THE OYSTER. 35 



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 

 walls are filled with blood-vessels in which the blood 

 comes into close contact with the water. 



The way in which the current of fresh water is kept 

 up to bathe the gills continually with a new supply is 

 more complicated. 



When one of the ridges on the surface of the gill is 

 examined with a high power of the microscope, it is 

 found to be fringed on each side by a row of fine 

 hairs, Plate III, P'ig. 2, c, c, each one less than -^\-^ inch 



