34 THE OYSTER. 



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-cham- 

 ber, 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 a 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 

 small piece of one of the gills be cut out, and spread 

 flat upon a glass slide so that its surface may be 

 examined under a microscope, it will be found to be 

 thickly covered with parallel ridges running from top 

 to bottom, like the lines on the sheet of paper, each 

 ridge being separated from the next one by a deep 



