Plan don 



297 



all the animals of both the plancton and the necton, 

 depends. 



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

 and ehlorophyl-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 numerous 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: 



1. 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 flota- 

 tion as those of Stephanodiscus (fig. 35 1) 

 and Rhizosolenia (fig. 1 80) ; or we may 

 get such predominantly shore ward 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 : 



1 . Filamentous forms having : 



(a) Stiff, smoothly-contoured fila- 

 ments; Oscillatoria (see fig. 34 

 on p. 109) and Lyngbya, etc. 



(b) Sinuous nodose filaments, Ana- 

 baena (fig. 179), Aphanizomenon, 

 etc. 



Fig. 180. 



a, Rhizosolenia; 



b, Attheya. 



