210 



CORALS 



Fig. io8. — Halimeda opuntia 

 from Hulule Male, Indian Ocean. 

 Nat. size. 



gives rise to a number of branches usually arranged in one 

 plane, and is attached to the sand in which it grows by a 

 mass of long branched iilaments. 



The stem and branches are composed of a series of 



calcareous internodes with un- 

 calciiied nodes, and are conse- 

 quently very flexible (Fig. io8). 

 The joints are frequently flat- 

 tened and may be round, circular, 

 kidney - shaped, triangular, or 

 cylindrical in form. The indi- 

 vidual plants vary a great deal 

 in size, but the majority of the 

 common varieties are not more 

 than a few inches in height. 



In life these Algae are grass- 

 green in colour, but when dead 

 become white and break up into 

 coral-like beads or flakes. 

 In anatomical structure Halimeda is quite different from 

 the Lithothamnion group of corals and their allies, and the 

 principal differences can be easily recognised both in the 

 dried calcareous skeletal structures 

 and in the soft tissues, which can be 

 seen, with the help of a simple magnify- 

 ing glass, when the calcium carbonate 

 is dissolved away with acid. 



If a dried internode be examined 

 it will be found to be rough to the 

 touch, not smooth and greasy like a 

 Lithothamnion, and it is so brittle that 

 it can be crushed between the finger 

 and thumb. With a lens the surface 

 is seen to be perforated by a number 

 of round pores about 0-014 mm. in diameter, regularly 

 arranged at equal distances apart (Fig. 109). In this 

 respect, therefore, although Halimeda is undoubtedly a 

 plant, it is not a Nullipore. When seen in section these 

 pores are found to be the mouths of short cylindrical 



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I'iG. io(). — Surface view 

 of Halimeda opuntia when 

 dried, x 150 diains. 



