iv PHYLUM CCELENTERATA 195 



the corallites and of the connecting coenenchyma, 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 coenosarc, and placing the polypes of the 

 colony in communication. 



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

 corallum having 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. 

 Tabuke 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 life. Every one must 1 lave 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 

 Alcyonida? are remarkable for their elaborate patterns and gor- 

 geous colouration; 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 Algas have 

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

 purpose as the similar structures which we have already studied 

 in Radiolaria (p. 61). 



Many Actinozoa, like many sponges (p. 116), 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 the Sea-anemone Adamsia 

 palliata (Fig. 144). This species is always found on a univalve shell 

 such as that of a Whelk inhabited by a Hermit-crab. The 

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

 in this way secures a more varied and abundant food-supply than 

 would fall to its lot if it remained in one place. On the other 

 hand, the Hermit-crab is protected from the attack of predaceous 

 Fishes by retreating into its shell and leaving exposed the Se;i- 

 anemone, which, owing to its toughness, and to the pain caused 

 by its poisonous stinging-capsules, is usually avoided as an article 

 of food. 



Other Sea-anemones^such as the gigantic Discosoma of the 

 great Barrier Reef are found associated with Small Fishes or 



