206 HIGHWATER MARK. 



to be made before one of these atoms would be trans- 

 formed into a decent oyster, even in miniature. 



One of the most obvious of such modifications 

 would occur in the breathing organs ; and as these 

 constitute in our little Tubulipora its chief feature of 

 beauty, as well as the most important agent in its 

 maintenance, and as in both animals their structure 

 is highly curious, we may find interest in examining 

 them in detail. 



If we take a Mussel, and, inserting a thin penknife 

 between the dark-blue shell-valves, sever the muscular 

 bands which pass from one to the other and form 

 their power of cohesion, we shall display the whole 

 yellow anatomy within. First of all, each valve is 

 lined by a membrane of thin flesh, the edges of which 

 are free, and are cut into festooned fringes. These 

 two leaves constitute the mantle, and are the agents 

 by whose instrumentality the shell itself is made, and 

 from time to time augmented. This process is a very 

 curious one, but, as it is not relevant to our present 

 inquiry, I will not now touch it farther. 



Lifting these membranes, then, there appear laid 

 along beneath them two other leaves on each side, 

 thinner and more delicate. These four leaves, which 

 are the breathing organs that I mean to describe, 

 with the two leaves of the mantle, bear the same re- 

 lation to each other as the leaves of a book, a book of 

 six leaves, of which the two shell-valves are the boards 

 or covers. These gill-leaves at first sight seem mere 

 thin, lax, motionless lamime of membrane, so fragile 



