n8 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 Phyllirrhoe 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. Lasscea and 

 Sphcerium (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 (Ensis, 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 serves them as a burrowing 



