124 THE PLANTS 



which encloses a living body containing a number of 

 disc-like green structures (Fig. 40) . 



Blue - Green Algae or Cyanophycese. — A small 

 number of the members of this group are pelagic, 

 though most live in fresh water. They sometimes 

 occur in enormous numbers in tropical seas, forming 

 large floating masses. They are usually threadlike, 

 and arranged in groups or bundles. One form, Tricho- 

 desmium erythraeum (Fig. 42), is of a reddish colour, 

 and probably accounts for the special name of the 

 Red Sea, for periodically it occurs in enormous 

 quantities in those waters. 



Coccolithophoridse. — These forms, on the border 

 line between animals and plants, are very minute, 

 and are only met with occasionally in plankton caught 

 in the ordinary way, since their size allows them 

 to pass easily through the meshes of the finest 

 silk net. 



There are two chief forms, known in oceanic 

 waters — Coccolithophora (Coccosphaera) and Rhabdo- 

 sphaera. When fresh they contain one or two brown 

 or greenish plates ; frequently, however, they appear 

 as mere skeletons, consisting of a number of closely 

 arranged plates forming a spherical body. The plates 

 are perforated in the centre in Coccolithophora, and 

 have each a central projection in Rhabdosphaera 

 (Figs. 43, 44). The plates consist of calcium carbonate 

 (chalk), so that the organisms are lost if the material 

 is preserved in any acid fluid. 



As only the waters of the Mediterranean have been 

 investigated for these forms by methods other than 

 ordinary tow nets, it would be of great interest to 

 discover their distribution in oceanic waters generally. 



For the minute forms, with which a filter is used, 

 set the filter so that it never runs quite dry, but that 



