METAZOA COELENTERA. 133 



these were the entosepta. In time, according to von 

 Koch, the septa fork at their ends and later these ends 

 fuse to form the cups or thecae. According to Lacaze- 

 Duthiers the theca arises as a ring-like thickening of the 

 base entirely independent of the septa. 1 Sometimes the 

 central ends of the septa fuse and form the columella. 



While these changes are taking place, the ectoderm at 

 the free margin of the young coral animal secretes lime 

 whereby the epitheca is formed. This is a thin imperfo- 

 rate layer which is originally free from the theca but which 

 secondarily fuses with it. 



Many of the imperforate Madreporaria occur in the 

 early geologic formations. The structure of the skeleton 

 of these ancient corals also called Rugosa and Tetraco- 

 ralla does not differ from that of recent corals. The 

 tetrameral symmetry (or having the parts in multiplies of 

 four) peculiar to many of them is not a constant charac- 

 ter, and a hexameral symmetry is not by any means char- 

 acteristic of recent corals. 2 



The relationship between these ancient and modern 

 forms causes them to be placed together under the head 

 of the Madreporaria Aporosa. 



It is probable that the earliest ancestors 'of our coral 

 animals were disc-like in form, for the reason that this is 

 the first condition of the skeleton of existing species. In 

 time this disc-shaped coral probably became cup- or horn- 

 shaped, a very common form among ancient species. 



The primitive disc-shaped ancestors are not known with 

 certainty, so that we must pass to the cup-shaped forms; 

 of these there are a number in the Collection. 



Cystiphyllum americanum E. & H. (No. 212), has septa 

 that are only slightly developed, being indicated by mere 

 ridges. The coral is vesicular throughout but towards 



1 For a discussion of these views, see Fowler, Quart. Journ. Micr. 

 Sci., XXV, 1885. 



2 Treatise on Zoology, ed. by E. Ray Lankester, Part 2, 1900, 



p. 70. 



