THE MUSSEL OR CLAM AND OTHER BIVALVES 149 



mantle at the posterior end and extruding a little from the shell 

 (Fig. 83, 7 and 8). If a little powdered carmine is placed near 

 the openings of these tubes, it will be drawn into the lower and 

 expelled from the upper one. (See arrows in Figure 84.) This 

 indicates that a current of water is continually entering, passing 

 through the mantle cavity within, and then flowing out again. 

 It is easy to understand from this how the mussel gets its food. 

 Very small particles of animal or vegetable matter floating about 

 in the water are drawn into the mantle cavity through the lower 

 incurrent opening (siphon) and waste matters pass out through 

 the upper excurrent opening (siphon). Oxygen is also taken 

 from this fresh current of water and carbon dioxide passes out 

 through the excurrent siphon. 



Principal Parts of the Body. — To study the mechanism which 

 creates these currents one must open up the shell by cutting the 

 large adductor muscles; this is easily accomplished by inserting 

 a sharp knife near either end of the hinge. The parts of the body 

 remind one of the leaves of a book with the valves of the shell 

 representing the covers (Fig. 85). Just within the shell on 

 either side is a thin flap, the two lobes of the mantle that se- 

 crete material which forms the shell (Fig. 84, ma). Inside of 

 the mantle cavity hang down the thick, muscular foot in the 

 center (Fig. 85, 7) and a pair of leaf-like gills on either side. 

 Some of the inner organs are inclosed by the foot and the rest 

 are contained in the soft mass above it. 



Respiration. — The gills are delicate structures, each consist- 

 ing of two thin layers of gill filaments connected by longitudinal 

 crosspieces which break it up into tubes (Figs. 84, g, and 85). 

 If we cut off a small piece of the gill of a living mussel, an opera- 

 tion that does not cause pain to the animal, and examine it under 

 a compound microscope, we shall find it covered with minute 

 hairlike projections, the cilia, which are waving back and forth. 

 A little powdered carmine placed in the water near the piece of 

 gill will be driven in one direction by these cilia. Considering 

 the fact that all the gill filaments are covered with cilia, it is 



