PELAGIC PLANT LIFE 



331 



always occupy the whole internal space, but lies sometimes, as 

 it were, at the bottom of a hollow hemisphere or up at the 

 mouth-opening in a conical sac. The shields of lime can be 

 dissolved by the weakest acids, and the cell then remains as 

 an insignificant mass with undefined boundaries. Still, these 

 shields are very characteristic, and have been found in such 

 enormous quantities in the deposits on the ocean-bottom that 

 they aroused the attention of scientists 

 long before the algae themselves were 

 known. The commonest forms {Cocco- 

 lithophora, Pontosphcera) have an almost 

 globular lime-covering, and are there- 

 fore without special suspension-organs, 

 though their surface is big in proportion 

 to their bulk, if we consider their extra- 

 ordinarily minute dimensions (5 to 20 \x 



if' 



Fig. zyj.—PvKOCYSTis noctilvca. (From Chun.) 



Fig. 238. 

 pvrocvstis fusiform is (^j"). 



( From ' ' Challenger " Narrative. ) 



in diameter). But in forms like Rhabdosphcsra the calcareous 

 shields have each a more or less large spike in the middle. In 

 Discosph(^ra we find trumpet-shaped spines, in Scyphosplicsra 

 barrel-shaped outgrowths, and during the "Michael Sars" Expedi- 

 tion I succeeded in discovering even stranger forms. Ophiaster 

 has a tuft of slightly spiral flexible calcareous filaments. 

 Michaelsarsia carries in the front of its cell a sort of parachute 

 or pappus of hollow jointed calcareous tubes arranged in a 



