28 HYDROIDS, ETC. 
surrounded by the reef (Fig. 26), and cut up by currents 
into deep channels. Such is the atoll forming at Tortu- 
gas, Florida, where the lagoon is fast filling up, and will 
eventually become dry land. The shape of atolls is due 
to the foundation upon which they are formed, currents, 
winds, and the sinking or elevation of the crust. 
Note.—An important factor in mud-making is the Holothurian. 
“Those at St. Solomon Islands have been found to eject two fifths of a 
pound of mud a day, so that sixteen animals would grind up a tona 
year ; and as in some places the bottom is fairly covered with them, the 
amount of work of this kind done by a single animal can be imagined. 
Works on Actinozoans for further reference. 
Dana's “Corals and Coral Islands” ; “‘ Our Sea-Anemones,” by A. 
E. Verrill ; ‘American Naturalist,” vol. ii, p. 251; “ Sea-Anemones,” 
“Popular Science Monthly,” vol. vii, p. 1; “ Arachnactis Brachiolata, 
a Floating Actinia,” A. Agassiz, “ Boston Journal of Natural History,” 
vol. vii, p. 525 ; “ Animal Life,” Semper ; “ Sea-Side Studies,” Agassiz ; 
“Smithsonian Reports”; ‘Challenger Reports” ; “ The Atlantic, and 
Depths of the Sea,” Sir Wyville Thomson; Darwin’s “ Structure of 
Coral Reefs”; ‘Fauna Americana,” J. B. Holder; “ Transactions 
American Academy,” vol. xi, 1883, Agassiz; ‘‘ Proceedings of Royal 
Society of Edinburgh, 1879-’80,” Murray. 
VALUE oF STony CoRALS.—The stony corals form islands. The 
fossil coral, Favosztes, is polished and much used in jewelry. Fossil 
coral found in Cuba is much used in building, Calcined coral is used 
as a dentifrice and as an antacid. 
Order II. Alcyonarians.—The animals that secrete 
a horny or calcareous stock without true dividing parti- 
tions or septa, are called Alcyonarians. Such are the sea- 
fans (Gorgonias), yellow, lilac, and black, sea-pens, the red 
coral of commerce, and many others. 
Red Coral.—In the red coral, Corallium rubrum (Fig. 
27), the axis p is calcareous, and composed of fused 
spicules, varying in color. When alive, the polyps, II, B, B, 
which are pure white, are connected by a series of com- 
plicated tubes, f, The tentacles are eight in number, and 
