^2 CORALS AND CORAL ISLANDS. 



Occasionally they make large, massive corals, from the 

 growing of plate over plate. One of these massive species, 

 Escharella variabilis., is common on the coast of the United 

 States from Cape Cod southward. 



The first of the foregoing figm-es, from Smitt's work on the 

 Bryozoans, represents one of the delicately branching species, 

 of natural size ; and the second, a portion of the same, much 

 enlarged. The latter figure shows that the branches are 

 made up of minute cells. From each cell, when alive, the 

 bryozoum extends a circlet of tentacles, less than a line in 

 diameter. 



The incrusting kinds are common in all seas. The crust 

 of cells they make is often thinner than paper. A portion 

 of such a crust is represented, enlarged, in figure 3. When 

 expanded, the surface is covered over with the delicate flower- 

 like bryozoa. A low magnifying power is necessary to observe 

 them distinctly. The animals, unlike true polyps and the 

 Hydroids, have two extremities to the alimentary canal, and 

 in this, and other points, they are Molluscan in type. 



The cells of a group never have connection with a common 

 tube, as in the Hydroids ; on the contrary, each little Bryo- 

 zoum, in the compound group or zoothome, is wholly inde- 

 pendent of the rest in its alimentary canal. 



Bryozoans occur in all seas and at all depths ; and in early 

 Paleozoic time they contributed largely to the making of lime- 

 stone strata. The two specimens figured on the preceding 

 page occur on the coast of New England, as well as in the 

 seas of North Europe. 



IV. NULLIPORES. 



The more important species of the Vegetable Kingdom that 

 afford stony material for coral reefs are called NulHpores. 

 They are true Algae or sea-weeds, although so completely stony 

 and solid that nothing in their aspect is plant-like. They form 

 thick, or thin, stony incrustations over surfaces of dead corals, 

 or coral rock, occasionally knobby or branching, and often 



