THE OYSTER. 37 



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 

 furrow. Fig. 3 of Plate III is a greatly magnified 

 drawing of a cross-section of a small part of a gill, 

 including one water tube, w, and the partitions a, a, 

 between it and the adjacent tubes ; r, r, r are the 

 ridges, and p, p water pores. In the bottoms of the 

 furrows there are many minute openings the water 

 pores, which pass through the wall of the gill into 

 the water tubes, and thus form the channels of com- 

 munication between the two divisions of the mantle- 

 chamber. 



The ridges themselves are hollow, or rather, each 

 one contains a minute blood-vessel, which runs 

 throughout its entire length, so that each wall of each 

 gill is practically a grating of parallel, vertical blood- 

 vessels, in which the blood is purified by contact with 

 the water which fills the gills and the chamber in 

 which they hang. The purified blood is then forced 

 into larger vessels, which carry it to the heart, by 

 which it is pumped to all parts of the body, to be 

 again returned to the gills after it has become impure. 



The gills are therefore easily intelligible, so far as 

 they are simply organs of respiration; they hang in 

 the water which fills the mantle-chamber, and their 



