SEA-ANEMONES AND CORALS. 



499 



not quite so simple. The layer each polyp secretes is not a smooth flat disc, evenly 

 secreted by the whole surface of its foot. Some parts of the foot secrete more 

 than others, hence those parts rise up as spines, walls, and rings, which protrude 

 into the body of the polyp without, however, breaking through the skin. These 

 probably help to fasten the polyp to its pedestal, and prevent it from being swept 

 off' by strong currents. The figure on p. 505 is a good illustration of one of these 

 plates. Each genus of corals has a pattern of its own, each one perfect and beauti- 

 ful in its way, and it is frequently a puzzle to discover how it is made. When a 

 crowd of polyps grow in contact, their pedestals will also grow in contact and 

 form continuous masses ; this growing in contact being ensured by their ordinary 

 method of multiplication. For a coral-polyp does not have to wait until another 

 takes up a position beside it, but as soon as it can feed freely, it begins to bud or 

 divide, producing a number of young polyps close around it. These also bud in 

 their turn and are soon surrounded by young polyps, and in this way such compact 

 colonies are formed that it is a struggle among the inner ones to avoid being 

 suffocated. We thus have densely crowded colonies of polyps struggling upwards, 

 each individual secreting a more or less beautiful and complicated pedestal. The 

 pedestals are fused together in a hundred different ways, and from these different 

 patterned pedestals, with their various ways of fusing together, are produced the 

 almost countless different kinds of coral which together build up coral-reefs. 



In a growing polyp-stock the individuals usually remain in organic connec- 

 tion, that is to say, each first provides for itself, and then shares its superfluity 

 with the others, sometimes by means of a continuous reticulated system of canals 

 running from polyp to polyp, perforating the stony substance which often separates 

 the members of the one stock from another. The whole stock may thus be 

 physiologically one creature with many mouths. Where, however, the secretion of 

 the pedestal is very rapid and the budding very slow the 

 polyps may separate, each standing at the end of a branch ; 

 the illustration of Caulastrcea showing an example of this. 

 It will be understood from this description that only the 

 layer of growing polyps with their intercommunications 

 can be spoken of as living ; and as this layer rises higher 

 and higher by secreting fresh layers of carbonate of lime, 

 the living linings of the communicating canals are either 

 withdrawn or die away, and all beneath the living layer 

 is mere dead matter built up and left behind by the 

 coral animals. 



Before passing to our survey of the corals themselves, 

 two other points deserve attention. Not all corals form 

 stocks. Some remain single, like the mushroom-corals 

 (Fungidce), which grow to a very large size with a heavy 

 solid skeleton ; and although these form new polyps by 

 budding, the latter become detached and live as solitary 

 individuals. Again, although coral-reefs are due to the great power of multiplying 

 by division or budding, yet all corals, so far as is known, also at certain times 

 produce eggs. The further development of these eggs gives rise ultimately to a 



OUTLINE OP Caulastrcea 

 (uat. size). 



