[CHAP. XVIII.]
PROTOZOA.
The Foraminifera.—The Amœbæ—Their Wonderful Simplicity of Structure.—The Polycystina.—Marine Infusoria.—Sponges—Their Pores—Fibres and Spiculæ—The Common Sponge of Commerce.
Think not, reader, that the life of the ocean ends with the innumerable hosts of fishes, molluscs, crustacea, medusæ, and polyps we have reviewed, and that the waters of the sea or the sands of the shore have now no further marvels for us to admire. The naked eye indeed may have attained the limits of life, but the microscope will soon reveal a new and wonderful world of animated beings.
Take only, for instance, while wandering on the beach, a handful of drift-sand, and examine it through a magnifying glass. You will then not seldom find, among the coarser grains of inorganic silica, a number of the most elegant shells; some formed like ancient amphoræ, others wound like the nautilus, but all shaped in their minuteness with a perfection which no human artist could hope to equal in the largest size.
Nummulina discoidalis.
a. Natural size. b., c. The same, highly magnified.
The knowledge of these charming little marine productions is of modern date, for they were first observed in the sand of the Adriatic by Beccaria in 1731, and for some time believed to belong exclusively to that gulf. At a later period some species were discovered here and there in England and France, but their universality and importance in the economy of the ocean were first pointed out in 1825, by the distinguished French naturalist Alcide d'Orbigny.
The sand of many sea-coasts is so mixed with Foraminifera, as they have been called from the openings with which their shells are pierced, that they often form no less than half its bulk. Plancus counted 6000 in an ounce of sand from the Adriatic, and d'Orbigny reckoned no less than 3,849,000 in a pound of sand from the Antilles. Along the whole Atlantic coast of the United States, the plummet constantly brings up masses of foraminiferous shells from a depth of ninety fathoms, so that the vast extent of ocean-bottom, which itself forms but a small part of the domains they occupy, is literally covered with their exuviæ.