204 MICROSCOPY FOR BEGINNERS. 



the gizzard, which they are not. The best word to ap- 

 ply to them is mastax. 



The mastax is the most hard-working part of the 

 creature's anatomy, except, perhaps, the cilia. "When 

 the currents produced by the latter bring an acceptable 

 morsel of food to the mouth, it is passed down to the 

 mastax, where it is crushed and allowed to go on to the 

 stomach. In some Rotifers this part is very compli- 

 cated. In the simpler forms it consists of two appar- 

 ently semicircular plates surrounded by a thick en- 

 velope of powerful muscle, the flattened sides acting 

 against each other and crushing the food between them. 

 The surface of each plate very often bears several trans- 

 verse parallel ridges, to be seen with a high power, each 

 ridge projecting a short distance beyond the straight in- 

 ternal edge, to form short teeth. These ridges, when 

 the mastax is closed, are received in the depressions be- 

 tween those on the opposite plate, thus making an ef- 

 fectual crushing instrument. In other forms the mastax 

 consists of three parts, one being immovable, and used 

 as an anvil on which the other two pound the food as it 

 passes by. In the nibbling Eotifers the entire mastax 

 is protruded through the mouth, and bites, tears, and 

 nibbles at acceptable food masses. 



If the beginner finds it difficult to make out the 

 form and structure of the mastax, as he probably will 

 when it is examined in action within the body, he 

 may succeed by killing the Rotifer with a strong so- 

 lution of caustic potassa allowed to run under the cover- 



