94 



OELENTERATA. 



consequently no ccenenchyma ; and the compound coral may 

 consist simply of a congeries of corallites directly united to, 

 or springing from, one another. 



Three chief forms of gemmation may be distinguished 

 amongst the compound Zoantharia viz., basal \ parietal, and 

 calicular. 



In basal gemmation the mode of increase is by means of 

 a rudimentary coenosarc, which is put forth by the original 

 polype, and from which the young polype-buds are produced. 

 It " affords very different products according as the coenosarc 

 remains soft, or deposits a ccenenchyma ; appears under the 

 form of stolons, or of stouter connecting stems ; or even 

 spreads out in several directions as a continuous horizontal 

 expansion ; " in which last case the youngest polypes are, of 

 course, those nearest to the periphery of the mass. 



The parietal mode of gemmation is the commonest, and 

 it gives rise chiefly to dendroid, or tree-like, corals. In this 

 method the buds are produced from the sides of the original 

 polype, and they often repeat the process indefinitely. 



Calicular gemmation is not known to occur in any recent 

 coral, but it was a common mode of increase amongst extinct 



forms. In this method "the 

 primitive polype sends up from 

 its oral disc two or more simi- 

 lar buds ; these, in their turn, 

 produce other young polypes, 

 and thus the process is repeated 

 until an inverted pyramidal mass 

 of considerable size is produced, 

 all the parts of which rest 

 upon the narrow base of the 

 first budding polype" (fig. 47). 

 Fission in the Actinozoa differs 

 from gemmation chiefly in the 

 fact, that the polypes produced 

 fissiparously resemble one an- 

 other in organisation, and often 

 in size, as soon as they become 

 distinct. In gemmation, on 

 the other hand, the polype-bud consists primarily of a mere 

 process of ectoderm and endoderm, enclosing a caecal process 

 of the somatic cavity, and a mouth and other structures are at 

 first wanting. Amongst the coralligenous Actinozoa fission is 

 usually effected by " oral cleavage," the divisional groove com- 

 mencing at the oral disc, and deepening to a certain extent, 



Fig. 47. Calicular gemmation as 

 seen in Lonsdalea Jlorifonnis. Upper 

 Silurian. 



