iv PHYLUM OELENTERATA 123 



It is by this last-named method, the ccenosarc attaining 

 great dimensions, and the individual corallites being small 

 and very numerous, that the most complex of all corals, 

 the Madrepores (Fig. 63, B), are produced. 



The Actinozoa are remarkable for the variety and brill- 

 iancy of their colour during life. Every one must have 

 noticed the vivid and varied tints of sea- anemones ; but in 



Fig. 62. — Astraea pallida, the living colony. Natural size. Fiji Islands. (After 



Dana.) 



life the corals also exhibit a marvellously varied and gor- 

 geous colouring ; and the same holds good of many of the 

 Alcyonaria. 



Many Actinozoa, like many sponges (p. 89), 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. 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 



