i6o 



Aquatic Organisms 



FIG. 70. Arcella dentata. 

 Through the central opening 

 there is seen a diatom, re- 

 cently swallowed. 



bubbles of gas within their 

 shells whereby they are caused 

 ^^^^^ to float. Thus they are often 



j^ taken in the plancton net from 



|O mf open water of the ponds and 



streams. 



H^ Other protozoans that have 



the body more or less cov- 

 ered with vibratile cilia (Cil- 

 iata), are very common in 

 freshwater, especially in ponds 

 and pools. Best known of 

 these is Paramecium, (fig. 

 7 1 a) another familiar biolog- 

 ical-laboratory "type" that 



grows abundantly in plant infusions. It is found in 



stagnant pools, swimming near the surface. There 



are many species of Paramecium. Some of them and 



some members of allied genera are characteristic of 



polluted waters. Other allied genera are parasitic, 



and live within the bodies of the 



higher animals. Stentor is (as the 



name signifies) a more or less 



trumpet-shaped ciliate protozoan, 



that may detach itself and swim 



freely about, but that is ordi- 

 narily attached by its slender 



base to some support. Its base 



is in some species surrounded by 



a soft gelatinous transparent 



lorica, as shown in the figure. 



Some species are of a greenish 



color. Stentor and Paramecium, 



tho unicellular, are quite large 



enough to be seen (as moving 



specks) with the unaided eye. 



FIG. 71. Ciliate pro- 

 tozoans. 



A, Paramoscium; n, nu- 

 cleus; v, v, vacuoles; /, 

 food-ball at the bottom of 

 the rudimentary esopha- 

 gus; C, Stentor; I, lorica. 



