FORMATION OF CORAL REEFS. 169 



In the diagram of the geological periods intro- 

 duced in a previous article, I have represented all 

 the three classes of Radiates, Polyps, Acalephs, 

 and Echinoderms as present on the first floor of 

 our globe which was inhabited at all. But it is 

 only recently that positive proofs have been found 

 of the existence of Acalephs or Jelly-Fishes, as 

 they are called, at that early period. Their very 

 name indicates their delicate structure ; and were 

 there no remains preserved in the rocks of these 

 soft, transparent creatures, it would yet be no 

 evidence that they did not exist. Fragile as they 

 are, however, they have left here and there some 

 faint record of themselves, and in the Museum 

 at Carlsruhe, on a slab from Solenhofen, I have 

 seen a very perfect outline of one which remains 

 undescribed to this day. This, however, does 

 not carry them farther back than the Jurassic 

 period, and it is only lately that I have satisfied 

 myself that they not only existed, but were 

 among the most numerous animals in the first 

 representation of organic life. 



The earliest Corals correspond in certain fea- 

 tures of their structure to the Millepores. They 

 differ from them as all early animals differ from 

 the succeeding ones, every geological period hav- 

 ing its special set of representatives. But still 

 they are always true to their class, and have a 

 certain general correspondence with animals of 



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