is Connection and Locomotion. © ° 533 
into a vesicular transparent mass, that sticks to the head of 
the animal, at the opening of the shell. ‘This raises and sus- 
tains it while it pleases to continue on the surface ; but when 
it wants to return, it throws off its bladder, and sinks. I have 
taken up many of these insects alive, with the bladder yet 
affixed to the aperture of the shell, and still preserve some 
with it on in spirits. I have also observed many of the vesi- 
cula themselves swimming upon the surface of the water 
about that place, which induced me to think they were thrown 
off as the creature retired.” 
The Mollasca with bivalve shells can none of them, 
according to Dr. Fleming, float on the water, neither can any 
of them swim ; but many have the power of moving from one 
place to another by means of a muscular foot, which they 
protrude and retract at will. ‘This organ varies in form in 
the different genera; but in the locomotive tribes it is in 
general of an oblong shape, often with a bend in the middle, 
and more or less compressed. It is of a firm semi-cartila- 
ginous texture, composed of interlaced fibres, and drawn 
inwards, or exserted by other muscles, which run towards 
different points of the shell where they are inserted. Its leneth 
is often surprising. I have seen a small individual of the 
Modiola discrepans put forth a foot at least six times longer 
than the shell, which, nevertheless, when not in action, was 
so neatly folded up and contracted within it that no part was 
visible. 
Bivalve Molltisca proceed at a rate even slower than that 
of any snail, and, perhaps, seldom attempt the exercise, un- 
less driven by some urgent want. One species only (Psam- 
mobia aurantia Lamar.) is certainly known to creep like the 
Gasteropodes, although, from the structure of the foot, it has 
been conjectured that some A’rce likewise do so.* The rest, 
when bent on change, leisurely protrude the motive organ, 
extend it to the utmost, apply it with hesitation and care to a 
solid surface, and then, by contracting it, as with a painful 
effort, they drag on the body and its testaceous envelope. 
Now the foot is again extended in the same cautious manner, 
and the shell again dragged forward to the point of fixture. 
Such is the manner in which I have seen the Cyclas, an inha- 
bitant of our ponds, and some of the lesser bivalves which 
inhabit our shores, move along; and, I presume, it is in a 
similar manner that the other and larger species proceed ; 
though I am aware that a somewhat different and compli- 
cated mode of progression has been attributed to the fresh- 
* Blainville, ut sup, cit. ps 151. 
