PLANT-ANIMALS, OR ZOOPHYTES. 173 



phenomenon will be more striking in proportion to 

 the quantity of water put into commotion." l 



The growth, or increase, of the polypary is com- 

 paratively rapid, for the size of the little animals that 

 are concerned in building it up. Mr. Couch has 

 expressed the opinion that a large specimen of one 

 kind of Sertularia may be formed, under favourable 

 circumstances, in the course of fourteen days. 



Another observer mentions the fact of the bottom 

 of a boat which became covered in a fortnight. Some 

 of the species undoubtedly continue through several 

 seasons, and increase in size from year to year, whilst 

 others have a much briefer career, and some only 

 last during a single season. Those kinds which 

 attach themselves to rocks are considered to be more 

 persistent than those which affect sea- weeds. In 

 perennial species the polypes are much reduced in 

 number during the winter, reappearing again in the 

 spring. Although several kinds have been known to 

 thrive with apparent vigour in aquaria, yet the rate 

 of increase, under such artificial conditions, would 

 hardly be equal to what it would be in a state of 

 nature. The enormous rate at which many of them 

 multiply, and establish new colonies, might almost 

 be anticipated from the large number of individuals 



1 "Annals of Natural History," first series, vol. viii. p. 343 

 (1842): 



