322 ANIMAL REPRODUCTIONS. 
by the diminution of fize and an internal tranf; 
parency. I have often been aftonifhed at the 
number of fucceffive opercula produced: by de- 
capitated fnails ; however, all do not produce 
them ; at the fame time, the fhells of very few 
continue open. 
My decapitated {nails were kept in boxes, 
Some remained at the bottom, others attained the 
fides, againft which they applied the mouth of 
the thell; others, afcending higher, reached the 
covering, where they fixed in the fame manner. 
‘Thefe feemed to be the moft vigorous, or had fuf- 
fered leaft from the operation. 
When I wifhed to learn the ftate of the de- 
capitated {nails from week to week, I had only 
to take the opercula carefully from the mouth of 
the fhells, and then immerfe them in very limpid 
water ; thus the {nails are forced to appear foon- 
er or later, but they have fometimes remained 
within for feveral hours after immerfion. This 
method, to which I have always had recourfe, is 
the beft in my opinion, for the fnails, then ex- 
tending to the utmoft limits, expofe the ante- 
rior part fo completely, that nothing can efcape 
the obferver’s eye. They endeavour to leave 
the water, and gradually attain their purpofe, 
if the depth is not too great; or, crawling 
flowly over the bottom and fides of the veflel, 
they advance until they reach a dry pofition,’and 
fix 
