180 Proceedings of Boy al Society of Edinburgh, [march 5, 
The right and left palps are usually of the same size, hut occasion- 
ally one is half the size of the other and I have met with a left 
inner palp only Jth the size of its fellow. 
Since the palps may sometimes change their contour when de- 
tached, so that it is not easy to tell which is the outer and inner 
margin, this may always be determined by noting that, on both 
inner and outer palps, the outer margin is transversely striped. 
Inner Palps — Descrijption . — Each inner palp lies inside the 
inner gill of either side, to which it is attached by its anterior basal 
corner, which also forms the angle of the mouth. This connection 
with the gill is very slight, and yet sufficient to allow the palp to 
come away with the gill when detached (fig. 5), The two inner 
palps are connected with each other at their base by a thin mem- 
branous portion forming a lower lip to the mouth, just as the two 
outer are similarly connected to form an upper lip (fig. 2 c). If a 
palp is detached as near its base as possible, having the form shown 
in fig. 2 a, and laid on a plate with the liquid from the shell, then 
its movements may be easily observed. 
Nature of Movement . — The movement is one of regular rotation, 
the palp revolving about one end in a steady manner and in a 
definite direction. There may be forward or backward or lateral 
movements combined with this, but when once the palp has fairly 
become accustomed to its free condition of existence, rotation is its 
characteristic movement. This rotatory motion is probably due to 
the fact that the basal (cut) end is destitute of cilia, and so there is 
a tendency to turn round that spot as on a pivot. The palp, how- 
ever, can also rotate upon its tip, and we can hardly account for 
its being made the pivot, on purely mechanical grounds. Muscular 
contraction also takes place in these palps, assisting the cilia or 
modifying the direction of motion of the palp. 
The usual movement of rotation is as follows : — The broad base 
of the triangular palp forms the pivot round which it turns, while 
the exceedingly sensitive tip is directed in the opposite direction to 
that of the motion, just as the rudder is constantly directed in 
turning a boat in motion. 
The nature of the movements, when the palp is completely 
immersed, will be considered later. 
Direction of Movement . — The right and left inner palps, detached 
