WHEEL- BE ARERS. 



and the rami of the incus with the maxillae ; 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 Rotifer a 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 permanently 

 inclosed, analogy requires us to consider the condition as 

 induced by a similar inversion, but of permanent duration. 

 If we imagine the head of a soft-bodied Insect-larva re- 

 tracted 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 



