ACTINOZOA. 183 



they subsequently, by gradual development, tend 

 to assume. 



Further observations are wanting on the occur- 

 rence of fission and gemmation among the Cteno- 

 phora. In none of these animals do we find 

 colonies of zooids resulting, as in the Hydrozoa, 

 from a process of continuous budding. 



But in other Actinozoa continuous gemmation 

 abundantly takes place, and in this manner are 

 formed those composite structures, consisting of 

 numerous poljrpes, met with among so many 

 genera of Zoantharia, Alcyonaria and Rugosa. 

 In a few of these organisms, discontinuous gem- 

 mation may also be noticed. 



Young polypes may be budded either (i) from 

 the base of the primitive structure, or (2) from 

 the sides of the polypes, or (3) from their oral 

 discs. Since these surfaces are but parts of a 

 common integument it might be anticipated that 

 intermediate positions of buds would now and 

 then occur. Nevertheless, it has been found con- 

 venient to distinguish three principal modes in 

 which gemmation of polypes may be effected, as 

 the basal, the parietal, and the calicular, respec- 

 tively. 



In basal gemmation the polype sends forth a 

 rudimentary coenosarc, from which, after a time, 

 the young polype-bud is produced, and so on for 

 all the zooid forms subsequently evolved. 



The extent to which a coenosarc may be deve- 

 loped varies, however, considerably. It must not 

 be inferred that in every composite Actinozoon 

 such a structure is present ; for the mass may 

 exhibit nought else save a congeries of polypes 

 in immediate mutual connection. In this case 



N 4 



