MICROSCOPIC OBJECTS. 



57 



as shown in fig. 8, which represents the Dytiscus without 

 the elytra and wings. The largest spiraculse are nearest 

 the posterior extremity of the body. Fig. 9 is a mag- 

 nified view of one of these valves or openings, and fig. 

 10 a portion of the same highly magnified, to show the 

 mechanism by which foreign substances are prevented 



from entering. 



Fig. 8. 



Fig. 9. 



^ig. 10. 



Scales of Fish. — ^The study of the dermal covering of 

 fishes has of late years become of vast importance, 

 arising from the investigation and discovery of M. Agassiz, 

 who has demonstrated that the nature and form of the 



