WHEEL-BEARERS. 



235 



SIOUTH OF BKACHIONUS. 



and the rami of the incus with the maxilla? ; while the 

 walls of the mastax with the two edges of its orifice 

 correspond 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 es- 

 sentially the same structure, in 

 which it is placed at the front 

 margin of the body, from which 

 the jaws can be freely protruded. 



The difficulty will seem less if you weigh the following 

 considerations : — 



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 ordi- 

 narily, to a greater or less degree, retracted within the 

 body, by the inversion of the surrounding parts of the 

 exterior ; while, in those genera in which it is per- 

 manently inclosed, analogy requires us to consider the 

 condition as induced by a similar inversion, but of per- 

 manent duration. If we imagine the head of a soft- 

 bodied Insect-larva retracted to a great degree (as is done 

 partially by many Dipterous larva?), the skin of the 

 thoracic segments would meet together in front, around 

 a purse-like opening, which would be the orifice of such 

 a buccal funnel as exists in most Rotifera. In the latter, 

 it is the normal, or proper condition ; in the former, it 

 is merely accidental and temporary. 



We need not devote any more minute consideration to 

 the digestive apparatus in our little Brachion, but there 



