352 ‘TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
depression of the mantle (fig. 1, Lg. P.). Into this 
depression the ligament dips, lying transversely across it. 
There are no fusions of the mantle edge to form separate 
inhalent and exhalent apertures, and consequently there 
are no siphons. The inhalent and exhalent currents are, 
however, confined to special regions, so that physiologi- 
cally the fusions are not needed for the separation of the 
currents. By scattering some carmine into water in 
which a Pecten is living, the particles of colour can be 
seen entering all round the shell between the two folds of 
the mantle, except for an area extending from the 
posterior end of the hinge line for a short distance 
forward. Here there is a very definite exhalent current 
sometimes accelerated by the animal closing the shell 
suddenly and forcing the water out at this point only, to 
eject the faeces. 
The free margin of the mantle lobes is much 
thickened and presents three typical folds (fig. 4). The 
outer one, the shell fold (fig. 4. Sh. /’.), is small and bears 
long tentacles. The median one, the Ophthalmic fold 
(fig. 4, Op. F’.), is not so distinct and also bears tentacles 
and the eyes which form conspicuous objects when the 
animal is alive. The most internal fold is much the 
largest and is turned inwards to form a flap, known as the 
“velum” (figs. 1 and 4, V.). It is usually pigmented 
either continuously or at regular intervals. List (6) has 
shown that the storage of pigment in the mantle cells is 
directly influenced by light, and that removal of a piece 
of the shell causes a deepening in colour of the tissue 
exposed, due to formation of pigment. This curtain-like 
velum becomes reduced in size as it approaches the base of 
the angle forming the ears, and it is this inner portion of 
the mantle on both sides that fuses as mentioned above. 
The outer folds remain free, with their eyes and tentacles, 
