212 AN INTRODUCTION TO ZOOLOGY. 



The flat gill-plates are very characteristic of the class. 

 The shell is lined by a loose muscular flap^ covered 

 with epidermis, called the mantle. The absence of 

 a head is doubtless correlated with the presence of a 

 shell. 



Two orders were formerly distinguished in this 

 group, the Asiphonia and the Siphordata, classed ac- 

 cording to the presence or absence of conspicuous 

 siphons, tubes formed by a posterior elongation of 

 the mantle, of which the lower is the " branchial 

 siphon," and draws in fluid, and the upper is the 

 " cloacal siphon," which expels it. The shell of the 

 Asiphonia has an even " pallial line," as the mark is 

 called, which is traced by the line where the mantle 

 is fastened to the shell. There is never more than a 

 rudiment of the siphonal structure, while in many of 

 the shells of the other order the long retractile siphons 

 leave a wavy mark on the, mantle line. The longest 

 siphons belong to forms that either burrow or lie 

 in sand ; and altogether the existence of the siphons 

 seems to be a provision adapted for that mode of life, 

 the Asiphonia being for the most part adapted to live 

 among stones rather than on sand, fixing themselves 

 by one valve, like the oysters, or mooring themselves 

 to stones by secreted threads (called byssus) like 

 the mussels, etc. This division includes the oysters, 

 which have one valve attached, and therefore always 

 lie on one side, the left, which is the deepest. The 

 Scallops [Peden) are the showiest of English bivalves. 

 They receive both their names from the fluted pattern 



