ACTINOZOA. 185 



as above stated, of comparatively rare occur- 

 rence. 



Every polype-bud is, therefore, at first, no more 

 than a protuberance from the two parent layers, 

 enclosing a caecal diverticulum of the somatic 

 cavity. Thus, the nutrition of the young zooid is 

 provided for, till it developes for itself a mouth ; 

 after which it may either still continue its primi- 

 tive connection with the common mass or, as 

 already stated, by deposition of tissue secretions, 

 become, physiologically, a separate organism, 

 though morphologically associated with other 

 zooids of the same composite fabric. 



The growth, gemmation, and fission of the 

 composite coralligenous Actinozoa require, in 

 addition to what has been said, the following more 

 particular explanations. 



The whole compound structure in the sclero- 

 basic species may be regarded as the result of a 

 peculiar modification of the process of basal 

 gemmation ; the rudimentary coenosarc developed 

 around the base of the primitive polype, instead 

 of spreading only at its circumference, shaping 

 itself into a slender and, at first, slightly elevated 

 stem which, gradually increasing in height, con- 

 tinues, at the same time, to excrete a succession 

 of epidermic layers. Thus the young coenosarc 

 enlarges in diameter, and soon a number of buds 

 are seen to spring from its surface. Some sclero- 

 basic Corals remain thus throughout life, the 

 coenosarc, with its axial excretion, merely becom- 

 ing taller. But, more frequently, branches are 

 sent forth, which resemble, in every essential 

 respect, the stem from which they originated. 



