BIVALVE MOLLUSKS. 99T 
dead at least fifteen hours, yet when I placed the 
torn fragments of the branchiz, one after another, 
beneath the microscope, the energy of the ciliary 
action, as the wave flowed with uniform regularity 
up one side and down the other of every filament, 
filled me with astonishment. [ven the next morn- 
ing, twenty-six hours after death, when the tissues 
of the filaments were partially dissolved, the ciliary 
motion was still going on, on portions that pre- 
served their integrity. 
In a sort of hood formed by the union of the 
gill-leaves at their basal part, is placed the en- 
trance of the stomach; a simple orifice without 
jaws, teeth, or tongue, but bordered by four thin, 
membranous lips. 
The force with which the valves resist our 
attempts to open them, during the life of the 
animal, depends on the presence of a large and 
powerful muscle, which is very apparent when 
they are opened, as it occupies a considerable 
portion of the interior, stretching directly from 
one valve to the other, and inserted by a broad 
base into each. By the contraction of the fibres 
which compose this muscle, the valves are strongly 
pulled together, and it is by cutting across this 
with an inserted knife that an oyster is commonly 
opened. When death ensues, the valves open spon- 
taneously from another cause; muscular contrac- 
tion then ceases, and the relaxed fibres can no 
longer resist the expansive force of a dense and 
highly elastic substance placed at the back of the 
shell close to the hinge, and known as the ligament. 
Around this great muscle are grouped the 
stomach, liver, intestine and other vital organs; 
while in the neighbourhood of the mouth there is 
