SHELL-BEARING ANIMALS /X I/ARJWRS 243 



never extends to liigh-tide mark, but always leaves a foot or 

 two in depth above the surface to be filled by the water at 

 high tide. The result is that the tidal channels in the man- 

 grove district are relatively much shallower than those beyond 

 the field.* 



In discussing the influence of vegetation on harbors we 

 have already had occasion, incidentally, to refer to the action 

 of certain forms of animals which dwell amid salt-loving plants 

 and contribute their remains to the marshy accumulations. 

 We have now to extend our consideration of the work done 

 by the various shell-bearing animals to those parts of harbor- 

 ages where they dwell apart from vegetation. Wherever 

 an inlet of the sea is the seat of a considerable tidal flow we 

 are sure to find that the conditions favor the abundant devel- 

 opment of animal life. Along the shores of the sea, near the 

 mouth of the reentrant, the waves are continuously at work beat- 

 ing the remains of marine animals and plants into a finely divided 

 state, where they readil)' float in the water and afford large 

 amounts of nourishment to the living beings with which they 

 come in contact. A host of species dwell in this water and 

 draw their sustenance from it. With each oscillation of the 

 sea the food-giving fluid is swept past the multitudinous 

 mouths of the animals which dwell on the floor of the harbor, 

 and which thus at once obtain the shelter of its recesses and 

 a supply of nutriment fitted to their nurture. 



The most important sediment-producing animals of havens 

 are found in the group of moUusks. The most of the species 

 in this type are shell-bearing. They grow rapidly, and at 

 death a large part of their bodies is contributed to the sedi- 



* For further information concerning mangrove marshes, see Tenth Annual 



Report of the United States Geological Survey. 



