ROTIFERS. 217 



are present the specimen is- a Rotifer, and can be noth- 

 ing else. By some observers these remarkable organs 

 are called the gizzard, which they are not. The best 

 word to apply to them in mastax. It is really the ani- 

 mal's true mouth, the passage leading to it from the 

 frontal region being only a part of the preoral tract. 



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 preoral aperture, it is passed 

 down to the mastax, where it is crushed and allowed 

 to go on to the stomach. In some Rotifers the part is 

 very complicated. In the simpler forms it consists of 

 two apparently semicircular plates surrounded by a 

 thick envelope of powerful muscle, the flattened sides 

 acting against each other and crushing the food be- 

 tween them. The surface of each plate usually bears 

 several transverse parallel ridges, to be seen with a 

 high power, each ridge projecting a short distance be- 

 yond the straight internal edge, and forming low 

 teeth. These ridges, when the mastax is closed, are 

 received in the depressions or furrows between those on 

 the opposite plate, thus making an effectual 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 Rotifers the entire mastax is protruded 

 through the preoral aperture, and bites, tears, and 

 nibbles at acceptable food-masses. 



If the observer 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 suc- 

 ceeded in isolating the organ by killing th^ Rotifer 



