l-l COELENTERATA CNIDARIA SIB-BRA:STH n 



Frequently, however, individuals formed by fission become only imperfectly 

 separated, remaining proximally more or less closely in contact. In such cases 

 tin- mlices form continuous, straight, curved, or labyrinthic furrows, with more 

 or less clearly distinguishable centres. 



The compound corallum of a polyp stock remains practically the same as in 

 solitary individuals, excepting that the conditions are more complicated when 

 the separation of the individuals is incomplete. Dendroid and astraeiform 

 colonies frequently develop a common connective matrix or tissue (coenenchyma) 

 which unites the various corallites into a whole ; it is secreted externally to the 

 theca by an inverted external part of the polyp over the top of the corallum 

 (Jin mljiliiffe), and is sometimes dense in structure (Oculinulae), or it may consist 

 of a vesicular or tubular tissue. The separate corallites are often also united 

 by means of the septa, which are produced over and beyond the thecae, and 

 fused with those of neighbouring individuals. In such cases the interseptal 

 loculi are almost always filled with strongly developed dissepiments. All 

 structures developed in the included space within the theca, with the exception 

 of the septa, are designated collectively as endotheca ; those lying without the 

 theca as exotheca. 



The Anthozoa are exclusively marine forms, and predominate in shallow 

 water. Many of the Actiniaria, Antipatharia, and Madreporaria occur also at 

 greater depths, ranging from 50-300, and sometimes even to 1500 fathoms. The 

 so-called reef-corals inhabit depths not exceeding 30-35 metres, and require a 

 temperature of the water of about 18 to 20 C. Hence, existing coral-reefs 

 are restricted to a zone extending about 30 on either side of the equator ; 

 they are distinguished according to form as fringing reefs, barrier reefs, and 

 atolls. While the stony corals (Forties, Madrepora, Turbinaria, Pocittupnw, 

 numerous Astraeidae and Fungidae) and the Alcyonarians (Hdiopora) are the most 

 important, they are not the only agents concerned in the formation of reefs, 

 as an active part is also played by the Hydromedtwc (Milleporidae), calcareous 

 algae (Litliothamnium, Melobesia), mollusks, echinoderms, bryozoans, and worms. 

 Of the ancient coral-reefs which have been formed in nearly all of the great 

 geological periods, those of the Cenozoic and Mesozoic periods are composed in 

 part of genera similar to those now living; while those of the Palaeozoic 

 represent genera and families that are now principally extinct, and whose 

 relation to living forms is often quite uncertain. 



The Anthozoa are divided by Haeckel into three sub-classes Tef 

 i, and Octoeoralla. 



Sub-Class 1. TETRACORALLA. Haeckel. 1 



(Zoantharia Rugosa, Milne-Edwards ; Pterocorallia, Freeh.) 

 Extinct, palaeozoic, simple, or composite sderodermic corals, with septa, arranged 

 according to a tetrameral system, and either bilaterally or radially si/mmetrirnl ; 



1 Literature (cf. also p. 67) : 

 Kunth, A., Beitrage zm Kemitniss fossiler Korallen. Zeitschrift der deiitschen geoloeisclien Gesell- 



schaft, Hd. XXI.. isw. ;,.,.! XXII., 1870. 

 Dybotosl-i. It'. .V.. Monographic -<ler Zoautlmria Rugosa, etc. Archiv fur Naturkunde Liv-, Est-, un.l 



Kurlands. lid. V., 1874. 



/:>'/'i/it;>; F., Lethaea Palaeozoica, 1883, pp. 324-416. 

 >V///;//,>,-. (i,-m., Anthozoen dt-s rliciuischeii Mittel-Devons. Aldiaiulhmgeii der preussischen <*eolo- 



gfechen Lmides-Austiilt, lid. VII I.. 1889. 



