726 THE ANIMAL KINGDOM. 



It may be (i) discontinuous in the shape of calcareous spicules, or (2) con- 

 tinuous and then (i) organic and horny, (ii) both organic and inorganic, 

 i. e. calcareous, or (iii) solely calcareous. The discontinuous skeleton co- 

 exists with the continuous either (i) or (ii) supra, but never with the lamel- 

 late form of (iii) supra. When the skeleton forms calycles, or tubes lodg- 

 ing the zooids of a colony, the deeper parts of the tubes or calycles may be 

 successively closed off by calcareous partitions, usually horizontal and 

 known as tabulae. Tabulae occur in Tubipora, and Heliopora among 

 Alcyonaria, in many extinct Rugosa and a few living corals among 

 Zoantkaria 1 . The skeleton -producing cells appear to be derived from 

 the ectoderm (except in Pennattdidae ?) ; they may remain in continuity 

 with it or be detached from it, in this case becoming mesoglaeal. The 

 individual or colony may be either of separate sexes, or hermaphrodite 2 . 

 Fission and gemmation may occur in solitary forms. There are two per- 

 fectly distinct sub-classes, the Alcyonaria and Zoantharia. 



SUB-CLASS i, ALCYONARIA (= Octactiniae). 



Anthozoa which are colonial with the exception of a single family. 

 The colony is sometimes free. The tentacles are eight in number, similar and 

 pinnate-, the mesenteries also eight and complete. The retractor muscles 

 are well developed and placed on the ventral aspect of each mesentery. The 

 siphonoglyphe, when present, is single and ventral. The zooids are sometimes 

 dimorphic and then are knovjn as autozooids ( = polypes) and siphonozooids 

 ( = zooids), the latter being of simplified structure. Calcareous spicules in the 

 mesoderm are very rarely absent. 



The colonial Alcyonaria are distinguished from one another by the 

 form of the colony and the character of the skeleton. As to the first, two 

 principal groups are distinguishable, (i) The zooids originate from a 

 system of basal tubular stolons (Clavularid], from a narrow band-like 

 stolon (Sarcodictyon\ or a disc-like expansion (Sympodium and Tubi- 

 poridae\ as well as from tubes connecting the zooids at different heights as 

 in Clavularia viridis, or from platforms s. external tabulae as in Tubi- 

 poridae. The consequence is that the individual zooids remain inde- 

 pendent and separate. In certain fossil forms (Favosites) which appear to 

 belong here the zooids are closely apposed. (2) The zooids are imbedded 



1 They occur also in the Hydrocorallina among Hydrozoa, and are consequently of no 

 systematic importance. 



2 Scytophorus striatus among Hexactiniae, certain Zoantkidae, the Cereantheae, as well as 

 some specimens of Corallium among Alcyonaria, are undeniably hermaphrodite. The question of 

 sex is complicated by the fact that it is possible for the male and female organs to be developed at 

 different times or on different mesenteries. Such at least is the case with certain Hexactiniae, 

 according to de Lacaze Duthiers : see A. Z. Expt. i. 1872, pp. 309, 371. The instance of 

 Corallium shows that one individual or one colony may be uni-sexual, another hermaphrodite. 



