268 EVENINGS AT THE MICROSCOPE 



der foot-stalk (fulcrum). But when you get a lateral view 

 the foot-stalk is seen to be only the edge of a thin plate, 

 to the upper edge of which are jointed the rami, in such 

 a manner that they can open and close, like the blades of a 

 pair of shears. Each ramus is a thick, three-sided piece, 

 with the upper side hollow, and the inner flat, and in con- 

 tact with that of its fellow, in a state of repose. The 

 uncus of each malleus falls into the concavity of its corre- 

 sponding ramus, and is fastened to it by a stout triangular 

 muscle, which allows some freedom of motion. 



Many muscles are inserted into various parts of these 

 organs, and into the walls of the mastax, which impart 

 various and complex motions to all the parts. Thus, as 

 we have seen, they are adapted to the various functions 

 of mouth-organs, those of grasping, holding, bruising, and 

 chewing food. 



The mallei correspond with the mandibles of Insects; 

 and the rami of the incus with the maxillae; while the 

 walls of the mastax with the two edges of its orifice corre- 

 spond with the mouth, with its labrum and labium. 



It is true we are somewhat startled to find a mouth 

 placed far down within the cavity of the breast; but there 

 are other forms in this class, some of which I may be able 

 to show you, where the mastax has essentially the same 

 structure, in which it is placed at the front margin of the 

 body, from which the jaws can be freely protruded. The 

 difiiculty will seem less if you weigh the following con- 

 siderations: 



The integument in the Rotifera is very flexible, and, 

 especially in the frontal regions, is extremely invertible. 

 In those" genera in which the mouth apparatus can be 

 brought into contact with the external water, it is ordina- 



