30 THE OYSTER. 



close attention to all the 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 to the various 

 organs of the body the blood which has been purified 

 in the gills. 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 mem- 

 brane 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 conditions, for days. If the pericar- 

 dium 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 

 a 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 



