114 THE SEAS 



extremely delicate and easily destroyed by the net ; others, 

 however, possess wonderfully designed skeletons made up 

 of little plates which may carry spines and wings, giving the 

 plants a beautiful appearance (Plate 40). Some of these 

 peridinians are the cause of the coloured water mentioned 

 above. The Goniaulax figured in Plate 40 has been known 

 to occur in such profusion as to colour the water red, and 

 another species has been reported to be so thick that when 

 they died and decayed they caused the death of great 

 numbers of fish. 



There is yet another group of plankton plants known as 

 Coccospheres. These also are unicellular but are charac- 

 terized by the presence of numerous calcareous plates 

 embedded in the cell, the different shapes of which serve 

 as a means of identification of the numerous species. 



These three groups, the Diatoms, the Peridinians, and 

 the Coccospheres, are the most important constituents of the 

 drifting plant life. The Diatoms are most abundant in the 

 colder waters of the temperate and polar seas, while the two 

 latter are characteristic of the warm waters of tropical and 

 sub-tropical regions. 



These plants are at times extremely abundant ; they 

 have often been reported to be so thick in the Baltic that 

 a thimbleful of water would contain more than a thousand 

 individuals. Were it not for these countless myriads of 

 small plants we can safely say that the great oceans and 

 seas of the world would be valueless to us as reservoirs from 

 which to draw much of our food in the form of fish and 

 other edible marine animals, for the small plants of the 

 plankton are indeed the pasturage of the sea. They form 

 the food of millions of small animals living in the drifting 

 community on which larger animals prey. On land, man 

 and beast alike are ultimately dependent on the grass and 

 herbs of the field for food, the animals which feed on the 



