LOWER INVERTEBRATES WHICH BREATHE IN WATER 419 



In spite of what has been said it would appear that Protozoa 

 also breathe to some extent by means of internal surfaces, for 

 water is taken in with each portion of food, and this mixture of 

 food and drink lies in a temporary hole (food vacuole) within 

 the living substance of the body, which no doubt takes up dis- 

 solved oxygen from the watery part of it. The body also contains 

 one or more clear fluid-filled spaces (pulsating vacuoles) which 

 alternately increase and diminish in size, and have been proved 

 in some cases to communicate with the exterior. It is usually 

 held that this is a sort of pumping arrangement, of respiratory 

 nature, whereby pure water containing plenty of dissolved oxygen 

 is taken into the body, and impure water containing waste pro- 

 ducts ejected to the exterior. 



