Many corals branch, while others form compact masses, as in the kind above described, and also in the brain coral, where the boundaries between the individuals are not clearly marked.
Corals are widely distributed, some living even in cold latitudes, and others on the floor of the deep sea. Coral reefs, however, made up of the skeletons of such animals, are only found in the warmer parts of the ocean, where the water is clear, particularly favorable conditions being afforded by the Pacific and Indian Oceans. (See also [Coral Reefs] and [Islands]).
Jelly-fish (Medusæ).—All agree in having a more or less bell or umbrella shaped body, with a proboscis hanging down in the place of the handle of the umbrella or the tongue of the bell. The mouth is at the end of the handle and leads into a stomach which divides and sends out branches, like the ribs of the umbrella, to the margin. The common name is due to its gelatinous consistency. Most of the species start in life as buds from attached animals, which later separate and henceforth lead a free existence, swimming by opening and closing the bell.
SPONGES
SPONGES (Porifera) are animals of peculiar structure, which resemble zoophytes in many respects, but possess neither tentacles not thread-cells. Some are simple, but most of them are compound. A simple sponge may be compared to a cup or vase with a wall perforated by numerous small holes, through which currents of sea-water stream into the central cavity, to make their exit by the main opening. They are set up by ciliary action.
Venus Flower-basket.—In the majority of cases the skeleton of a sponge is mostly or entirely made up of sharp needles of lime or flint, which may be welded together, as in this form. Often the opening of the vase is provided with a convex perforated covering. Another elegant form is the Glassrope Sponge native to the Japanese seas. It is rooted in the mud by a bundle of long, glassy spicules, which are slightly twisted.
Most sponges are marine, and, despite their fixed habit and apparent helplessness, are pretty free from the attacks of most other creatures, partly because of the innumerable sharp spicules they contain, and partly because their taste and smell are unpleasant. These deterrent qualities are often associated with bright warning colors, generally red, yellow or orange.
Most of us little realize that the sponges we see or use daily are in reality dead animals.
CROSS-SECTION OF LIVING SPONGE