188 Proceedings of Boyal Society of EdinhurgJi. [march 5 , 
variably turn in one direction (upon anterior end). Such is the 
normal way in which the gill, or portions of it, turn, but sometimes 
the entire gill will turn upon its anterior end in succession, and 
I have met with the anterior half also turning upon its anterior 
end. 
In some cases, however, the exact reverse is the condition, the rota- 
tion taking place on the anterior end. The three currents of the gill 
Surface. 
A Alntra-lamellar. 
drive it according to the directions of the arrows and the intra- 
^^■“^Marginal. 
lamellar currents, by their action or inaction, could turn the scale 
either in favour of the anterior or posterior end turning round. 
Minor variations were noted as follows : — Pieces of sea-weed laid 
upon the surface and carried to the margin were then thrown off 
and carried posteriorly by the counter current, necessarily created 
by the forward marginal current. At the anterior end the currents 
were normal, but at the posterior end the surface current drove 
particles forward for a short distance instead of towards the 
margin. 
Rate of Movement . — The rate of movement \yas determined both 
in the entire gill and in small portions of it ; also when moving 
horizontally, vertically, and upside down ; as well as when immersed 
and merely kept moist. 
The rate of rotation has already been mentioned in connection 
with the different gills, but as forward movement in the direction 
of the cut surface is most characteristic of the gill, that is what we 
have now to consider. 
In some cases the gill would move several inches in succession 
without departing from the horizontal, but usually there was a 
movement of rotation which interfered with this horizontality. So 
I adopted the plan of starting the gill level for each inch, and fixing 
upon a point about the middle of the outer margin, from which to 
reckon the forward distance travelled. 
Entire Gill. — As the result of numerous determinations at 
different times, I have found that 2 minutes to the inch is a good 
average rate of speed, both for the outer and inner gills, when 
travelling horizontally. Sometimes the gill is sluggish at first, and 
especially in large specimens the rate may be slower, but the ordi- 
nary rate is as above. Thus in the first specimen of the left inner 
