Ciliary Mechanisms 



57 



finally reaching a sharply defined tract parallel with the 

 mantle edge. Here the stream is directed backward to 

 a point just below the opening of the incurrent siphon 

 (/ s). It will be remembered that the siphon is a pair 

 of tubes formed as a growth from the mantle, and that in 

 the living clam, a strong stream of water enters the 



Fig. 13. — Ciliation of the mantle of the round clam or "little neck" 

 (Venus mercenaria). The arrows indicate the directions taken 

 by foreign particles in the mantle chamber that come in contact 

 with the mantle wall. The palps are represented at p. 



lower one. If a mass of material being moved by the 

 mantle cilia were brought in contact with this stream, 

 it would be swept far back again into the mantle cham- 

 ber, so it comes to rest in this small bay below the siphon, 

 where a considerable quantity of it may collect. What 

 is to become of it? It could not easily be carried di- 

 rectly outward between the edges of the mantle, because 

 the entire shell is covered in the bottom. The only 



