Chap. II] 



MARINE VEGETATION 



803 



phosphorescent Algae, apparently allied with the Flagellatae, are common in warm 

 seas. Bacteria are very abundant in hemiplankton near the coasts, and some of 

 their species cause diffuse phosphorescence of the sea. But they seem to be very 

 rare in the open sea. 



Denizens of the plankton must be able to spend their existence either 

 wholly or for the most part in a free-swimming condition. Associated 



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Fig. 492. Antelminellia ijigas, (Castr.") 

 Schiitt (Diatomaceae). Magnified 26. 

 After Schiitt. 



Fig. 493. Planktomella Sol, (Wall.) 

 Schiitt (Diatomaceae). Magnified 190. 



After Schiitt. 





Fig. 494. Gossleriella tropica, Schiitt. Magnified 

 1 =;o. After Schiitt. 



Fig. 495. Ornithocercus splendens, Schiitt 

 (Peridinieae). Ventral side. Magnified 150. 



with this need are found various adaptations, which are usually much more 

 complete in the forms of true plankton than in the hemiplankton, whose 

 constituents consequently sink to the bottom at certain times, in particular at 

 the reproductive period. These adaptations have developed in two direc- 

 tions, (a) to decrease the specific gravity, and (b) to increase the surface 



3 F 2 



