74 COELENTERATA [CH. 



mum- in very many* points. The tentacles, hollow as before, are 

 never feather-like but always perfectly simple and round, and there 

 is usually a large number of them arranged in several concentric 

 circles. The mesenteries also are numerous, and extend inwards 

 to different lengths, so that we can distinguish primary mesen- 

 teries which join the gullet from secondary ones which do not. 

 Both primary and secondary are usually arranged in pairs, but 

 there is much variety and all that can be universally asserted 

 is that they never exhibit exactly the arrangement shown in 

 Alcyonaria. A very common arrangement is to have six pairs 

 of primary mesenteries and two siphonoglyphs, one at each end. 

 Spicules are never developed and in the ordinary anemones of 

 our coasts there is no skeleton whatever. These commoner forms 

 sometimes, though rarely, bud, but there is another large class 

 of anemones which do form colonies, the buds occasionally arising 

 as in the Hydromedusae from the body of the parent directly but 

 often from the stolons connecting different persons. These colonial 

 anemones form the hard stony masses called coral (Fig. 32). If we 

 look at a piece of coral we can see in it cups with partitions radiating 

 inwards, the whole reminding one of the structure of a sea-anemone : 

 and it was a natural mistake to suppose, as the earlier naturalists 

 did, that the hard skeleton was formed inside the body of the polyps, 

 and that the partitions represented the mesenteries. Of course it is 

 difficult to imagine how the animal could move if it had all that 

 mass of stone inside it. How the corallum or stony skeleton is 

 formed is a matter of dispute. It is certainly situated outside the 

 ectoderm, but whether it is secreted by the ectoderm as a kind of 

 sweat which hardens, or whether the ectoderm cells are calcified and 

 thrown off, or whether the ammonium carbonate, secreted by all 

 animals, precipitates the calcium carbonate of the sea water and so 

 forms the skeleton, is not finally decided. At any rate a calcareous 

 cup is formed in which the polyp sits and the partitions of the cup 

 indent the base of the animal, pushing before them folds of the body 

 wall, which project into the coelenteron between mesenteries, so that 

 the action of the longitudinal muscles is not interfered with. 



Under the name coral the skeletons of quite a number of 

 different kinds of Coelenterata were included besides 



Coral. 



Zoanthana. 



The so-called millepore corals belong to the first division of 

 the Coelenterata, the Hydrozoa, for Millepora itself gives off quite 

 typical Anthomedusae and the other genera have gonophores. 



