192 



COELENTERATA. 



In the colonial forms, fission by division of the polyps into two may occur 

 (Oculinae, Astraea), or the division may be confined to the oral disc, so that 

 a complex polyp is formed, with several mouths and oesophaguses opening into 

 a common coelenteron (Maeandrina, Fig. 144). In many cases, perhaps the 

 majority, the colony is increased by budding from the extra-thecal coenosark. 



The hard structures or corallum follows more or less closely the shape of 

 the polyp, and were at one time thought to be actually contained in the 

 tissue of the polyp. They consist of a cup or theca, from which the polyp 

 projects and into which it can shrink, and in some forms of a connecting 

 substance, which may or may not be porous, connecting the cups this is the 

 coenenchyma. The cup has, projecting inwards from its walls, a number of 

 radiately and vertically arranged calcareous plates, which suggest calcified 

 mesenteries. These are the septa; 

 they are not mesenteries, but occur 

 between mesenteries. The cup 

 presents a basal plate below, from 

 which rise the walls. 



FIG. 153. Basal plate of a larva of 

 Astroides ccdycularis, soon after at- 

 tachment. With 12 radial ridges 

 (after Lacaze Duthiers, from Bal- 

 four). 



FIG. 154. Diagram to exhibit the relations of 

 the polyp to the corallum. T tentacles ; ec 

 ectoderm ; ed endoderm ; st oesophagus ; mf 

 mesenterial filaments; r extra-thecal coelen- 

 teron ; m mesentery ; TO' extra-thecal portion 

 of inesentery ; Ep basal plate ; ep epitheca ; 

 Th theca ; cy calicoblasts (after G. C. Bourne). 



The hard structures or corallum are secreted by, and on the outer side of, 

 the ectoderm. 



The first part of the skeleton to appear (Astroides calycularis) is an annular 

 basal plate (Fig. 153), incomplete at first in its central part, between the basal 

 ectoderm and the surface to which the young polyp is attached. Next twelve 

 radially arranged folds of the basal body-wall rise up and project into the 

 enteron. The ectoderm of these folds secrete calcareous deposits, which 

 constitute the first trace of the septa (Fig. 153). The folds of the septa 

 differ from those of the mesenteries (between which they are placed) in being 

 folds of the whole body-wall, and not of the endoderm alone. 



The septa, therefore, arise as rod-shaped structures in continuity with the 

 basal plate. They increase in thickness and height as the polyp grows, and 

 their outer ends, which do not reach to the body-wall of the polyp, become 



