250 ZOOLOGY 



foot is adapted for burrowing and consequently without 

 a flat surface or sole. The shell is hinged above, and the 

 mantle adds material all along the margins, producing 

 concentric lines of growth. Within the mantle, between 

 it and the foot, are the leaflike gills. The mantle edges 

 are usually united posteriorly to form more or less 

 tubular organs called siphons. The upper of these, the 

 anal siphon, is for the purpose of getting rid of waste 

 water and food materials. The lower or branchial 

 siphon is the one through which water enters, carrying 

 oxygen in solution, which is absorbed through the sur- 

 face of the gills. When the valves of the shell are ex- 

 amined, it will be seen that there is an apical point, 

 representing the earliest stage of the shell ; this is the 

 umbo. Below the umbo is the hinge, which in some 

 species is large and complicated. Within are seen the 

 anterior and posterior scars of the adductor muscles, 

 which close the shell. Passing from one to the other, 

 but variously curved, is the pallial line, marking the 

 attachment of the mantle. Many species of bivalves, 



Pearls particularly the large fresh-water mussels, have the 



shell lined within with a beautiful pearly substance, the 

 nacre. In the region of the Ohio and Mississippi rivers 

 the shells of these mussels are used as a source of pearl 

 buttons, while occasionally the nacre forms around some 

 object in a globular fashion, and is then a true pearl. 

 It has been found that pearls result from the presence 

 of parasites, which are inclosed and rendered harmless 

 by the secretion of nacre. 



Cephalopoda 6. The Cephalopoda or head-footed mollusks include 

 the octopus, squid, nautilus, and the extinct ammonites. 

 Although the shell is spiral, the animals are symmetrical. 

 The foot forms a series of appendages surrounding the 

 mouth ; thus the octopus derives its name (eight- 



