534 CILIOBRACHIATE POLYPES. FOSSIL CORALS. 



upon this little specimen. If each of these tentacula has only 

 100 cilia upon its edges (and there are probably many more), 

 the whole polypidom will have 39,600,000 of these minute but 

 important organs. Other species certainly contain more than 

 ten times these numbers. Dr. Grant has computed about 

 400,000,000 cilia to exist on a single Flustra foliacea. In the 

 Alcyonella, a fresh-water species composed of long membranous 

 cells arranged side by side so as to form a spongy mass, the 

 number of tentacula on a moderate-sized specimen may be com- 

 puted at nearly five millions and a half ; and the cilia are cer- 

 tainly not less than a hundred times that number. The figure in 

 the preceding page represents another very beautiful fresh- water 

 polype of this Order, which is by no means uncommon. It is 

 found upon the surface of leaves, &c., of aquatic plants. 



1106. From the very imperfect degree in which the greater 

 part of the polypes of this group have been yet examined, the 

 subdivisions of it are at present necessarily founded only upon 

 the arrangement of the cells in regard to one another. Such an 

 arrangement is liable to this great objection, that the mode in which 

 they are disposed will vary extremely with the surface upon 

 which they grow. Hence it is not advisable for us to enter more 

 particularly into the classification of this group ; but the study 

 of it may be especially recommended to those who have oppor- 

 tunities at their command, as a most interesting pursuit, which, 

 with but a moderate amount of previous information, and of skill 

 in observation, is sure to be productive of much that will be 

 alike interesting, novel, and important. 



1107. The most extended survey we can take of the opera- 

 tions of the Polypifera upon the surface of the globe at the 

 present time, will give us but a very inadequate idea of the 

 important part which they performed, in the remoter epochs of 

 the history of the earth. Our wonder is excited when we hear 

 of a continuous reef of coral more than a thousand miles in 

 length; yet what is this to the formation of limestone strata, 

 covering superficial areas, not only of thousands, but of tens of 

 thousands, of square miles, to a thickness, in many instances, of 



