32 THE OYSTER. 



details of a long, complicated and minute description, 

 which from the nature of the case cannot be stated 

 briefly, although it may all be put in simple language. 



A gill is, of course, a breathing organ, for aerating 

 the blood by exposing it to the oxygen in the water ; 

 and the oyster has a heart for driving the blood which 

 has been purified in the gills to the various organs of 

 the body. It is easy to see and study the oyster's 

 heart, but in order to do so the animal must be opened 

 with great care, by cutting the muscle with a thin sharp 

 blade, as near the shell as possible. If this is done, 

 a small semi-transparent space will be seen close to the 

 inner edge of the muscle. The thin membrane which 

 covers the space is the pericardium, or the chamber 

 which holds the heart, Plate I, d, and through its 

 transparent wall this may be seen slowly pulsating ; for 

 an oyster is not killed by opening its shell, and its heart 

 continues to beat for hours, or, under favorable condi- 

 tions, for days. If the pericardium be gently lifted 

 and cut with sharp scissors, the heart, Plate II, d, with 

 its blood-vessels, will be exposed. It consists of two 

 chambers, the auricle, which receives the pure blood 

 from the gills, and the ventricle, which drives it 

 through arteries to the various organs of the body. 



While the gill of an oyster is a breathing organ, 

 like the gill of a fish or crab or conch, this is only one 

 of its many uses. The fish and the crab and the conch 

 have other organs for supplying the gills with a stream 

 of fresh water, but the gills of the oyster, besides puri- 

 fying the blood, keep up a circulation of water for 



