CLASS I ANTHOZOA 79 



corallum is formed, consisting of a succession of cups placed one within the other, 

 of which only the youngest and uppermost is occupied by the living animal. 



The beginning of reproduction by fission is marked by an elongation or 

 distortion of the parent calice, accompanied by the contraction of the Avail at 

 opposite points along the margin. The constriction may proceed until it 

 divides the oral disk into two halves ; or two opposite septa may unite to form 

 a new theca. By this method branching, massive or " astraeiform " colonies 

 are produced, which do not differ essentially from those formed by budding. 

 Frequently, however, individuals formed by fission become only imperfectly 

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

 the calices form continuous, straight, curved or labyrinthic furrows, with more 

 or less clearly distinguishable centres. 



The compound corallum of a polyp stock I'emains practically the same as in 

 solitary individuals, excepting that the conditions are more complicated when 

 the separation of the zooids is incomplete. Dendroid and massive colonies 

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

 unites the various corallites into a whole ; it is secreted by the common 

 colonial flesh, called coenosarc, which extends as a carpet between the polyps, 

 The coenenchyma is sometimes dense in structure (Oculinidae), 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 and columella, 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 Adiniaria, Antipatharia and Madreporaria occur also at 

 greater depths, ranging from 50 to 300 and sometimes to over 3000 fathoms. 

 The so-called reef-corals inhabit depths usually not exceeding 45 metres, and 

 require a temperature of the water of 20° C, or higher. 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 (Porites, Acrop)ora, Tnrhinaria, 

 Pocillopora, numerous " Astraeidae" and Fungidae) and the .Alcyonarians 

 (Heliopora) 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 Hydromedusae 

 (Milleporidae), calcareous algae (Lithothamnium, Melohesia), mollusks, echino- 

 derms, 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 Paleozoic 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 subclasses : Tetracoralla, 

 Hexacoralla and Alcyonaria or Odocoralla. Of these the two first-named groups 

 are by some authors collectively termed Zoantharia. 



