3 6 



MANUAL OF ZOOLOGY. 



the parts of which rest upon the narrow base of the first bud- 

 ding polype" (fig. 47). Fission m the Adinozoa differs from 



gemmation chiefly in the fact, 

 that the polypes produced fissi- 

 parously 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 caecal pro- 

 cess of the somatic cavity, and 

 a mouth and other structures are 

 at first wanting. Amongst the 

 coralligenous Actinozoa fission is 

 usually effected by " oral cleav- 

 age," the divisional groove com- 



Fig. 47. Calicular gemmation as seen 6 ' . & , .. 



in Lonsdahia flori/ormis. Upper mencing at the oral disc, and 

 Sllunan - deepening to a certain extent, 



the proximal extremity always remaining undivided. Ac- 

 cording to Dana, in fission a new mouth is formed in the 

 disc near the old mouth, and a new stomach is formed for the 

 new mouth, round which the new tentacles are then developed. 

 This, therefore, is not, strictly speaking, a subdivision into 

 halves ; since one half carries off the old mouth and stomach. 

 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 ccespitose, 

 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 case 

 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." 

 Both these forms of corallum "are liable to become massive by 

 the union of several rows or tufts of corallites throughout the 

 whole or a portion of their height. An illustration of this is 

 afforded by the large gyrate corallum of Meandrina, over the 

 surface of whose spheroidal mass the calicine region of the 

 combined corallites winds in so complex a manner as at once 



