36 THE OYSTER. 



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 edge of 

 one W to the inside of one cover, and the opposite edge 

 of the other W to the other, and you will have a rough 

 model of the coarse anatomy of the oyster's gills, like 

 the diagram in Fig. I of Plate III. The space between 

 the covers is the mantle-chamber, divided by the gills 

 into a lower portion or gill-chamber, in which the 

 gills hang, and an upper cloacal chamber, into which 

 the pockets open. 



So far I have spoken of the gills as if the pockets 

 reached, without interruption, from end to end, but 

 this is not the case. Each pocket is divided up, by a 

 series of vertical partitions, into a number of small 

 cavities the water tubes, each of which ends blindly 

 below, and opens above into the cloaca. 



To represent them in our model we must gum the 

 two leaves of each pocket together from top to bot- 

 tom along the series of vertical lines about an inch 

 apart. Our model is very much larger than the actual 

 gill, of course. 



The spaces between the partitions which are thus 

 formed will represent the water tubes, w, in Figs. I 

 and 3 of Plate III, closed below and opening above 

 into the cloaca, and our model will now illustrate the 

 anatomy of the gill, so far as it can be made out without 

 a microscope. 



I must now speak of the minute anatomy. If a 



