52 EVENINGS AT THE MICROSCOPE 



where a slight alteration of position prevents the waves 

 from taking the tooth -like appearance. Sometimes one 

 here and there of the ovals cease to play, while the rest 

 continue ; and, now and then, the whole are suddenly 

 arrested simultaneously as if by magic, and presently all 

 start together again, which has a most charming effect. 

 A still more singular circumstance is, that while in general 

 the ciliary wave runs in the same direction in the different 

 ovals, there will be one here and there in which the course 

 is reversed; and I think that the animal has the power of 

 choosing the direction of the waves, of setting them going 

 and of stopping them, individually as well as collectively. 



The object of these ciliary wheels is to keep up a con- 

 stant current in the water. This fluid, as I have said, 

 enters from without, through the upper orifice of the body, 

 and is hurled over the whole surface of the breathing-sac 

 by means of the ciliary waves, parting with its oxygen, as 

 it goes, to the blood, which streams, as we saw, everywhere 

 between the rows of wheels. But the water has another 

 function: it carries particles of organic matter with it, 

 which are suitable for the nourishment of the creature; 

 these atoms are carried by the currents with the effete 

 water to the bottom of the sac, and are poured into the 

 stomach, where they are digested; the innutritive remains, 

 together with the waste water, being discharged through 

 the lateral orifice. 



Thus we see how closely connected are the three car- 

 dinal processes of circulation, respiration, and digestion. 



