Structure of Bivalve Molluscs. 



FIG. 50. 



is sometimes fringed, and the irregularities secrete 

 corresponding processes on the shell in the forms of 

 ridges, spines, &c. 

 On the inside of the 

 shell a line of demar- 

 cation shows where 

 the nacre-secreting 

 surface ends, and the 

 prism-secreting por- 

 tion begins, this is 

 called the pallial line 



(fig- 5)- 



Hinge. The two 



Valves Of the Shell shell of Galathea, showing the umbo, hinge, 



in Lamellibranchs are mantle line and the two adductor scars - 

 usually similar to each other, they are disposed late- 

 rally, one on the right and one on the left, and are 

 united by a hinge of interlocking teeth at the dorsal 

 margin. A highly elastic ligament unites the valves 

 outside the hinge, and is so arranged that it keeps 

 the valves slightly open. On the inside of a bivalve 

 shell there are to be seen one or two oblong scars 

 in each valve to which are attached muscles running 

 from valve to valve for the purpose of closing the shell, 

 and hence called adductor muscles (fig. 49 #, e). 



Soft Parts of Bivalves. The lobes of the mantle 

 are usually more or less united along the under bor- 

 der, and are often prolonged backwards into a long 

 tube or siphon which projects at the hinder end of 

 the body ; when this tube exists, the pallial line is 

 indented posteriorly into a sinuosity called the pallial 



sinus. 



G 2 



