CORAL 



471 



> lenterates the Actinozoa that the format ion 

 <>r uriil strut-turn becomes emphitflMd and per- 

 I'i'.-i.-d. lii many simple and in most colonial 

 I'm m-, .-i -k'-l'-i"ii i- I'piined, as isolated spiculea or 

 <-iintinuuu-. When continuous it may t>e horny, 

 H limy \\iili an organic l>.-i-K or to all appear- 

 in. -. .-iiiiii-lv limy, (a) Among the Alcyonaria, 

 ii \\liii-li Alrvonium ((j.v.) is a convenient type, 

 Tuliipora, with tin- sm'cules united to form tubes 

 anl crossing platform** ; Corallium, with a branched 

 ;i\is of fused spiculea ; Isis, with a jointed axis 

 alternately limy and horny ; Heliopora, with limy 

 cups iBolonngtne individual animals, and traversed 

 .-is they grow by successive horizontal doors at 

 >li.u r lit intervals, are examples of different forms of 

 <Miial occurring in distinct groups. (6) Among the 

 /oiintharia, of which Actinia or any sea-anemone 

 is a familiar example, the Actiniaria have no 

 skeleton, and therefore include no coral forma ; the 

 Antiputharia have a varied non -calcareous sup- 

 porting skeleton, and include some forms known 

 as ' black corals ; ' the third subdivision, the Madre- 

 poraria, always have a continuous liniv skeleton, 

 and include the best-developed corals. Most madre- 

 pores are colonial, and the common connecting 

 skeleton is either solid (apart from mere gaps) or 

 perforated by canals which establish community 

 of life between the individual memtars. Hence it 

 is usual to distinguish two sections of Madrepores 

 as Aporosa and Perforata. (3) The earliest corals 

 of Palaeozoic strata the Rugosa or Tetracoralla 

 are distinguished .by four primary partitions. 



General Structure. In discussing further the 

 general structure, it will be convenient to leave the 

 widely separated Hydrocorallina out of account. 

 The most general fact that can be stated at the 

 outset is, that with the possible exception of the 

 sea-pens, the cells which form the skeleton are at 

 least originally epidermic. Free spicules, con- 

 nected lattice works, solid cups, illustrate various 

 degrees of lime deposition ; while supporting axes, 

 nsheathing tubes, and what look like tolerably 



complete casts of 

 the entire animal , 

 are the main 

 types of coral 

 architecture. In 

 detail, however, 

 those main types 

 vary widely, and 

 in the cup-corals 

 especially, the 

 structure is often 

 very complex. In 

 a simple madre- 

 pore coral, repre- 

 senting a single 

 individual, tlie 

 following struc- 

 tures are at once 

 obvious. The 

 main cup -like 

 mass, known as 

 the theca, is usually attached by its base to 

 some foreign object. A terminal depression 

 the calyx contains the animal, the soft parts of 

 which extend more or less over the lips. Within 

 the cup, when the animal has died away, radial 

 partitions are seen, forming various systems of septa. 

 Some of these often meet in the middle of the cup 

 to form a central pillar or rolnnn-lln. When the 

 coral is colonial, as is generally the case, complica- 

 tions arise by the fusion of individuals. The deeper 

 parts of growing corals are sometimes separated off 

 Dy cross partitions ( tabul(e), but this is rare in the 

 .Zoantharia. In large masses of madrepore certain 

 portions of the skeleton are often left behind by 

 the animals, and being exposed to the water, 



t'ig. "2.CaryophyUia borealit, single 

 individual. 



become corroded and altered. In regard to tlie 

 development of corah), it in hardly powuble yet to 

 make any useful general statements. For the 

 animalH themselves, it must lie enough here t 

 refer to the articles on AbCYONIUM and ANEMONE, 

 which, though not themselves 'coral*,' are good 

 examples of the two sub-clauses to which mod 

 corals belong. 



General Life. Corals are predominantly passive 

 forms, and are much modified by their environ- 

 ment (currents, &c.); the great majority are 

 permanently fixed, except in embryonic life. 

 Flabellum may be noted an an instance of a coral - 

 cup which becomes free as an adult. The limy 

 material which forms the skeleton seems, in many 

 cases at anyrate, to be derived not from the water, 

 but from the minute animals which constitute the 

 food. The great majority are colonial, yet cases of 

 division of labour are very rare. Many corals are 

 richly coloured, but the meaning of the numerous 

 pigments is still imperfectly known. Masses of 

 corals furnish browsing ground for crowds of 

 animals, not a few of which intrude into the 

 colonies, while others have become established 

 parasites. Some corals also include parasitic 

 Algte (see SYMBIOSIS). Semper describes some 

 very interesting cases of the constant association of 

 worms and corals, and notes how the guest has per- 

 manently modified the host (see CoMMENSALlSM). 

 It is interesting to notice, apart from actual modi- 

 fication of form, how various worm-organisms act 

 upon the ' sea-meadows of coral ' much as earth- 

 worms do upon land-meadows (see Huxley's Inverte- 

 brates, p. 171 ). 



In most corals the sexes are separate, and even 

 the colonies may be entirely male or female. In 

 Corallium, hermaphrodites may occur, or different 

 branches be of either sex. As is common in sessile 

 passive organisms, asexual or vegetative reproduc- 

 tion is a marked feature. Semper considers the 

 budding of the Fungia colony as illustrating 

 ' alternation of generations. ' 



Distribution. Of the madrepore corals, the 

 ' solitary ' forms, which never form great masses, 

 are for the most part widely distributed deep-sea 

 animals, while the reef -builders are as markedly 

 dwellers in shallow water, from the low-tide mark 

 to about 20 fathoms, and are limited to warm 

 waters. They are not found on the western coasts 

 of Africa and America. In the waters of the 

 Central Pacific reef-corals are found in greatest 

 profusion and variety ; but those of the Red Sea, 

 Indian Ocean, and fiast Indies are not far behind. 

 Over eighty fossil genera of the Paheozoic Rugosa 

 are knowji, and of the other Actinozoan corals, 

 about 1800 fossil, and as many living species. Of 

 these, the vast majority are madrepores, and only 

 a small number Alcyonarian. Both sets begin in 

 Silurian times, but were for long outnumbered by 

 the Rugosa. Most existing genera date only from 

 the Tertiary period. The 'areas of past distribu- 

 tion of coral-reefs in no way correspond with those 

 distinctive of the modern seas.' See Heilprin, Dis- 

 tribution of A nimals ( 1 887 ). 



Important Forms. The most important corals 

 are the reef-builders (Porites, &c.). The delicate 

 colour, exquisite architecture, and labyrinthine 

 complexity make the limy skeletons oeautiful 

 objects. Yet more beautiful, however, is the 

 wealth of colour often exhibited by the living 

 forms. Caryophyllia is a simple and solitary 

 cup-coral found on the south coast of England. 

 Bathyactis and Deltocyathus are almost world- 

 wide. Antipathes the black coral has a valu- 

 able hard axis like ebony. Fungia is worth noting 

 on account of its flat mushroom-like shape and 

 Semper's observation (see above) in regard to 

 alternation of generations ; among Alcyonarian 



