. A manual of zoology. Zoology. 258 COSLENTERATA. the sarcosepta, and the theca inside and at some distance from the col- umn, the outer surface of which secretes only the inconstant epitheoa (fig. 209). From the above it would appear that the sclerosepta correspond in number to the sarcosepta, but this is not always the case. Thus the Helioporidse, which on the grounds of the skeleton were regarded as Hex- acoralla, are shown by the soft parts to be undoubted Octocoralla. By means of their skeletons the Anthozoa produce large accumulations of carbonate of lime, the well-known coral reefs, on
. A manual of zoology. Zoology. 258 COSLENTERATA. the sarcosepta, and the theca inside and at some distance from the col- umn, the outer surface of which secretes only the inconstant epitheoa (fig. 209). From the above it would appear that the sclerosepta correspond in number to the sarcosepta, but this is not always the case. Thus the Helioporidse, which on the grounds of the skeleton were regarded as Hex- acoralla, are shown by the soft parts to be undoubted Octocoralla. By means of their skeletons the Anthozoa produce large accumulations of carbonate of lime, the well-known coral reefs, on the bottom of the sea. These are formed by many species, the Madreporaria playing the most important role. When the reef reaches the surface it produces an island, the most noteworthy form being the atoll, a ring-like structure with a central lagoon. The origin of these atolls, as well as that of fringing and barrier reefs, was for a long time explained by Darwin's and Dana's theory of coral reefs. Later investigations, notably those of Mr. Agassiz, afford another explanation. Order I. Tetracoralla (Rugosa). Extinct forms from the paleozoic rocks with the parts arranged in fours (fig. 211). The present tendency is to regard them as modi- fied Ilexacoralla. Order II. Octocoralla (Alcyonaria). These forms, which have eight single septa, are externally re- cognizable by their feathered tentacles, eight in number (fig. 206).. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Hertwig, Richard, 1850-1937; Kingsley, J. S. (John Sterling), 1854-1929. New York, H. Holt and Company
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