THE MANTLE-COVERED ANIMALS. 109 
consists of its stomach, digestive tube, and dark 
coloured liver (Iv), an ovary where the oyster eggs are 
formed, and a heart (//), with two chambers, which 
pumps the blood through the channels of the body, 
while fine nerves spread in all directions, not yet 
arranged in pairs along a cord as we shall find them 
afterwards in insects, but straggling to the various 
parts from t\vo chief centres. 
But where is the mouth ? Placing the oyster with 
its deep shell downwards, and the rounded part to- 
wards you, you will find an opening (0) in the right 
hand corner near the hinge, and over it four thin lips 
(lp}* If you could watch the oyster alive, you would 
sse that all the water passing over the gills flows 
towards this mouth, and the reason is made clear if 
you put a small piece of a gill in water under the 
microscope ; for then you will see a whole forest ot 
lashes waving over the surface of the gills like reeds 
in a stream, and striking strongly in one direction, 
namely, towards where the mouth would be. By 
means of the action of these lashes, or cilia, the oyster, 
as he lies gaping in the water, has a constant current 
flowing over him, which not only provides him with 
breath, but drives the helpless microscopic plants 
and animals past his thin lips, to be drawn in and 
swallowed. 
But though the oyster has little trouble in obtain- 
ing his food, he has much in preserving himself from 
danger. When he first comes out of the egg, he re- 
mains for some time lying safely between the gills of 
* In opening oysters at the shops, they turn them on the flat valve, 
and remove the round one, so that the mouth will then be seen on the 
left side. 
