WHEEL-BEAEEES. 271 



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 Motifera is very flexible 

 and, especially in the frontal regions, is extremely in- 

 vertible. In those genera in which the mouth appara- 

 tus can be brought into contact with the external water, 

 it is ordinarily, 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 retracted to a great degree (as is 

 done partially by many Dipterous larvae), 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 naost Motifera. In the 

 latter, it is the normal 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 are some other points in its structure which are 

 worth noticing. In the central line of the body, just 

 above the mouth, as you see the animal in a dorsal 

 view, there is a square speck of a rich crimson hue, the 

 edges of which, when we view it under reflected light, 

 glitter and sparkle like a precious stone. But when 

 we obtain a perfectly lateral view, we perceive that the 

 situation of this gem-like speck is considerably nearer 

 the dorsal side of the shell than the mouth, and that it 

 forms a wart-shaped prominence on a large turbid mass 

 which occupies the whole front portion of the animal. 



