UNICELLULAR ORGANISMS 



61 



and forth. Ordinarily in life, they are directed somewhat back- 

 wards, and as a result of this position, when they beat back and 

 forth they cause the propulsion of the animal forward through 

 the water with a uniform motion. When the cilia are directed 

 forward, their beating back and forth will cause the animal to 

 move backward. At the same time with their back-and-forth 

 motion they beat slightly to one side, causing the animal to 

 rotate slowly on its long axis as it moves either forward or back- 

 ward. Exactly how these cilia are able to move is not known, 

 but a power of automatic vibration is always characteristic of 

 these organs. Lining the tube called the oesophagus, leading 

 from the mouth, there are special cilia, longer than the rest and 

 united to form a vibrating membrane known as membranella ; 

 Fig. 21 mb. The function of this mass of fused cilia is to guide 

 the food from the mouth down through the oesophagus into the 

 body cavity. The direction in which the cilia point, and con- 

 sequently thejlirection of the motion they produce, are affected 

 by a variety of external condi- 

 tions, for the Parametium, like 

 the Amoeba, is irritable and its 

 motions are regulated by the 

 surrounding conditions. 



Structure. The ectoplasm of 

 the Paramedum is somewhat 

 clearer than the endoplasm, 

 but it contains large numbers 

 of minute threadlike organs 

 known as trichocysts (Gr. trix 

 = hair + cystis = bag); Fig. 

 22 tr. These may be discharged 

 from the animal; and they ap- 

 pear to be organs of offense or 



defense, since they apparently contain a small quantity of poison 

 by which the animal may kill or paralyze its prey or its enemies. 

 On the very outside of the ectoplasm is an extremely thin mem- 



FlG. 22. A BIT OP THE OUTER 

 EDGE OF THE PARAMECIUM 

 (HIGHLY MAGNIFIED) 



(Modified from Maier.) 



cu, cuticle; /, food vacuole; 



ec, ectoplasm; tr, trichocyst. 

 en, endoplasm; 



