124 MANUAL OF ZOOLOGY. 



In basal gemmation the mode of increase is by means of 

 a rudimentary ccenosarc, 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 ccenosarc 

 remains soft, or deposits a ccenenchyma ; appears under the 

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

 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 similar buds ; these, in their turn, 

 produce other young polypes, and thus the process is repeated 

 until an inverted pyramidal mass of considerable size is pro- 

 duced, all the parts of which rest upon the narrow base of the 

 first budding polype (fig. 32, a). Fission in the Actinozoa differs 

 from gemmation chiefly in the fact, that the polypes produced 

 fissiparously resemble one another 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 csecal process 

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

 first wanting. Amongst the coralligenous Actinozoct fission is 

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

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

 the proximal extremity always remaining undivided. More 

 rarely, fission " is effected by the separation of small portions 

 from the attached base of the primitive organism, whose form 

 and structure they subsequently, by gradual development, tend 

 to assume." 



" The coral-structures which result from a repetition of the 

 fissiparous process are of two principal kinds, according as 

 they tend most to increase in a vertical or in a horizontal 

 direction. In the first of these cases the corallum is caspitose, 

 or tufted, convex on its distal aspect, and resolvable into a 

 succession of short diverging pairs of branches, each resulting 

 from the division of a single corallite." In the second c<n,se 

 the coral becomes lamellar. " Here the secondary corallites 

 are united throughout their whole height, and disposed in a 

 linear series, the entire mass presenting one continuous theca." 



