146 THE ANATOMY OF INVERTEBRATED ANIMALS. 



stone-corals are essentially Actinice^ which become converted 

 into zoanthodemes by gemmation or fission, and develop a 

 continuous skeleton. 



The skeletal parts ^ of all the Actinozoa, consist either of 

 a substance of a horny character ; or of an organic basis im- 

 pregnated with earthy salts (chiefly of lime and magnesia), 

 but which can be isolated by the action of dilute acids ; or, 

 finally, of calcareous salts in an almost crystalline state, form- 

 ing rods or corpuscles, which, when treated with acids, leave 

 only an inappreciable and structureless film of organic matter. 

 The hard parts of all the Ajjorosa, Perforata^ and Tahulata 

 of Milne-Edwards are in the last-mentioned condition ; while, 

 in the Octocoralla^ except Tuhipora^ and in the AntipatTiklce^ 

 and ^oai'^^A^V?c??, among the Hexacoralla^ the skeleton is either 

 horny ; or consists, at any rate, to begin with, of definitely 

 formed spicula, which contain an organic basis, and frequently 

 present a laminated structure. In the organ-coral {Tiihipora)^ 

 the skeleton has the character of that of the ordinary stone- 

 corals, except that it is perforated by numerous minute canals. 



The skeleton appears, in all cases, to be deposited within 

 the mesoderm, and in the intercellular substance of that layer 

 of the body. Even the definitely shaped spicula of the Octo- 

 coralla seem not to result from the metamorphosis of cells. 

 In the simple aporose corals the calcification of the base and 

 side walls of the body gives rise to the cup or theca ; from 

 the base the calcification extends upward in lamelU^, which 

 correspond with the interspaces between the mesenteries, and 

 gives rise to as many vertical septa^ the spaces betw een which 

 are termed loculi ; while, in the centre, either by union of the 

 septa or independently, a column, the columella^ grows up. 

 Small separate pillars between the columella and the septa are 

 termed p)aluli. From the sides of adjacent septa scattered 

 processes of calcified substance, or synapticulce, may grow 

 out toward one another, as in the Fimgidm ; or the interrup- 

 tion of the cavities of the loculi may be more complete in 

 consequence of the formation of shelves stretching from sep- 

 tum to septum, but lying at different heights in adjacent 

 loculi. These are interseptal dissepiments. Finally, in the 

 Tabidata, horizontal plates, which stretch completely across 

 the cavity of the theca, are formed one above the other and 

 constitute tabular dissepiments. 



1 See Kolliker, " Icnnes Histolo.oricoe," 1866. 



2 Lacaze-Duthiers's investifirntions on Astrcea calycularis prove that the septa 

 begin to be fonned before the theca. 



