54 ON CORAL REEFS AND ISLANDS. 



sod, it sprouts again, and continues to grow and flourish as be- 

 fore. The sod, however, has roots which are still unhurt ; while 

 the zoophyte, which may be dead at base, has a root — a source 

 or centre of life — in every polyp that blossoms over its surface. 

 Each animal might live and grow if separated from the rest, and 

 would ultimately produce a mature zoophyte. 



We close this review of the characters of coral animals, which 

 is a mere abstract of the fuller descriptions in the General Re- 

 port on Zoophytes, by alluding briefly to a second division of 

 Zoophytes, not yet touched upon, and also to the Hydroidea and 

 Bryozoa, which are likewise coral-making animals. 



The Alcyonoidea. — The polyps of the Alcyonium* group of 

 zoophytes differ from those which have been occupying us, in 

 having but eight tentacles, and these are fringed with minute 

 papillae. The organ-pipe coral (Tubipora) is of this kind. When 

 expanded in the sea, a clump resembles a bed of pinks, or looks 

 like a lilac-cluster that had been dropped in the water; and this 

 resemblance extends to the color and size of the flowers as well 

 as their form. 



Some of these zoophytes secrete lime and form a tube ; and 

 of this kind is the Tubipora. Others secrete only scattered 

 granules of lime through the tissues : and still others are fleshy 

 throughout. Many of them, besides forming granular calcareous 

 secretions within the body of the polyp, give origin to a horny 

 secretion at base, analogous to the epidermic secretions (hair, 

 nails) of other animals; and this secretion receiving constant ad- 

 ditions from the polyps as they are successively budded out, 

 forms the axis of the growing branch. Of this character is the 

 horny axis of the Gorgonia or sea- fan, which was long taken for 

 a vegetable production. The crust which covers the axis con- 

 sists of united polyps, which expand over its surface ; and when 

 expanded, each branch becomes a spike of flowers. 



The Hydroidea. — The Hydroidea include the groups Hydra, 

 Sertularia, Tubularia and the allied. Some species form thready 

 tufts and plumes of extreme delicacy and others (the Hydrae) are 

 simple polyps. The fine branchlets of the feathery species con- 

 sist when dead of one or two series of microscopic cells arranged 

 like tiny cups, tubes or goblets along the stem ; and when alive, 

 each cell is the site of a minute flower-animal. A coronet of ten- 

 tacles surrounds the mouth, as in the Actiniae, though somewhat 

 different in character. The internal cavity is a simple tube with- 

 out radiating lamellae or special organs of reproduction, and the 

 gemmules grow out singly or in bunches from the sides of the 



* This name is derived from Alcyone, the fabled daughter of Neptune, and al- 

 though from a Greek word having the aspirate breathing to the first letter, it was 

 usually written by the Latins as here, (and by Linnaeus and others,) without the H. 



