MUSSELS 437 



possible by the division of the shell into two halves (valves), movable 

 upon each other at the back, like the covers of a book. This movable 

 ' back ' is formed by an elastic band known as the ligament. The closure 

 of the shell is effected by two muscles passing transversely across from 

 one valve to the other, and firmly attached near the anterior and posterior 

 ends of the valves (adductor muscles). Their places of attachment (' scars,' 

 or muscular impressions) can be distinctly seen, and the force exerted 

 by them is familiar to anyone who has ever tried to open a living lamelli- 

 branch (e.g., an oyster). Further, any lateral displacement of the two 

 valves of the shell is prevented by several tooth and ridge-like elevations 

 of one valve which fit into corresponding depressions on the other valves, 

 forming what is known as the hinge. In the dead mussel, the action of 

 the adductor muscles ceases, and the valves of the shell gape, being drawn 

 apart by the elastic ligament. The same, of course, happens in the living 

 animal when it ceases to put these muscles into action. 



The shell is formed and grows, as in gastropods (see p. 430), by 

 secretion, from the mantle which forms a soft lining membrane upon the 

 inner surfaces of the valves, and surrounds the whole body. 



3. Respiration and Nutrition. — In a fresh-water mussel which is left 

 entirely undisturbed, we notice at the posterior end, between the valves 

 of the shell, two apertures — a lower, larger one surrounded by tentacular 

 processes, and an upper, smaller one. If we repeat the experiment made 

 in the case of the crayfish (see p. 418), by bringing a finely-powdered 

 pigment near these openings, we shall see some of the pigment particles 

 being drawn in by the lower aperture disappearing in the interior of the 

 animal, but after a short while being once more expelled from the upper 

 opening. Hence a current evidently enters the animal through the lower 

 opening, and makes its exit by the upper one. 



(a) How this current is originated we can discover by means of the 

 microscope. If we examine a small portion of a branchia, or of the 

 inner mantle surface of a freshly opened animal, under a fairly high 

 magnifying power, we shall see that the surface of these parts is covered 

 with millions of extremely fine filaments, moving rhythmically like corn 

 waving in the wind (cilia). If now we introduce into the water con- 

 taining our fragment a small quantity of the pigment, we shall further 

 see that the movement of the cilia is such as to drive the grains of 

 pigment all in one direction. In exactly the same manner these cilia 

 produce the current of water which traverses the whole mantle cavity. 



(b) It is easy to understand the importance of this current when we 

 reflect that the mussel is a sessile creature. For respiratory purposes, 

 it requires a constant renewal of the water which bathes the gills, and it 

 also requires food which, in the absence of an effective organ of lpco- 



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