THE OYSTER. 33 



held vertically, with the opening above, will form a 

 pretty good model of a single gill. 



The closed portions of the four gills hang down 

 into the mantle-chamber, side by side, but their upper 

 edges are fastened to each other, and to the inside of 

 the mantle, in such a way that they form a folded par- 

 tition, something like a double W, which divides 

 the mantle-chamber into two parts : a lower chamber, 

 in which the gills hang, known as the gill-chamber, 

 and an upper chamber, into which the pockets open. 

 This chamber is known as the cloaca, the Latin word 

 for a sewer, or channel for waste water, and I hope to 

 show you the fitness of the name soon. 



The partition between the two chambers is formed 

 somewhat in this way. The upper edge of the outer 

 leaf of the outer gill is united, along its whole length, 

 to the inner surface of the mantle. The upper edge of 

 the inner leaf of the outer gill is united to the same 

 edge of the outer leaf of the inner gill. The upper 

 edges of the inner leaves of the two inner gills are 

 united to each other on the middle line of the body. 



If you were to make four pockets out of four sheets 

 of paper, and were then to gum two of them together 

 along their free edges, you would make a double 

 pocket, which might be opened out so that a section 

 through it would be like a W. This would serve as 

 a model of the two gills on one side of the body, and 

 two more sheets, treated in the same way, would 

 make a model of the other two gills. Now gum two 

 W's together, side by side, and the double W will be 

 a model of the four gills. Now open a very large 

 book-cover, just far enough to gum the upper outer 



