10 ESSENTIALS OF ZOOLOGY 



It is clear that while the endoplasm is very similar in 

 constitution and function to that of the Amoeba, the ecto- 

 plasm is a more definite, formed structure. It is a distinct 

 membrane and is relatively impermeable, and it has been 

 necessary therefore to provide an opening into it which acts 

 as a mouth or entrance for the food. The opening consists 



Contractile vacuole 



Macronucleus, 

 Micronucleui 



Pharynx. 

 Trichocys 



Contractile vacuole 



Peristome 



Mouth 



I Food vacuoles 



FIG. 4. Paramecium caudatum. The oblique lines indicate the 

 myonemes and also the rows of cilia associated with them. 

 (After Biitschli.) 



of a peristome, mouth, and pharynx, the last provided with 

 an undulating membrane. It must be understood that while 

 such popular terms are employed, they have no relation- 

 ship to the terms employed with reference to the Metazoa, 

 except that they refer to cell modifications which perform a 

 similar function. To avoid them it has been suggested to call 

 the mouth of such a modified cell the cytostome, and the 

 cavity which leads from it to the opening into the endoplasm 

 the cytopharynx. It has also been necessary to provide for 

 the discharge of undigested material, and this is done in the case 



