CORAL ALGAE 209 



from that of the other segmented coral Algae, being much 

 more profuse, not confined to one plane, and very irregular. 

 It is in the structure of the plant, however, as seen with a 

 lens, that Galaxaura differs from the other Algae that have 

 been described most conspicuously. When it is fresh or 

 preserved in spirit the branches show a smooth surface 

 without pores or markings of any kind, but when felt with a 

 needle or probe are found to be soft and yielding. When 

 dried the calcareous framework seems to collapse, leaving 



Fig. 107. — Galaxaura. Nat. size. 



only flattened shrivelled strands of granular chalky sub- 

 stance cemented together by the dried vegetable tissues. 



Class Chlorophyceae 



This large and heterogeneous group of the green sea- 

 weeds includes a few genera in which the thallus is strengthened 

 by the deposit of calcareous matter, and one of these — the 

 genus Halimeda — is so widely distributed in the tropical 

 seas and so abundant in many localities, that it must be 

 regarded as an important constituent of the coral reef flora. 



Halimeda.^ — This plant consists of a short stem which 



1 For a full account of this important genus see E. S. Barton, Siboga- 

 Expcditie, livr. 2, igoi. 



P 



