MICROSCOPIC OBJECTS. 



55 



respiratory system would be objectionable, inasmuch as 

 the animal would be constantly required to leave the 



water to breathe. 



The beautiful arrangement shown in 



Fig. 3. 



fig. 7 is therefore adopted. This system belongs to the 

 larva of a Dytiscus seen at fig. 5 of the natural size. 



Fig. 4. 



For the admission of air there are only two openings, 

 both situated near the tail, which it brings to the surface 

 of the water whenever it is necessary to take a fresh sup- 

 ply. Fig. 6 is a magnified portion of the trachea, to 



