26 LIFE IN THE SEA [CH. 



they are present in extraordinary numbers. It is 

 quite probable that if we could suddenly dry up, 

 say, one square mile of sea, we should find that the 

 benthic and nektic animals and plants were really 

 less in bulk than were the invisible organisms of the 

 plankton. Now two great sub-kingdoms of life the 

 Algae among the plants, and the Protozoa among 

 the animals form the major part of the permanent 

 plankton. The Algae are represented principally by 

 the Diatoms, minute unicellular plants, possessing 

 an outer skeleton of silica. Generally speaking the 

 Diatoms constitute the bulk of the plankton during 

 the spring, and they are nearly always present at any 

 time of the year. Many sub-groups of Protozoa are 

 always present, among them we may mention the 

 Flagellates, Foraminifera and Radiolaria. These 

 latter groups, with the Diatoms, are responsible for 

 the formation of the abyssal sea-bottom deposits. 

 Among the Flagellates one group the Peridinians 

 is notable. It contains unicellular organisms which 

 we may regard either as animals or plants, for they 

 have the structure of the animal unicellular organism, 

 but they have the same method of nutrition as the 

 unicellular plant. They are creatures of fantastic 

 shapes, with outer skeletons of a substance resembling 

 excessively thin paper impregnated with silica ; and 

 possessing chlorophyll, the colouring matter of the 

 green plants. 



