762 THE LIVING ANIMALS OF THE WORLD 



in a condition of healthy growth, yielding a spectacular effect that vies with that of any 

 floral parterre. Sometimes large areas, acres upon acres in extent, may be covered with one 

 almost uniform purple, green, brown, or other coloured growth of the branching stag's-horn 

 species. The aspect presented is not unlike that of a heath-covered common. 



In addition to the solid, calcareous-skeletoned Madrepores, or " Stony Corals," as they are 

 often termed, there are a number of species in which a skeleton composed only of loosely 



aggregated calcareous spicules 

 is produced. The so-called 

 FLEXIBLE CORALS, or SEA- 

 FANS, belong to this category, 

 as also the precious CORAL 

 OF COMMERCE. In the last- 

 named species the solid, 

 brilliantly coloured skeleton 

 so much prized as an article 

 of jewellery is deposited as a 

 supplementary basis outside 

 the tissues by which the star- 

 patterned skeletons of the 

 stony corals are secreted. 



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 or- 

 ganisms 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 FRESH- 

 WATER 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. 



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Pht> by 



Savillt-Ktnt, F.Z.S.] 



A QUEENSLAND STAR-CORAL 



This species in life is of a pale lemon-yellow tint 



[Milford-on-Sta 



