Lowest in the scale of animals, and scarcely distinguishable from a vegetable, we find the sponge, attached to and passing its life upon a rock, exhibiting, indeed, less signs of feeling than many of the vegetable tribes. The chemical differences between vegetables and sponges are, however, very decided; and we find in the tissues of the sponge a large quantity of nitrogen, a true animal element, which exists, but in smaller quantities, in vegetables. These creations, standing between vegetable and animal life, possess the singular power of decomposing carbonic acid, as plants do; and the water in which they live always contains an excess of free oxygen. The polypes are a remarkably curious class. “Fixed in large arborescent masses to the rocks of tropical seas, or in our own climate attached to shells or other submarine substances, they throw but their ramifications in a thousand beautiful and plant-like forms; or, incrusting the rocks at the bottom of the ocean with calcareous earth, separated from the water which bathes them, they silently build up reefs and shoals, justly dreaded by the navigator; and sometimes giving origin, as they rise to the surface of the sea, to islands, which the lapse of ages clothes with luxuriant verdure, and peoples with appropriate inhabitants.”[263] Most of the polypes are fixed and stationary; but the hydra and some others have the power of changing their positions, which they do in search of the light of the sun. They do not appear to have organs of sight requiring light; but still they delight in the solar influences. The most extraordinary fact connected with the hydra is its being multiplied by division. If an incision be made in the side of a hydra, a young polype soon developes itself; and if one of these creatures be divided, it quickly restores the lost portion of its structure. The varieties of the polypes are exceedingly numerous, and many of them are in the highest degree curious, and often very beautiful. The actiniæ, like flowers, appear to grow from the rocks, unfolding their tentacula to the light; and, in the excitement due to their eagerness for prey, they exhibit a beautiful play of colours and most interesting forms. Microscopic zoophytes of the most curious shapes are found,--all of which attest, under examination, the perfection of all created things. Infusoria and animalcula,--animals, many of them, appearing under the microscope as little more than a transparent jelly,--must be recognized as the most simple of the forms of life. They exist in all waters in uncountable myriads; and, minute creatures as they are, it has been demonstrated that many of the great limestone hills are composed entirely of their remains.