iv PHYLUM CCELENTERATA 205 



as Dendrophyllia (Fig. 157, A), in which a common calcareous stem, 

 the coemnchyma, is formed by calcification of the coenosarc (cs.), and 

 gives origin to the individual corallites. It is by this last-named 

 method, the coenosarc attaining great dimensions and the indivi- 

 dual corallites being small and very numerous, that the most 

 complex of all Corals, the Madrepores (Madrepora, Fig. 157, B) 

 are produced. 



The microscopic structure of corals presents two main varieties. 

 In what are called the aporose or poreless corals, such as Flabellum, 

 Astraea, &c., the various parts of the corallite are solid and stony, 

 while in the perforate forms, such as Madrepora, all parts both of 

 the corallites and of the connecting ccenenchyma have the charac- 

 ters of a mesh-work, consisting of delicate strands of carbonate of 

 lime united with one another in such a way as to leave interstices, 

 which in the living state are traversed by a network of interlacing 

 tubes, representing the ccenosarc, and placing the polypes of the 

 colony in communication. 



The Blue Coral (Heliopora), one of the Alcyonacea, has a massive 

 corallum of the same general appearance as a Madreporarian. The 

 lobed surface bears apertures of two sizes, the larger being for the 

 exit of the ordinary polypes, the smaller for the siphonozooids. 

 Tabulae are present, and septum-like ridges, which, however, have 

 no definite relations to the mesenteries and are inconstant in number. 



Colour. The Actinozoa are remarkable for the variety and 

 brilliancy of their colour during lif e. Everyone must have noticed 

 the vivid and varied tints of Sea-anemones ; but most dwellers in 

 temperate regions get into the habit of thinking of Corals as white, 

 and have no conception of their marvellously varied and gorgeous 

 colouring during life. The Madrepores, for instance, may be pink, 

 yellow, green, brown, or purple ; Tubipora has green polypes, con- 

 trasting strongly with its crimson skeleton ; and the effect of the 

 bright red axis of Corallium is greatly heightened by its pure white 

 polypes. In Heliopora the whole coral is bright blue ; the tropical 

 Alcyonidae are remarkable for their elaborate patterns and gor- 

 geous coloration ; and Pennatula, in addition to its vivid colours, 

 is phosphorescent. 



In most cases the significance of these colours is quite unknown. 

 In some species, however, " yellow cells " or symbiotic Algae have 

 been found in the endoderm, where they probably serve the same 

 purpose as the similar structures which we have already studied 

 in Kadiolaria (p. 65). 



Many Actinozoa, like many sponges (p. 127), furnish examples of 

 commensalism, a term used for a mutually beneficial association 

 of two organisms of a less intimate nature than occurs in symbiosis. 

 An interesting example is furnished by various Sea-anemones 

 (Fig. 158) which live on univalve shells inhabited by Hermit-crabs. 

 The Sea-anemone is carried from place to place by the Hermit-crab, 



