A QUEENSLAND STAR-CORAL.
This species in life is of a pale lemon-yellow tint.
A group which demands brief notice is that of the Hydroid Polyps. These include the majority of the Jelly-fishes, a few coral-secreting species, and the organisms whose seaweed-like horny skeletons, known as Sea-firs, are, in common with those of Sea-mats, included among the flotsam and jetsam on every sea-beach. In the Common Hydra, or Freshwater Polyp, an exceptional fresh-water representative of this group is presented. It may be likened to a tiny sea-anemone, having, when extended, a slender foot-stalk and long thread-like tentacles. Like a sea-anemone, it will shrink up when disturbed into a mere button of jelly. Its organisation is more simple than that of the anemone, its body-cavity being a simple sac, without any intucking of the orifice, or strengthening by supplementary membranous partitions. A similar simple structural plan is characteristic of all the organisms belonging to the series. An interesting phenomenon connected with the fresh-water hydra is the circumstance, demonstrated now over a century ago, that, if one of these animals be cut up into little pieces, each separate fragment is capable of repairing itself and growing into a new polyp.
The Jelly-fishes, or Medusas, and their allies would appear at first sight to possess but little structurally in common with the Coral-polyps and Sea-anemones. In their most familiar form they are represented by a more or less translucent bell-shaped body, which drifts with the current or propels itself through the water by its alternate expansions and contractions. In the centre of the lower surface, occupying the position of the bell's clapper, a polyp-like, tubular mouth is usually discernible, and this is frequently surrounded by a circle of tentacles, sometimes simple and sometimes elaborately ramified. Long, thread-like tentacles are also commonly developed around the margin of the swimming-bell.
Photo by W. Saville-Kent. F.Z.S.] [Milford-on-Sea.
A GIANT ANEMONE FROM THE GREAT BARRIER REEF.
From 18 inches to 2 feet in diameter when expanded. Bright apple-green in colour, and with almost spherical bead-like tentacles.
The larger number of the jellyfishes are, as a matter of fact, transitional phases only of the fixed hydroid polyps previously referred to. In certain instances the body of the fixed polyp becomes elongated, and splits up horizontally into a series of jelly-fishes, or medusas, resembling a pile of saucers, which consecutively break away and lead a free-roving existence. In other forms a compound tree-like growth gives birth to medusa-like buds, like the flowers on a plant, which ultimately become detached and swim away. What are known as the Comb-bearing Jelly-fishes—their locomotive organs consisting of comb-like bands of vibratile hairs—are especially noteworthy. In some of these the body is nearly spherical or ovate, one of the species, in reference to its shape, being popularly known as the Sea-lemon. A notable feature of these medusas is their remarkable glass-like transparency, their presence in the water in many instances being recognisable only by the prismatic glimmerings of their rows of vibratile hairs when the light falls upon them at a favourable angle. The most remarkable member of this particular group is undoubtedly the form known as Venus's Girdle. This species takes the form of a long, ribbon-like band of transparent jelly. The edges of the ribbon are clothed with vibratile hairs, and the mouth is situated in the centre of one of the edges. The animal progresses by the action of its hairs alone, or may be assisted by the twistings and undulations of its ribbon-like body.
