POLYZOA ; FLUSTRA. 443 



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

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



c b a 



FIG. 705. PLUMATELLA: a, natural size ; b, a group enlarged; c, anal orifice. 



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. 



1078. After the details which we have just given of the 

 structure and mode of life of these interesting little creatures, it 

 will be unnecessary to dwell at any length upon the peculiarities 

 displayed by the different groups into which they are divided. 

 There are, however, one or two points to which attention may 

 be called. The class is divided into two Orders, in accordance 

 with the arrangement of the tentacula ; and it is somewhat re- 

 markable, that of these two types of structure, one occurs almost 

 exclusively in the sea, and the other in fresh water. The marine 

 Polyzoa exhibit the arrangement of the tentacles seen in the 



