MUSSELS, CLAMS, OYSTERS, SNAILS, SQUIDS 107 



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mantle lobes along the entire length of their dorsal edges, 

 and the inner walls of the inner gills are attached to the 

 sides of the body (Fig. 57). Thus the mantle cavity is 

 divided into two chambers : a dorsal, or cloacal chamber and 

 a ventral, or branchial chamber. The fresh sea water 

 carrying oxygen is brought into the branchial chamber 

 through the inhalent siphon (Fig. 56). The gill cilia then 

 cause it to flow through the 

 holes in the sides of the gills 

 into the troughs of these 

 organs, whence it passes into 

 the cloacal chamber and out 

 through the exhalent siphon. 

 The blood in the gills is sepa- 

 rated from the surrounding 

 water by the very thin, mem- 

 branous walls of the gills, 

 and the carbon dioxide read- 

 ily passes through this tissue 

 by osmosis, and the oxygen 

 is readily absorbed from the 

 water in exchange. 



The foot and locomotion. — 

 At the anterior end of a living mussel a white, flexible, 

 muscular organ, the foot (Fig. 58), is often seen protruding 

 forward and downward between the gaping ventral edges 

 of the shell. This is the organ of locomotion. In its nat- 

 ural position the mussel stands with the anterior end of 

 the shell considerably deeper in the mud than the posterior 

 end. This leaves the' siphons clear of the mud and in direct 

 communication with the water above. In this position the 

 mussel plows slowly along through the mud. The foot is 



Foot 

 Fig. 57. — Cross section of clam. 



