XIV ZOANTHARIA SKELETON REPRODUCTION 37 I 



theca towards the stomodaeum. In the colonial forms the theca 

 of the individual zooids is continuous with a common colonial 

 skeleton called the " coenosteum." This is solid in the Imper- 

 forate corals, and it supports at the surface only a thin lamina 

 of canals and superficial ectoderm. In the Perforate corals, 

 however, the coenosteum envelopes and surrounds the canals 

 during its formation, and thereby remains perforated by a 

 network of fine channels. In the colonial Madreporaria the 

 skeletal cups which support and protect the zooids are called 

 the " calices." 



The skeleton of the Antipathidea is of a different nature. 

 It is composed of a horny substance allied to keratin. When 

 it is old and thick, it usually has a polished black appearance, 

 and is commonly termed " black coral." The surface of this 

 kind of coral is ornamented with thorny or spiny projections, 

 but it is never perforated by calices or canal systems. It forms 

 a solid axis for the branches of the corals, and all the soft parts 

 of the zooids and coenosarc are superficial to it. 



It was formerly considered that this type of coral, which 

 shows no trace of the shape and form of the living organisms 

 that produce it, is of a different character to the calcareous 

 skeleton which exhibits calices, septa, pores, and other evidence 

 of the living organism, and it was called a " sclerobase " to 

 distinguish it from the " scleroderm " of the Madreporaria. 



It is now known that both the sclerobasic skeleton and the 

 sclerodermic skeleton are products of the ectoderm, and conse- 

 quently these expressions are no longer in general use. 



Asexual reproduction in the Zoantharia may be effected by 

 continuous or discontinuous fission or gemmation. 



In the Edwardsiidea, Actiniaria, and Cerianthidea, that is to 

 say in the animals popularly known as Sea-anemones, asexual 

 reproduction does not commonly occur, but nevertheless a good 

 many instances of it are now known in individual genera. In 

 Actinoloha (Metridium), for example, Parker has described a case 

 of complete longitudinal fission, and Duerden states that it 

 occurs in the West Indian Anemones Actinotryx and Ricordea. 

 A still more remarkable form of asexual reproduction known as 

 transverse fission has been described in the genus Gonactinia} 

 In this case, the body of the Anemone becomes constricted ■ in 

 ' H. Prouho, Arch. Zool. Expir. 2iidser. ix. 1891, p. 247. 



