These are four parallel plates which occupy the ventral halt 
of the mantle cavity and extend from the posterior nearly to 
the anterior end of the body. Their ventral edges are free, 
hut their dorsal edges are united to each other, to the mantle 
and to the body. The space above or dorsal to the posterior 
ends of gills, is occupied by the oval, firm, adductor muscle, 
the so-called “ heart.” For some time I was at a loss to know 
how the muscle came to be called the heart, but a friend told 
me that he had always supposed that this was the heart, since 
the oyster dies when it is injured. The supposed “ death ” is 
simply the opening of the shell when the animal looses the 
power to keep it shut. Between this muscle and the hinge 
the space above the gills is occupied by the body, or visceral 
mass , which is made up mainly of the light colored repro- 
ductive organs and the dark colored digestive organs, packed 
together in one continuous mass. 
If the oyster has been opened very carefully, a transparent 
crescent-shaped space will be seen between the muscle and the 
visceral mass. This space is the pericardium, and if the deli- 
cate membrane which forms its sides be carefully cut away 
the heart may be found without any difficulty, lying in this 
cavity, and pulsating slowly. If the oyster has been opened 
roughly, or if it has been out of water for some time, the rate 
of beating may be as low as one a minute, or even less, so the 
heart must be watched attentively for some time in order to 
see one of the contractions. 
The heart is made up of two chambers, a loose spongy 
transparent auricle , which occupies the lower part of the 
pericardium, and receives blood from the gills through trans- 
parent blood vessels, which may usually be seen without dif- 
ficulty running from the gills tourards the heart, and a more 
compact white ventricle , which drives the blood out of the 
pericardium through transparent arteries, which are usually 
quite conspicuous. 
The visceral mass is prolonged backwards over the pericar- 
dium and the adductor muscles, and here contains the rectum 
surrounded by prolongations of the white reproductive or- 
