CGELENTERATA : ACTINOZOA. 119 



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 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, 

 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 c&spitose, 

 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 

 to suggest that resemblance to the convolutions of the brain 

 which its popular name of Brain-stone Coral has been devised 

 to indicate." 



CHAPTER XIV. 

 ALCYONARIA. 



ORDER II. ALCYONARIA. The second great division of living 

 Actinozoa is that of the Alcyonaria, defined by the possession 

 of polypes with eight pinnately -fringed tentacles, the mesenteries 

 and somatic chambers being also some multiple of four. The co- 

 rallum, when present, is usually sclerobasic, or spicular ; if"thecce" 

 are present, as is rarely the case, there are no septa. 



