PARAMECIUM: NUCLEI 



49 



radiating canals, whose wide ends lie under it (Fig. 23). These are 

 the formative vacuoles. 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. Over each con- 

 tractile vacuole there is a 

 minute gap in the pellicle, 

 through which the contents 

 of the vacuole are discharged. K ^ /^ M 



NUTRITION 



O 



>o< 



Successive stages of the contractile 

 vacuole of Paraynecium. 



The food consists of 

 bacteria and other minute 

 organisms. These are drawn 

 towards the mouth by the 

 current set up by the cilia Fig. 23 

 of the peristome and driven 

 down the gullet by the 



working of the cilia. Particles appear to be forcibly sucked in by the 

 mouth. A drop of water containing the food particles is now pinched 

 off by a contraction of the endoplasm and becomes a food vacuole, 

 which is carried by a streaming of the endoplasm around the 

 body, passing first backward along the ventral side, then forward 

 nearly to the middle of the body, then through several turns of 



a short circuit in this region of the 

 body, and finally forward to the 

 front end and back so as to com- 

 plete the circuit of the body 

 (Fig. 24). During these wanderings 

 the food is digested. The un- 

 digested remains are then expelled 

 at a spot just behind the end of 

 the gullet, where a passage through the ectoplasm, known 

 as the temporary anus, is formed when it is required. As in 

 Amceha, digestion takes place in an acid vacuole and protein is 

 the chief food. The formation of the vacuoles and their acidity 

 are easily demonstrated if a suspension of i g. of yeast and 10 



«. c. 



Fig. 24. — A diagram of the course 

 of the circulation of the food 

 vacuoles in Paramecium. 



I.e., Long circuit ; s.c, short circuit. 



