Plancton 



297 



all the animals of both the plancton and the necton, 

 depends. 



These are diatoms, blue-green and true-green algae, 

 and chlorophyl-bearing flagellates. Concerning the 

 limnetic habits of the last named group, we have spoken 

 briefly in Chapter IV (pp. 102-108). Being equipped 

 with flagella, they are nearly all free-swimming. 

 Most important among them are Ceratium, Dinobryon 

 and Peridinium. 



Most numerovis in individuals of all the plancton '- 

 algae, and most constant in their occurrence throughout 

 the year, are the diatoms (see fig. 35 on p. 1 1 1). Wher- 

 ever and whenever we haul a plancton net in the open 

 waters of river, lake or pond, we are pretty sure to get 

 diatoms in the following forms of aggregation: 



I. Flat ribbons composed of the thin cells of Dia- 

 toma, Fragillaria, and Tabelaria. 



2. Cylindric filaments composed of 

 the drum-shaped bodies of Melosira and 

 Cyclotella. 



3. Radiating colonies of Asterionella. 



4. Slender single cells of Synedra. 

 And we may get less common forms 



showing such diverse structures for fiota- 

 tion as those of Stephanodiscus (fig. 35 -'") 

 and Rhizosolenia (fig. 180); or we may 

 get such predominantly shoreward forms 

 as Navicula and Meridion. 



The blue-green algae of the plancton 

 are very numerous and diverse, but the 

 more common limnetic forms are these : 

 I . Filamentous forms having : 



(fl) Stiff, smoothly-contoured fila- 

 ments; Oscillatoria (see fig. 34 

 on p. 109) and Lyngbya, etc. 

 (&) Sinuous nodose filaments, Ana- 

 baena (fig. 179), Aphanizomenon, 

 etc. 



Fig. 180. 



a, Rhizosolenia; 



b, Attheya. 



