414 SUPPLEMENT 
the interior of the sea; but it appears that whether free or 
aggregated, they can move, probably, without any determined 
direction, by means of the water which they cause to enter 
into their mantle for respiration and nutrition. ‘This water 
penetrates through an aperture provided with lips and valvules, 
and issues forth by an opposite one. ‘This alternative action 
has been sometimes called systole, and diastole, and its result 
is, that the body is carried in an inverse direction to that of 
the rejected water. 
We have no knowledge respecting the digestion of these 
singular animals. Itis probable that it must be easy, in con- 
sequence of the fluid form in which their nutriment is con- 
veyed. It is not likely that some foreign bodies which are 
often found in the cavity of their mantle are digested there ; 
for this does not constitute their stomach. The function of 
respiration takes place by the introduction of water into the 
cavity of the mantle. 
Circulation appears to be somewhat singular. The motions 
of the heart are made spirally ; they take place by a twisting 
of its parietes, and always begin from one of its extremities. 
If it be that which touches the nucleus, a name given to the 
little mass of the digestive apparatus, the motion of the blood 
is made into the aorta, and into its principal ramifications ; 
if it be the other, the march of the fluid proceeds in an oppo- 
site direction. These motions of the heart are easily per- 
ceived, first pushing the blood in one direction, stopping, con- 
tracting, and then impelling it in an opposite direction. We 
then see this fluid fall back, as it were, by its own proper 
weight, and assuming a direction opposite to what it had at 
first. But as the two systems of vessels which issue from the 
heart communicate together, it happens, after a certain time, 
that these sorts of oscillations send the blood into all parts of 
the body. 
These motions of the blood are more perceptible, as it is 
