528 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
aeration comes into contact with the constantly changing 
water supply of the mantle cavity, and is only separated 
therefrom by the very delicate tissue of the gill filaments. 
Now, each filament is not a plain sheet, but is crinkled 
or folded in such a way as to form half cylinders, which 
alternate with one another as do the leaflets themselves, 
on each side of the tissue which forms the axis of the 
filament. This may be understood from the figure 
where the crinkled filament is shown from the external 
surface (fig. 62, f,5 f,, f,). This sheet of alternating 
semicylindrical folds does not extend along the full 
length of the supporting pillar, but only along the 
anterior two-thirds im the case of all the filaments on the 
internal side of the gill, while those on the external side 
bear the aerating folds all along their length, and 
consequently there are more of these folds here (fig. 60, 
L,). The number of aerating folds also varies, of course, 
with the size of the leaflet. 
Now, if we consider each filament as an element 
of the first order, each of these is crinkled into 
alternating elements of the second order, these again, in 
the same way, into elements of the third order, and so on 
until the eighth series of elements is reached, these 
ultimate folds being microscopic. Accessory gill leaflets 
are situated on the vertical septum interior to the efferent 
vessel, and occur in pairs between successive leaflets (fig. 
60, L.acc.) all along the gill. Probably they originally 
were attached to these leaflets, and now have moved 
slightly away. They equal one of the folds of the true 
leaflets in size. In shape they are triangular, the apex 
of the leaflet pointing downwards. Tach is crinkled into 
five or six pairs of secondary elements, whose further 
structure exactly resembles that of the ordinary secondary 
element of the gill, 
a 
