160 VIGNETTES FROM INVISIBLE LIFE. 



connecting zones or girdles, which overlap each 

 other at their free extremities. These zones are of 

 great importance, as we shall presently see, and 

 therefore their structure must be clearly understood. 

 One zone would be enough if the object were 

 merely to keep the valves in position, and therefore 

 the fact of there being two is very suggestive. On 

 close examination it is seen that, although not ac- 

 tually united to the valves, yet that on one side the 

 zone is more firmly adherent to the valve with which 

 it is in contact than to the zone it overlays. This 

 overlapping sometimes gives the zones the appear- 

 ance of an intermediate valve, or the whole frustule 

 that of being a compound triplicate ; the double 

 structure, where they lie over each other, being of 

 course less transparent than on the parts imme- 

 diately in connection with the valves, on which 

 there is no overlapping. 



This bivalve shell with its girdling zones 

 encloses a soft organic substance, the nature of 

 which has been the subject of much dispute. 

 Sufficient for our purpose here to say, that it is 

 generally of a bright golden yellow, which colour 

 it imparts to the external shell, and that it is the 



