XI MOLLUSC A 143 



Most of our pearls are obtained from the large pearl 

 oyster (Mekagrina margaritifera), which is common round the 

 coasts of tropical countries, but the fresh- water mussel of 

 North Britain {Unto margaritifera) also produces pearls of 

 value. 



The Body Beyond the shell there projects, when the 

 with Shell mussel is active, a large, fleshy, somewhat wedge- 

 mtact. siiaped "foot." When the animal is at rest this 

 foot, and often more than half of the shell, is buried in mud 

 or sand, and there can only be seen the back of the shell, 

 gaping slightly open, and exposing the thickened hinder edges 

 of the mantle-flaps which bound two apertures — an upper, 

 smaller, oval one, and, a little way below this, a vertical, slit- 

 like opening round which the mantle-edge is pigmented and 

 fringed with small tentacular processes. If a little carmine, 

 or some other finely divided coloured substance, be suspended 

 in the water, it can be clearly seen that there is a constant 

 stream of water passing into the lower, inhalent aperture, and 

 out of the upper, exhalent aperture. 

 Body with To understand the mode of nutrition and 

 Shell respiration in such a bivalve, it is desirable to 



removed. i-ginQye Qug valve of the shell from a mussel and 

 study the organs that are then exposed. For this purpose 

 it is necessary to wedge open the valves with some strong 

 object, such as a flat ruler, and then with a sharp penknife or 

 scalpel, cut through the muscles on one side which attach the 

 mantle to the shell (see Fig. 90), sliding the knife up carefully 

 between mantle and shell so as not to damage the mantle. 

 Bend back the valve thus separated and remove it, cutting 

 through the ligament along the hinge line. The soft body 

 which is lying in the remaining valve of the shell will now 

 be seen covered by the mantle-lobe of this side, and — in order 

 to see the organs lying below — this must be turned back, or 

 cut away along its line of attachment (Fig. 91, Im). Before 

 this is done, however, the formation of the exhalent and 

 inhalent apertures should be noted. The two margins of the 

 mantle-lobes fuse just above the exhalent aperture, and then 

 separate to form its opening, the lower limit of which is due, 

 not to a second fusion of the mantle-lobes below it, but 

 merely to a nipping in of the mantle, so that the two 

 margins are pressed closely together. The inhalent aperture 



