SUB-KINGDOM III.
Cœlenterata (hollow intestines). These are radiate animals, have a distinct digestive cavity, and always two layers of tissue in their walls. They have minute sacs containing a fluid, and barbed filaments capable of being thrown out as stings. The classes of the Cœlenterata are the Hydrozoa, Anthozoa, and Ctenophora. The best known representative of the former is the fresh water hydra (water animal). It gives but slight evidence of a nervous system, has no stomach, or digestive sac, but is a simple tube into which the mouth opens. The sensitive little body is in color and texture, to the casual observer more like a plant than an animal. It is attached at one end to a submerged aquatic plant, while the mouth at the free end is provided with tentacles,[2] by which it feeds and moves. It buds, and also produces eggs. The young hydra, when hatched or thrown off, attaches itself to plants, as did its parents. Some hydroids attach themselves to shells, and are supported by horny, branching skeletons, specimens of which are numerous, and may be seen in almost any museum.
HYDRA, SHOWING BUDS.
A second representative of this class is the jelly fish. It has a soft, gelatinous, circular body, that floats or swims on the surface in calm weather, with the mouth downward. Tubes radiate from the center to the circumference that is fringed all around with pendant tentacles, sometimes of great length and of considerable contractile power. They are of various sizes, some quite small, others as much as eight feet in diameter. They move about by flapping their sides, after the manner of opening and shutting an umbrella. When dried, the thin, filmy covering is very light. One variety, called Lucernaria, is found attached to grasses along the Atlantic shore. But the ordinary jelly fish is free, and borne on the surface of the sea.
The Anthozoa (flower animals) are small, but not microscopic animals, having a double cavity, the inner of which is the digestive sac. The best known of the class is the Actinia (rayed), or sea anemone, so called from its resemblance to a plant or flower of that name. The body is somewhat like a flower in shape. The disc has a central orifice, very contractile, and surrounded with tubular tentacles of various forms, which it elongates, contracts, and moves in different directions. They are so many arms by which the animal seizes its prey, and when expanded for the purpose, being tinted with brilliant colors, present an elegant appearance, and make vast fields of the ocean look like beautiful flower gardens. They feed voraciously on little crabs and mollusks, that often seem superior to themselves in strength and bulk, but they have power in their little tentacles to seize and hold their prey, and when they engulf large bodies the stomach is distended to receive them; and their digestion is good. The purple sea anemone is very common on the southern shores of England, and one species, found on the shores of the Mediterranean, is said to be esteemed a great delicacy by the Italians. At night, or when alarmed, the animal draws in its arms, shuts the door, and seems but a rounded lump of flesh, a huge oyster without a shell. The coral polyps (many footed), belonging to this class, are little folk, but of importance from their well-earned reputation as reef builders. They are very diminutive creatures, mere drops of animal jelly, often not larger than the head of a pin, but their works show unmistakable evidence of life, and an organic structure. They live in communities, closely united, but each, having a distinct individuality, by the sure process of secreting a portion of the calcareous matter within reach, prepares for himself a house, as all his ancestors have done, and his neighbors are now doing. They build together, their foundations having strong connections, and thoroughly cemented. There in his own little palace the polyp lives, his food being brought to his lips; and, having sent out a numerous progeny to do likewise for themselves, there he dies. Life and death, as in all mundane communities, being in close proximity, the old dying, a new generation builds houses over their sepulchres.
SEA ANEMONE.
The great variety of corals seen in any extensive collection, some very beautiful, others rough and unsightly, are from different branches of a very extensive family. Astrea (star shaped), from the Fiji islands, is a kind of coral hemisphere, covered with large and beautiful cells.
Mushroom coral is disc shaped, not fixed or attached, and is the secretion, not of many, but of a single huge polyp.