MADE AT THE PLYMOUTH LABORATORY. 251 
and ventrally to the head. The posterior parts of the fin-fringes 
were in constant motion, moving in a series of vibrations from before 
backwards, together with the part of the body to which they were 
attached, and the effect of this motion was to pump out the water 
from the space between the body and the glass, its place being 
supplied by water which entered in front. The subcaudal fin-flaps 
were perfectly motionless, and tightly pressed between the base of 
the tail and the surface of the glass, so that any movement of them 
was impossible. The question arose, however, whether the tail and 
these flaps formed a small sucker which helped in the adhesion, 
To test this I removed the flaps with a snip of the scissors, 
an operation which caused very little pain to the fish, and it 
adhered afterwards quite as well as when the flaps were in their 
natural condition. The subcaudal flaps are therefore certainly 
not necessary to the adhesion, nor to the pumping action of the 
muscles and fins, which went on as before. It seemed probable, 
therefore, that the pumping action was itself the cause of the adhe- 
sion. But the difficulty in accepting this view was that there was a 
distinct though gentle respiratory movement of the jaws and oper- 
cular flaps ; and if the pumping of the water from beneath the body 
caused a negative pressure there, and a positive pressure on the 
outer side of the body, it seemed equally certain that the respiratory 
movement must force water into the space beneath the body, and 
so cause a positive pressure there which would tend to force the fish 
away from the glass. The currents of water were now examined by 
means of the suspended particles in the water, and by putting 
carmine from a pipette at any spot at which it was desired to see 
the rapidity and direction of the flow. Particles were seen to pass 
in at the mouth and out at the lower respiratory orifice, but particles 
and carmine were also seen to pass into the space beneath the body 
above and below the head without passing through the respiratory 
channel. It was, therefore, satisfactorily proved that the amount of 
water pumped out in a given time at the sides of the tail was greater 
than the amount passed in anteriorly by the respiratory movements; 
and considerably greater, for the velocity of the stream above and 
below the snout as shown by the movement of the particles of carmine 
was by no means insignificant. It follows that the pumping action of 
the fins, continually withdrawing water from the space between the 
body and the surface to which it is applied, causes a negative pres- 
sure greater than the positive pressure due to the respiratory move- 
ments, and this keeps the body pressed against the vertical surface 
sufficiently to prevent its falling. The negative pressure is continually 
being neutralised by the water entering in front, and therefore the 
pumping action must be constantly kept up, as it is observed to be, 
