216 PALAEONTOLOGY. 



rooted by a calcareous base to some sub-marine body, con- 

 stitute the class Polypifera. Their skeleton assumes a vast 

 variety of forms, being horny or calcareous, globular or 

 branched, solid or tubular, stellate, porous, or reteform. The 

 gelatinous organised substance of the animal is enclosed in 

 ramified tubular sheaths, or expanded over the surface of the 

 calcareous skeleton which it encloses and secretes. The 

 mouth of the polyp is surrounded with numerous filaments 

 or tentacules, which, in the highest groups, are furnished with 

 vibratile cilia. Each polyp, or digestive sac, contributes a 

 moiety to the nourishment of the compound body with which 

 it is organically united. This physiological relation occa- 

 sions remarkable associations and singular groupings among 

 the Polypifera ; hence the stupendous results obtained from 

 their operations in the seas of inter-tropical regions, by 

 which the life of the individual is combined with the life of 

 the whole, and the nutriment prepared by each organism is 

 made to contribute to the nourishment of the community, of 

 which it forms a part, as in the red coral, fig. 151. 



The calcareous skeletons of some Anthozoa are very abun- 

 dant, and attain a great magnitude in the Pacific, where they 

 may be said to modify the ocean's bed, and contribute largely 

 to the formation of new continents. That there are masses of 

 rock many leagues in extent, founded in the recesses of the 

 ocean, and built up into gigantic structures, from eight hundred 

 to a thousand feet in thickness, by the secretions of polyps, 

 is a fact of deep interest to the naturalist, and of great 

 importance to the geologist, the study of which affords him 

 important data for reasoning on the operations of these ani- 

 mals in former periods of the earth's history. Dr. Darwin * 

 has recently shown that the zoophytic productions known as 

 coral reefs, and coral islands, may be classified into three 

 groups Atolls, Barrier-reefs, and Fringing-reefs ; that the 

 vital operations of the Polypifera are limited within the 

 range of thirty fathoms, and* that beyond that depth they 

 cannot live; that the forms which reefs assume depend 

 upon the elevation or subsidence of the ocean's bed, on 

 which the foundations of the zoophytic structure are laid. 

 These facts, previously ascertained* by Quoy, Gaymard, 



* On the Structure and Distribution of Coral Reefs, 8. 1842. 



