CORAL BUILDERS. 239 



of coral-reefs, but they add largely to the beauties 

 of the coral landscape. The animal of the commer- 

 cial Red Coral belongs to the same category, being 

 what is termed an Alcyonoid polyp, and all further 

 allusion to it will be found in our chapter on Sea- 

 fans. 



The Bryozoans, or animals of the sea-mats, are of 

 another and distinct type, but the structures they 

 form are usually small, and either thin and membra- 

 naceous, or thicker and more calcareous, forming 

 plates which may, by growing in a stratified manner 

 one over the other, ultimately produce a layer of 

 considerable thickness. Whatever they may have 

 done, in Palaeozoic times, in contributing to the for- 

 mation of limestone, they contribute but little to the 

 coral-reefs of the present age. In furtherance of 

 our design, if space permits, we hope to include 

 both these groups of animals in the present volume, 

 therefore are content to abandon them for the 

 present. 



It is also a fact that some undoubted plants, or sea- 

 weeds, secrete lime to such an extent as to contribute 

 to the accumulations which form coral-reefs. Some 

 of these are very delicate and beautiful, though fra- 

 gile, and their name of corallines suggests miniature 

 resemblances to coral. The larger kinds, or Nulli- 

 pores, not only contribute something in bulk to the 

 calcareous deposit of reefs, but Darwin says that 



