36 



NATURAL HISTORY. 



consisting of a large number of parallel tubes bound together at regular 

 intervals by horizontal floors. Each tube is the home of a single polyp, 

 with eight feathery tentacles. The color of the organ-pipe coral is a deep 

 red, and the skeleton is rather fragile. 



The group of sea-pens, one example of which is figured, bear, as 

 their common name indicates, a considerable resemblance to a quill pen. 

 There is a central axis corresponding to the quill 

 proper, and from the upper part of this on either j%. ^ 

 side are rows of leaf-like discs which bear the 



z&z&f 5 ?; 





ifigfe- i 





Fig. 37. — Organ-pipe coral (Tubipora). 



Fig. 38. — Sea-pen (Pteroides), 

 with a single polyp, enlarged. 



polyps, and which correspond to the barbs of the feather. The sea-pens 

 are brightly colored, red in one shade or another being the prevailing tint. 



Corals play an important part in the history of the earth, from the 

 fact that they form large reefs and islands. Not only is this the case 

 to-day, but so it was in geological time. In many places in the rocks we 

 find the remains of the old coral reefs of ages ago, one of the best known 

 being the large deposit of fossil corals at the Falls of the Ohio, at Louis- 

 ville, Ky., which dates from Devonian time. Here the resemblance to a 

 modern reef is perfect, and large coral masses six and eight feet in diame- 

 ter occur. 



To-day the distribution of reef-building corals is limited by tempera- 

 ture, no reefs being formed outside of the isotherm of 68°, a limit which, 

 however, does not at all agree with the parallels of latitude laid down 

 on the globe. Still, it never passes the latitude of 35°, and in the 

 Atlantic the Bermudas are the farthest north of all the coral islands. 

 The largest existing reef to-day is that on the northeastern coast of Aus- 



