UNINCELLULAR ORGANISMS 



487 



a little over three days, the excess serving as food for 

 animals or settling to the bottom. Whether these esti- 

 mates are of general application is unknown. It is cer- 

 tain, however, that the number of individuals per unit 

 volume of sea water varies according to seasonal and 

 other conditions. 



Unicellular Organisms and Bottom Deposits. — 

 Several thousand species of one-celled animals occur in 

 the plankton of all the oceans of the globe. The larger 

 number of these are related to the amoeba, but differ in 

 the presence of a supporting skeleton (Fig. 140). Those 



Fig. 140. — Microscopic One-Celled Marine Animals (Globirgerina) 

 with chambered cells, largely responsible for chalk deposits. 



with flinty skeletons, the radiolarians, comprise at least 

 5,000 species, living for the most part in deep tropical 

 waters of the Pacific ocean. Others with skeletons of 

 lime (the foraminifera) are likewise tropical forms of 

 wider distribution. The numbers of both of these groups 

 become incredibly great under favorable conditions, as 

 many as 100,000 individuals occurring in every liter of 

 water at or near the surface throughout thousands of 

 square miles. 



The bodies of these animals are heavier than water, 

 and while very many are buoyed up by oil drops and 



