106 MANUAL OF ZOOLOGY. 



from gemination 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 

 commencing 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 ba.se 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 

 to suggest that resemblance to the convolutions of the brain, 

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

 to indicate.' 



CHAPTER XIY. 

 ALCYONAEIA. 



OEDER II. ALCTONAKIA. 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 

 corallum, when present, is usually sclerobasic, or spicular ; if 

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



