386 = TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
therefore free. The attachment of the ventral side is 
situated on a small protuberance on the side of the 
gonads. ‘The dorsal short side passes into the upper lip. 
The outer labial palp is rather more triangular; the 
attachment is by the proximal side, and mainly to the 
visceral mass at the side of the digestive gland; the lower 
end, however, is prolonged slightly, and is attached to the 
mantle. 
The attached sides of the palps join one another, so 
that a palpar gutter is formed, along which food particles 
are conveyed to the mouth. The two apposed surfaces of 
the labial palps are grooved (fig. 40); the other two 
surfaces are plane. The ridges run across the surface 
almost at right angles to the attached side, that is directly 
across the direction taken by the food current, and have 
their crests directed orally so as to facilitate the passage 
of particles in that direction and prevent their return. 
The plane outer surfaces of the palps are bounded by 
a layer of cubical non-ciliated epithelial cells (fig. 40), 
which are pigmented and include a few scattered sense 
cells. The grooved surfaces are bounded by a layer of 
much elongated ciliated cells. Those on the summits and 
sides of the ridges are, however, much longer, and bear 
more cilia than the cells at the bottom of the grooves; 
scattered sense cells occur. The rest of the palp is 
composed of loose connective tissue with scattered nuclei, 
and numerous spaces with blood corpuscles. A few muscle 
fibres form a somewhat difiuse layer not far from the 
surface, and there are in addition nerves from the cerebro 
pleural ganglia. 
The Lips are continuous with the labial palps; the 
upper with the two external palps, and the lower with the 
internal. They are, in Pecten, very extensive, highly- 
developed structures, and very characteristic in shape 
