COELENTERA — STEOMATOPORES AND ALCYONARIANS. 49 



and, by secreting spicules of a horny substance impregnated Gallery X. 



with carbonate of lime, give it greater consistency. The 



common Alcyoniuin digitatitm (dead man's fingers) has such 



a skeleton ; it falls to pieces on the death of the animal, but 



isolated spicules have been found fossil. Sometimes the 



spicules become tightly wedged together and form a compact 



skeleton which cannot be disintegrated ; the precious coral 



{Cor allium ruhrum) is the hardest skeleton so formed, but 



has only been found fossil in some of the later rocks, and that 



rarely. The organ-pipe coral {Tuhipora) is also well known, 



but is not found fossil. Some of the Gorgonacea form an 



axis partly horny and partly calcareous ; and some calcareous 



segments referred to Isis, and a few other forms have been 



found in Tertiary and Cretaceous rocks. A horny axis 



supports the colony of the sea-pen, Pennatula, and in some 



allied forms this axis is partly calcified ; a few such axes 



have been detected in Triassic and later rocks. In the living 



Heliopora, the skeleton is formed, not from spicules developed 



in cells, but from lamellae of calcite crystallised in an organic 



matrix produced by the disintegration of ectoderm cells. 



This genus is found fossil back to the Albian Age. With 



the exception of a supposed Triassic Pennatulid, fossils that 



can with certainty be referred to Alcyonaria are confined to 



rocks of Upper Cretaceous and later age ; and yet, as was 



the case in Hydrozoa, there are a number of Palaeozoic 



genera that resemble many of the recent forms. 



In the Sub-Class ZOANTHARIA, the only living forms 

 with a skeleton capable of fossilization are several genera 

 differing a good deal in their structure but conveniently 

 grouped together as Madreporaria. Several of them might 

 be roughly described as sea-anemones with a skeleton. 

 This skeleton is quite different from that of the Alcyonaria, 

 except perhaps Heliopora. It consists of crystalline car- 

 bonate of lime secreted by special ectoderm cells, not within 

 the cells but outside them, and outside the whole of the 

 ectoderm. The skeleton of the Alcyonaria is internal, that 

 of the Zoantharia is external, as is the shell of a snail. 

 When the young coral-embryo settles down on the sea- floor, 

 it deposits a layer of skeletal substance between its skin and 

 the sea-floor, forming a plate, which soon is turned up at the 

 edges like a saucer. The soft body of the coral may be 

 w^hoUy supported within the saucer, or it may pass beyond 

 its rim. In either case the rim is still built up, and at the 

 same time laminae of lime stretch out from it to the centre 



E 



