112 Aquatic Organisms 



The slender cells of the" needle diatoms," Syyiedra (g), 

 are common in nearly all waters and at all seasons. 

 They are perhaps most conspicuously abundant when 

 found, as often happens, covering the branches of some 

 tufted algae, such as Cladophora, in loose tufts and 

 fascicles, all attached by one end. 



Gyrosigma (h) is nearly allied to Navicula but is 

 easily recognized by the gracefully curved outlines of its 

 more or less S-shaped shell. The sculpturing of this 

 shell (not shown in the figure) is so fine it has long been a 

 classic test-object for the resolving power of microscopic 

 lenses. Gyrosigma is a littoral associate of Navicula, 

 but of much less frequent occurrence. 



Encyonema (i) is noteworthy for its habit of develop- 

 ing in long unbranched gelatinous tubes. Sometimes 

 these tubes trail from stones on the bottom in swift 

 streams. Sometimes they radiate like delicate filmy 

 hairs from the surfaces of submerged twigs in still 

 water. The tubes of midge larvae shown in figure oo 

 were encircled by long hyaline fringes of Encyonema 

 filaments, which constituted the chief forage of the 

 larvae in the tubes and which were regrown rapidly 

 after successive grazings. When old, the cells escape 

 from the gelatine and are found singly. 



The group of diatoms having circular shells with 

 radially arranged sculpturing upon the valves is repre- 

 sented by Melosira (j and k) and Stephanodiscus (l) of 

 our figure. Melosira forms cylindric filaments, whose 

 constituent cells are more sohdly coherent than in 

 other diatoms. Transverse division of the cells in- 

 creases the length of the filaments, but they break with 

 the movement of the water into short lengths of usually 

 about half a dozen cells. They are common in the 

 open water of lakes and streams, and are most abundant 

 at the higher temperatures of midstunmer. Cyclotella 

 is a similar form that does not, as a rule, form filaments. 



