1 64 MANUAL OF ZOOLOGY. 



(4.) Fission. This process in the coralligenous Actinozoa is usually 

 effected by " oral cleavage," the divisional groove commencing at the oral 

 disc, and deepening to a greater or less extent, the proximal extremity 

 always remaining undivided. According 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 grad- 

 ual 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 be- 

 come 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 convolu- 

 tions of the brain which its popular name of Brain-stone Coral has been 

 devised to indicate " (Greene). 



The Zoantharia sderodermata are divided into the two follow- 

 ing groups, founded upon the characters of the corallum : 



1. Aporosa. The calcareous tissue of the corallum is more or less 

 compact and imperforate ; the septa usually constituting complete solid 

 plates, and the theca being as a rule not pierced by any apertures. Dissepi- 

 ments or synapticulae are usually present, but tabulse are rarely developed. 

 This section includes the most highly developed of existing corals ( Tur- 

 binolid(Z, Octdinidtz, Astrceidt?, Fungidce, &c.) 



2. Perforata. The calcareous tissue of the corallum is more or less 

 porous, loosely aggregated, spongy, or reticulate, the walls in all being 

 perforated with more or fewer apertures. The septa are generally well 

 developed, but they are also perforated by apertures, and may be simply 

 trabecular. Imperfect dissepiments may be present, and in some cases 

 there are well-developed tabulae ; but the visceral chamber is usually more 

 or less completely open from top to bottom. The three families comprised 

 in this section are the Eupsammida, the Madreporidce, and the Poritidcc, 

 to which must be added the great and almost extinct family of the 

 Favositidce. 



In addition to the above-mentioned groups of the Zoantharia sderoder- 

 mata two other groups have been established under the names of the 

 Tabulata and Tubulosa. The former of these included the so-called 

 "Tabulate Corals," distinguished by the imperfect development of the 

 septa, and the fact that the visceral chamber is divided into compartments 

 by horizontal plates or "tabulae" (fig. 76, D). Some of the so-called 

 "Tabulate Corals," however, such as Millepora, have been shown to be 

 Hydrozoa ; others, such as Pocillopora (fig. 76) belong to the Aporose 



