44 POPULAR HISTOEY OF THE AQUARIUM. 



the contrary, the cells and vesicles are similar in shape 

 and only ditl'er in size. The cells of some Sertidarim are in 

 pairs, some alternate, some at irregular intervals. 



Sertularia argentea 



Is a fine, feathered, bushy polype, sometimes called the 

 Squirrel' s-tail Coralline, which is found growing on the 

 rock-oysters at Sheppey and Sheerness. The ramifications 

 are in tufts, arranged spirally round the stem ; an arrange- 

 ment which gives a peculiarly graceful air to the whole 

 polypidom, which sometimes reaches several feet in length. 

 But when its upper branches reach this length, much of 

 the vitaKty of the under branches is impaired through 

 age, and these earlier parts die and become worn ; in this 

 state they fall away and leave the lower part of the stem 

 bare. 



The following experiment on the dead and dry polypi- 

 dom of a Sertiilaria will be found interesting, showing a 

 degree of elasticity in the horny substance of which it is 

 composed : — 



"About two years ago I detached two specimens of 

 Sertularia from an oyster-shell: they were about If inch 

 high, the side branches being from 4 to f of an inch in length. 



