-like skeletons grow up from the basal spongin plate ; the netted skele tons grow in from any part of the semi-solidified, sticky spongin cuticle (fig. I).
Of other sponges—wholly useless to man—there are many thousands of species. The skeleton usually contains crystalline spicules; these are of cubic opal (silicic acid with cubic sym metry) in the Glass Sponges, of cubic opal, cemented by varying quantities of spongin, the Needle-sponges, of tetrahedral opal in the Four-ray Sponges, and of carbonate of lime, in the form of calcite (Iceland spar), in the Chalky Sponges. Siliceous spicules of fossil sponges have formed our flints.
A microscopic examination shows that the living substance is composed of protoplasmic cells, which, in all groups except the Glass Sponges, surround and penetrate a clear glutinous semi fluid intercellular jelly. Through out the many forms there are always some tubes or cavities in the sponge lined with flagel late cells each bearing a fine whip (Latin flagellum) with which they lash on the water so that it keeps moving through the cavities and bringing fresh food for the sponge. These are also called collar-cells, because there is a little
collar (fig. 6) surrounding the butt of the flagellum and so pre venting its side-to-side movements from knocking away particles of food from the cell; it also acts as a valve to prevent water flow ing the wrong way.
Precisely similar collar-cells are to be seen in the Collar flagellates or Clioanoflagellata (a group of Protozoa, q.v.), and it seems certain that the Horny Sponges and their relations have originated from protozoan Collar-flagellates which have taken to sticking together into colonies, and then to secreting so much adhesive substance that it forms a jelly continuous under the bases of the cells (fig. 7) from which rations can be drawn by cells who are not themselves catching any food. Socialism and class-distinction are thus alike established, and sponges have therefore passed out of the grade of Protozoa, where cells are typically separate, similar and individualist. The title of Metazoa has been given to the further-developed animals with stomachs (Enterozoa) ; W. J. Sollas has therefore called the sponges Parazoa.
The Glass Sponges have probably emerged to the rank of Parazoa by another route from other Collar-flagellates. Their cells rarely form any intercellular jelly, but for the most part lie naked in the water as a branching network or cobweb (fig. 9). Opposed to the "True" gelatinous sponges, we may call these sponges." Sub-kingdom PARAZOA Grant. Sponges.
Colonial Collar-flagellates consisting of cells of more than one kind.
Phylum I. NUDA. Naked Sponges. Originating from branching colonies of naked cells.
Class I and Order I. Hexactinellida. Glass Sponges.
Phylum 2. GELATINOSA. True sponges. Originating from ad hesive colonies of cells bedded in secreted jelly.
Class 2 and Order 2. Monaxonellida. Needle-sponges'.
Families : Clavulidae (Pin-cushions) ; Desmacidonidae (Claw-sponges) ; Axinellidae (Fans) ; Renieridae (Spin dle-sponges).
Class 3. Ceratosa. Horny Sponges.
Order 3. Dendroceratida (Horny Bushes).
4. Dictyoceratida (Horny Nets).
Class 4. Calcarea. Chalky Sponges.
Order 5. Calcaronea (Banana-chalks).