202 ZOOLOfiY SFCT. 



formed of a flexible horn-like material. Moreover it is not meso- 

 gloeal, but ectodermal in origin : in close contact with it is an 

 epithelium, from the cells of which it is produced as a cuticular 

 secretion, and this epithelium is formed as an invagination of the 

 base of the colony. In addition to its axis, Gorgonia contains 

 numerous spicules in the mesoglcea of the ccenosarc. In some 

 of the Gorgonacea the axial skeleton is partly horny, partly 

 calcareous. 



In the Sea-pen (Pennatula, Fig. 148) and its allies the stem of 

 the colony is supported by a horny or calcareous axis which is 

 imbranched, not extending into the lateral branches. In this case 

 the axis is contained in a closed cavity lined by an epithelium, the 

 origin of which is still uncertain. Spicules occur in the mesoglcea, 

 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 Flabellum (Fig. 156, A, B). It has the form 

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

 Its wall or iheca (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, b. 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 tabulae, like 

 those of Millepora (p. 158), extending across the whole corallite. \ In 



