therefore free. The attachment of the ventral side is 

 sitnated ou 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 mainlj- 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 j)alp is 

 composed of loose connective tissue with scattered nuclei, 

 and numerous spaces with blood corpuscles. A few muscle 

 fibres form a somewhat diffuse 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 



