2 S 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 ton a 

 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 Aclinozoans for fiirther 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. I ; " 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, iSyg-'So," Murray. 



VALUE OF STONY CORALS. The stony corals form islands. The 

 fossil coral, Favosites, 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 / 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, /. The tentacles are eight in number, and 



