72 COELENTERATA—CNIDARIA SUB-BRANCH IT 
Frequently, however, individuals formed by fission become only imperfectly 
separated, remaining proximally more or less closely in contact. In such cases 
the calices form continuous, straight, curved, or labyrinthie 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 
(ftandplatte), and 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, 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 (Porites, Madrepora, Turbinaria, Pocillopora, 
numerous Astracidue and Fungidac) and the Aleyonarians (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), caleareous 
algae (Lithothamnium, 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—Tetracoralla, 
Hexacoralla, and Octocoralla. 
Sub-Class 1. TETRACORALLA. Haeckel.! 
(Zoantharia Iugosa, Milne-Edwards ; Pterocorallia, Frech.) 
Extinct, palaeozoic, simple, ov composite sclerodermic corals, with septa arranged 
according to a tetrameral system, and either bilaterally or radially symmetrical ; 
1 Literature (cf. also p. 67): 
Kunth, A., Beitriige zur Kenntniss fossiler Korallen. Zeitschrift der deutschen geologischen Gesell- 
schaft, Bd. XXI., 1869, and XXII., 1870. 
Dybowski, W. N., Monographie der Zoantharia Rugosa, ete. Archiv fiir Naturkunde Liv-, Est-, und 
Kurlands, Bd. V., 1874. 
Roemer, F., Lethaea Palaeozoica, 1883, pp. 324-416. 
Schliiter, Clem., Anthozoen des rheinischen Mittel-Devons. Abhandlungen der preussischen geolo- 
gischen Landes-Anstalt, Bd. VIIT., 1889. 
