SUPPLEMENT 
ON THE 
FOURTH CLASS OF THE MOLLUSCA. 
THE ACEPHALA. 
WE shall begin our review of this class with that well known 
animal the OYSTER, (OsTREA, L.) 
Oysters have been pretty generally regarded as almost in 
the last rank of animality, but most erroneously so, since there 
are a great number of animals below them, most certainly their 
inferiors in point of organization, and consequently in its 
results. What has caused them to be so considered is, that 
they live, for the most part, fixed to submarine bodies, or to 
individuals of their own species, and that it is thought that 
they are not capable of changing place. This, however, is 
an error; certain species can move, if not by means of a foot, 
of which they have no trace, at least by abruptly opening and 
closing their shell, as many other bivalves do, so as to turn 
themselves, when by any chance they happen to be upside 
down. If their sensibility is nothing, or at least extremely 
obtuse in the greater part of their body, it is not so with the 
papillary edge of their mantle or cloak. At the slightest con- 
tact of an external body on the tentacular threads, at the 
slightest rough motion of the water, it contracts, and the 
VOL. XII. Bb 
