224 TRANSACTIONS, NATURAL HISTORY SOCIETY OF GLASGOW. 
just where it meets the edge of the shell. It is, however, con- 
tinued beyond this, inwards towards the middle line, in the form 
of a pretty broad border. This, like the tentacles, extends all 
round the edges of both the shells, so that when the two boraers 
meet each other the whole opening caused by the gaping of the 
shell is covered over and the animal itself is completely shut in. 
The arrangement may be compared to a pair of bellows, with 
some little difference in detail. The two shells correspond to the 
two wooden boards. We must, however, close up the valved 
aperture by which the wind finds access to the bellows, and in its 
stead make a larger aperture by cutting the leather up the centre 
into two parts which would correspond to the two borders of the 
mantle described above. We must further remove the nozzle, to 
which there is nothing to correspond, morphologically at least, in 
Lima. It may be hinted that we have now spoiled the bellows 
for blowing purposes, but this is not so; the only difficulty now is 
that the wind, although finding access to the bellows readily 
enough, yet comes out again in an indiscriminate manner by the 
way it went in. That is, however, because the cut edges of the 
leather are applied edge to edge, and it is obvious that, if these be 
slightly folded in so as to meet with part of their flat surfaces 
opposed, the bellows will perform their function just as before, 
care being taken to leave a part of the cut edges near the hinge 
not folded in so that the wind can escape by this part as by a 
nozzle. This, at any rate, is the ingenious plan successfully adopted 
by Lima, and it is a fact that on the sudden closure of the shells a 
stream of water is driven out by this aperture at the hinge, the 
consequence of which is that the animal is propelled with a sudden 
impulse in the opposite direction, that is, away from the hinge. 
It is interesting to compare Pecten with our Mollusc, as it shows 
a complete failure to solve the problem of the mutilated bellows, 
and its mode of progression is merely by a sudden closure of the 
valves. This, as will be readily understood, is a less effective 
method, and sends the animal in the opposite direction to that in 
Lima, viz., towards the hinge. 
Another fact of interest in the domestic economy of Lima hians 
is that there is almost invariably, living in the nest along with it, 
a peculiar green gelatinous worm. So constant is this association 
that it is probably a case of commensalism, but in what way we 
are as yet unable to say. It is one more puzzle to be solved in 
