MOLLUSCA. 67 
feet. In few of the animals which inhabit fixed shells can 
a foot be observed. They are more simple in their orga- 
nization than the free shells, and are destitute of absorbing or 
ejecting syphons, the place of these being supplied by holes 
in the duplicature of the cloak. This last distinction, how- 
ever, is not peculiar to the fixed shells, although found in 
all of them. 
Among the free shells, a very important circumstance 
occurs, which we have already noticed, viz. that some of 
these adhere to rocks and stones by means of temporary 
threads produced by the animal. They are termed bysse- 
fere. Independent of the utility of this power of produc- 
ing threads of attachment, to the economy of the animals, 
the byssiferze must possess at least three organs of which 
the other testaceous mollusca are destitute. The first of 
these is a gland for the secretion of the substance of which 
the threads are formed ; the second, a foot so constructed 
as to be capable of spinning these threads and fixing them 
to the rocks or other bodies to which they are intended to 
adhere ; and the third is a muscle in the animal to which 
the inner end of these threads may be attached, and which 
muscle, in general, has the power of contraction and elon- 
gation. This character, then, appears perhaps of the very 
highest order, so that, in a natural arrangement, we might 
divide the molluscous bivalves into such as spin threads of 
attachment, and such as do not. We must, however, con- 
fess, that the byssiferee have scarcely any other subordinate 
characters in common, to warrant such an arrangement. 
In general, the valves of which the shell consists close 
upon each other in such a manner as to leave no opening. 
In a few genera, however, the valves do not close upon 
each other at one end, and sometimes at both; the point 
