186 



ACTINOZOA. 



These branches may be apparently quite irregular 

 in their arrangement, or they may arise in the 

 same vertical plane from opposite sides of the 

 stem, sometimes uniting one with another by the 

 formation of an intricate network, as in the well- 

 known and beautiful Fan-Corals. 



Such is the growth of the corallum in the fixed 

 Gorgonidw and Antipathidw. Among the Penna- 

 tulidce, which are free forms, the proximal end of 

 the coenosarc usually becomes produced into a 

 gently swelling or tapering mass, supported by a 

 comparatively slender, elongated sclerobasis. 



More varied are the modifications of the com- 

 posite sclerodermic corallum, the increase of which 

 may be the result of fission alone, or of gemmation 

 alone, or of gemmation and fission combined. 



Among coralligenous polypes fission is usually 

 effected by oral cleavage, as indicated above ; the 

 proximal extremity of each, for a greater or less 

 extent, remaining undivided. The two zooids thus 

 produced either supply by reparation those parts 

 which were wanting to render them complete, or, 

 it may be, continue through life in a more or less 

 imperfect condition. 



_ The coral structures which result from a repe- 

 tition of the fissiparous process are of two principal 

 forms, according as they tend most to increase in 

 a vertical or horizontal direction. In the first of 

 these cases the corallum is coespitose, 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 coral- 

 lite. In some parts of the mass the walls of the 

 corallites blend, so that a few of them may even 

 become compacted with one another. 



