FORMATION OF CORAL REEFS. 169 
In the diagram of the geological periods intro- 
duced in a previous article, J 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 
mysélf 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 hay- 
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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