ON THE FORMATION OF CORAL ISLANDS. 337 



as the shell ; being a secretion or deposition of earth in 

 its substance. And though in one sense this may be 

 viewed as an original colony, all the polypi are but 

 parts of one whole, as flowers are of a plant ; while 

 the resemblance would be perfect if the flowers 

 were to produce the plant instead of being produced 

 by it. 



This analogy, and the general nature of the corals 

 also, will be made more intelligible by examining the 

 Vorticella, possessing an essential resemblance to the 

 coral polypus in everything else but the production of 

 a shell. The simple vorticella swims at liberty ; re- 

 sembling a flower-bud, and consisting of a body pro- 

 vided with tentacula. Other species are fixed by a 

 pedicle which restrains their motions within narrow 

 limits. In others again, two or more are united by 

 one stem ; and, at length, we find highly ramified 

 kinds in which each branch is terminated by the active 

 polypus. In this case also, each polypus is partially 

 independent, yet all depend on the whole ; presenting 

 a singular analogy to the vegetable identity, especially 

 in such cases as that of Epidendron, where the 

 leaves combine to produce and nourish the plant ; so 

 as to give a sort of colour to that gradation so often 

 sought by visionary naturalists, between one depart- 

 ment of nature and the other. If the corals differ 

 from the vorticcllse in the secretion of a stony mat- 

 ter, so do they present an analogy to the Chara and 

 the Coralline, secreting a calcareous bark, and the 

 Equisetum, which produces a siliceous one. The 

 madrepore will serve to explain more distinctly the 

 mode in which the calcareous cell is constructed. 

 This polypus is surrounded by feet, or hands, sur- 

 rounding the body and divided at the extremities ; 

 each embracing a lamella of the star. As in the 



VOL. i. z 



