CH. XIV.] EPHEMERA OR MAYFLY. 205 



the gills of fishes present a familiar example of the 

 essential parts of a similar fact ; these are the breath- 

 ing organs of this class of animals, and in them the 

 blood is vitalized by the contact of such air as is 

 contained in the water, as the latter fluid is admitted 

 or ejected by the alternate opening and shutting of 

 these organs. 



In examining the structure of these organs in the 

 different kinds of ephemerae, the reader will be 

 astonished to find such a variety of form in parts 

 destined to perform the same function. In the two 

 following figures the construction of this singular 

 apparatus is admirably shown. Fig. 1 represents 



FIG. 1. FIG. 2. 



one of the tracheae or breathing-tubes magnified, 

 which has been removed from that kind of ephemera 

 that has been likened to a galley. A common trunk 

 gives rise to two large branches, from each of which 

 smaller ones, of a conical form, are given off diamet- 

 rically opposite each other and at equal distances. 

 VOL. II. S 



