118 THE LIFE OF THE MOLLUSCA 
In sedentary Gastropods like Vermetus, and in 
the parasite Stylifer, the foot becomes reduced and 
atrophied ; while in the free-swimming Phyllirrhoé it 
no longer exists as a distinct organ. 
The more primitive Pelecypoda, such as Nucula, 
with its kin, and Trigonia (Plate XXX., Fig. 7) have 
a flat under side to the foot with fringed margins, 
that can be used as a creeping disc. Lass@a and 
Spherium (Plate XVIII., Fig. 16), though they have 
no creeping disc, can also crawl on submerged 
objects, and some of them even on the surface film 
of the water. Most of the Pelecypoda, however, 
and the Scaphopoda, make use of the foot to force 
a way through the mud and silt in which they live. 
The foot is first extended in the direction in which 
the animal wishes to go, the blood is then forced 
into it so that it swells out and ; gives some hold 
on the surrounding ‘silt, when by contraction of the 
pedal _ muscle ‘the shell is hauled towards it. By 
a constant repetition of this process the creature 
slowly advances. The same method is adopted by 
those Bivalves, like Mya, Solen, etc., that live head 
downwards in sand. The Solens (Exsis, Plate XXX., 
Fig. 8, etc.), indeed, owing to the powerful develop- 
ment of the foot and elongate cylindrical shape of 
the body and shell, can bore down -with_ such-great 
rapidity that if the animal has been alarmed it is 
impossible to overtake them by digging. The foot 
of the Cockles not ‘only sei serves them as a burrowing 
