i6o 



Aquatic Organisms 



bubbles of gas within their 

 shells whereby they are caused 

 to float. Thus they are often 

 taken in the plancton net from 

 open water of the ponds and 

 streams. 



Other protozoans that have 

 the body more or less cov- 

 ered with vibratile cilia (Cil- 

 iata), are very common in 

 freshwater, especially in ponds 

 ^ A 11 . , , and pools. Best known of 



Fig. 70, Arcella dentata. -• • t-> • /r^ 



Through the central opening thcSC IS FarameCtUm, {tig. 



there is seen a diatom, re- yj^) another famihar biolog- 



cently swallowed. : /^ . , //, ,, ,1 , 



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 all'ed genera are parasitic, 

 and live within the bodies of the 

 higher animals. .S'/^/^/c^r is (asthe 

 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. Stent or and Paramecitim, 

 tho unicellular, are quite large 

 enough to be seen (as moving 

 specks) with the unaided eye. 



Fig. 71. Ciliate pro- 

 tozoans. 



A, ParamcBcium; n, nu- 

 cleus; V, V, vacuoJes; /, 

 food-ball at the bottom of 

 the rudimentary esopha- 

 gus; C, Stentor; I, lorica. 



