38 EVENINGS AT THE MICROSCOPE. 



circumstance is, that while in general the ciliary wave runs 

 in the same direction in the different ovals, there will he 

 one here and there in which the course is reversed ; and I 

 think that the animal has the power of choosing the direc- 

 tion 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, every- 

 where 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 to the 

 bottom of the sac, and are poured into the stomach, where 

 they are digested ; the remains, together with the waste 

 water, being discharged through the lateral orifice. 



Thus we see how closely connected are the three great 

 processes of circulation, respiration, and digestion. 



