and Teredo Navalis. 
285 
The ventricle may be said to be continued into an artery, 
which supplies the viscera and goes up to the muscle of the 
two boring shells. The heart is very loosely connected to 
the surrounding parts ; its action was very distinctly seen 
through the external covering, and was in some instances 
continued after it was laid bare. The first contraction is in 
the two auricles, which shorten themselves in that action. 
This produces a swelling of the ventricle, followed by a con- 
traction. The artery from the ventricle can be traced up to the 
head, and the vessels from the auricles are seen very dis- 
tinctly as far as the breathing organs. The auricles are lined 
by a black pigment, so that their contents cannot be seen 
through them, and the ventricle is too thick in its coats to 
be transoarent : but the muscle of the boring; shells is of a 
red colour, as well as the liver, and most of the surrounding 
parts, between the heart and the head. 
This structure of the heart admits only of a single circula- 
tion, as in other animals which breathe through the medium 
of water, but the mode of its being performed is different from 
that in fishes ; in the teredines the blood passes directly from 
the heart to the different parts of the body, and returns 
through the vessels of the breathing organs to the heart, 
while in fishes it goes first to the breathing organs, and then 
to the different parts of the body. 
This peculiar circulation becomes a link in the gradation of 
the modes of exposing the blood to the air in different ani- 
mals, it appears to be less perfect than in fishes, since the 
exposure to the air is carried on more slowly, but is more 
perfect than in caterpillars. 
It is common to animals that have the same general economy 
