The luminosity of the medusæ is clearly perceived, the so-called phosphorescence being due chiefly to the minute jelly-fish which abound near the surface of the sea. It appears impossible, for most, at any rate, if not all, these medusæ to sink beneath the surface, for they can be found in hundreds cast ashore, melting away into film. We might imagine that they would be provided with some means of sinking themselves, but being apparently only air and water, it is necessary for them to remain upon the surface to exist at all.
Fig. 834.—Sea cucumber.
The term Acalephæ, by which they are known, means “stinging” fish or sea-nettles, the Greek word meaning nettle.
The Actinozoa comprise corals and the popular sea anemones (actinidæ). They resemble the hydrozoa in possessing tentacles, and also the two inner and outer tissues of the body. But they differ from the hydrozoa in their interior arrangement in the possession of a kind of stomach between the “body cavity” and the mouth which the hydrozoa do not possess. The appearance of the sea anemone is well known. It fixes itself by the flat base and hangs out its tentacles to obtain food. When we touch an anemone with a stick we perceive how it contracts itself, but there is no nervous system nor any respiration. The reproduction of its species is carried on within, not as in other animals, like the hydra, by exterior budding.
Fig. 835.—Coral.
The corals belong to the same class as the sea anemones, and are called zoanthidæ. We have already in previous portions of this volume mentioned the “coral” building polypes, but we may again describe them here. We have the black coral or antipathidæ, which live in masses and are united by a stem. They grow upon this fleshy trunk and cover it in time “just as a trunk of a tree is covered by the bark.” This stem is called a cænosarc, which secretes the coral, or skeleton. The madrepores are the greatest producers of the coral of commerce.
Fig. 836.—Coral.