THE WHEEL ANIMALCULES (ROTATORIA) 561 



or a bunch of long cilia. The membrane or the bunch of cilia is 

 always in rapid movement, giving the appearance of a minute 

 flame, so that these structures are called flame cells. The cilia or 

 membrane doubtless serve to propel a current through the tubes. 

 In many rotifers a transverse tube in the head region unites the 



FIG. 861. Excretory organs. A, Ljcluularia socialis Ehr., showing the thin-walled tube a, the thick- 

 walled tube b, the transverse connecting tube c, and the flame cells fc. Modified from a figure by Hlava. 

 B, Excretory tubules of right side in Floscularia campanulata Dobie. cv, contractile vesicle; fc, flame 

 cells. (After Montgomery.) 



thin- walled tubes of right and left sides. Often all the tubes are 

 convoluted in their course. 



There is reason to believe that the walls of the tubes absorb the 

 nitrogenous waste matter from the fluid of the body cavity. This 

 waste matter passes backward, driven by the flame cells, to the region 

 of the cloaca (Figs. 856, 857, c). Here is found in most rotifers a 

 small sac into which the tubes from both sides enter. This sac opens 

 along with the intestine into a small cavity known as the cloaca. 

 The sac, or contractile vesicle (cz>), as it is called, contracts at 

 intervals, expelling to the outside the fluid with which the tubes 



