192 ZOOLOGY 



SECT. 



which is still uncertain. Spicules occur in the mesoderm, some of 

 them microscopic, others readily visible to the naked eye. 



In the Madreporaria we have a skeleton of an entirely different 

 type, consisting, in fact, of a more or less cup-like calcareous 

 structure, secreted from the ectoderm of the base and column of 

 the polype. When formed by a solitary polype, such a " cup- 

 coral" is known as a corallite : in the majority of species a large 

 number sometimes many thousands of corallites combine to 

 form a corallum, the skeleton of an entire coral-colony. 



The structure of a corallite is conveniently illustrated by that 

 of the solitary genus Fldbellum (Fig. 142, A, B). It has the form 

 of a short conical cup, much compressed so as to be oval in section- 

 Its wall or theca (th.) is formed of dense stony calcium carbonate, 

 white and smooth inside, rough and of a brownish colour outside, 

 except towards the margin, where it is white. Its proximal or 

 aboral end is produced into a short stalk or peduncle, by which the 

 Coral is attached in the young state, becoming free when adult : 

 in many other simple Corals there is no stalk, but attachment to 

 the support is effected by means of a flattened proximal surface 

 or "basal plate (C, &. pi.). From the inner surface of the theca a 

 number of radiating partitions, the septa (sep.\ proceed inwards or 

 towards the axis of the cup, and, like the mesenteries of a polype, 

 are of several orders, those extending furthest towards the 

 centre being called primary septa, the others secondary, tertiary, 

 and so on. Towards the bottom of the cup the primary septa 

 meet in the middle to form an irregular central mass, the columella 

 (col.). In some Corals the columella is an independent pillar-like 

 structure arising from the basal plate (D, col.). 



In many Corals there is a distinct calcareous layer investing the 

 proximal portion of the theca, and called the epitheca (C, e.th.). Some 

 species have the inner portions of the septa detached so as to form 

 a circlet of narrow upright columns, the pali. In others there are 

 horizontal partitions or dissepiments passing from septum to septum, 

 and in others, again, complete partitions or tabulce, like those of 

 Millepora (p. 145), extending across the whole corallite. In the 

 Mushroom-coral (Fungia), the corallite is discoid, the theca is con- 

 fined to the lower surface, and small calcareous rods, the synapticulw, 

 connect the septa with one another. 



In the living condition the polype fills the whole interior of the 

 corallite and projects beyond its edge to a greater or less degree 

 according to its state of expansion (C). The proximal part of the 

 body- wall is thus in contact with the theca, which has the relation 

 of a cuticle, and is, in fact, a product of the ectoderm. The free 

 portion of the body- wall is frequently, in the extended state, folded 

 down over the edge of the theca so as to cover its distal portion. 

 The septa alternate with the mesenteries, each lying in the space 

 between the two mesenteries of one couple, and each being in- 



