114 



KVENINGS AT THE MICEOSCOPE. 



ePIEACLE OF LEATHEE-COAT. 



geographical simile, we may saj that, instead of being 

 an island in the midst of a lake, it is a promontory. 

 Thus, the breathing space is a 

 crescent- shaped band, which is 

 crossed in every part by bars 

 passing from the margin to the 

 projecting plate. But, as if the 

 interstices left by these bars 

 would be too coarse for the pui-- 

 pose, they are further sublimat- 

 ed by a membrane, which is 

 stretched across them, and 

 which is perforated with a num- 

 ber of excessively minute roiind 

 holes, through Avhich alone the air is admitted. 



In many of the two-winged flies, which inhabit the 

 water in their earlier stages, there are some interesting 



contrivances and modifica- 

 tions connected with the or- 

 gans of respiration. It is ne- 

 cessary that the orifices of 

 the air-tubes should be 

 brought at intervals to the 

 surface of the water, in or- 

 der to come into contact 

 with the external air ; while, 

 at the same time, it is im- 

 portant that as small a por- 

 tion as possible of the ani- 

 mal's body be exposed to 

 danger, by being protruded from its sheltering element. 

 An example in point you may see in this vase. 



Here is a slender worm, an inch and a half in 



6PIRACLE OF COOKCnAFEK-GEUB. 



