. Animal forms; a second book of zoology. Zoology. Fig. 14.—Portion of wall of sponge, showing three layers, e, outer layer ; i, inner layer, consisting of collared cells ; m, middle layer, consisting of irregular cells, among which are the radiate spic- ules and egg-cells. is absorbed by all parts of the body in contact with the water. 27. Skeleton of sponges.—When it is remembered that the protoplasm composing the cells of the sponge has about the same consistence as the white of egg, it will be readily un- derstood why the greater number of sponges possess a skel- eton. Without such a suppo
. Animal forms; a second book of zoology. Zoology. Fig. 14.—Portion of wall of sponge, showing three layers, e, outer layer ; i, inner layer, consisting of collared cells ; m, middle layer, consisting of irregular cells, among which are the radiate spic- ules and egg-cells. is absorbed by all parts of the body in contact with the water. 27. Skeleton of sponges.—When it is remembered that the protoplasm composing the cells of the sponge has about the same consistence as the white of egg, it will be readily un- derstood why the greater number of sponges possess a skel- eton. Without such a support the larger globular or branched forms could not ex- ist, and even in the smaller members there would be danger of a collapse of the body walls and consequent stoppage of the food supply, owing to the closure of the pores. So in all but a very few thin or flat forms a skeleton appears in the young sponge almost before growth ..................^ .... has fairly begun, and ^N/£> ^^^ this increases with the body in size and com- plexity. It is formed by the activity of the cells of the middle layer, and may be composed either of a lime com- pound resembling mar- ble, or of flint, or of a horn-like substance resembling silk, or these may exist in combination in certain species. When consisting of either of the first-named substances it is never formed in one continuous piece, but of a vast multitude of variously shaped crystal-like bodies termed spicules (Fig. 15). These occur everywhere throughout the body, firmly bound together. ) Fig. 15.—Different types of sponge Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Jordan, David Starr, 1851-1931; Heath, Harold, 1868- [from old catalog] joint author. New York, D. Appleton and company
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectzoology, bookyear1902
