1 32 



manual of elementary zoology 



Excretion. 



There are two contractile vacuoles, which lie in the cortex 

 of the dorsal side, one towards each end. At 

 its full size each is a large spherical space 

 surrounded by from six to ten pear-shaped radiating canals, 

 whose wide ends lie under it. These are the formative 

 vacuoles. Contraction or " systole " affects only the central 

 vacuole. After it has taken place, the formative vacuoles 

 flow together at their inner ends and thus form the 

 beginning of a new contractile vacuole, round which new 

 canals appear, starting as mere slits and swelling to a 



pear shape by the 

 enlargement of 

 their inner ends. 

 It is stated that 

 the supposed ex- 

 cretory granules 

 of the endoplasm 

 collect near the 

 formative vacuoles 

 and are gradually 

 dissolved. Over 

 each contractile 

 vacuole there ap- 

 pears to be a 

 soft spot in the 

 pellicle, through 

 which the con- 

 tents of the 

 vacuole are dis- 

 charged. 

 The food consists of bacteria and other minute organ- 

 Nutrition. isms - These are drawn towards the mouth 

 by the current set up by the cilia of the 

 peristome and driven down the gullet by the working 

 of the undulating membrane, which has a waving motion. 

 The pressure of the water driven into the gullet with the 

 food particles causes the naked endoplasm at the bottom 

 of the gullet to bulge inwards, and into the space thus 

 formed the food is forced. A drop of water containing 

 the food particles is now pinched off by a contraction 

 of the endoplasm and becomes a food vacuole, which 



O 



•O 



Fig. 8i. 



-Successive stages of the contractile 

 vacuole of Paramecium. 



