MOLLUSCA — PELECYPODS 



215 



ture. Thus the shell grows by the continual building out of 

 its edge through successive depositions of lime and of conchiolin. 

 In this way are formed the two outer of the three layers of which 

 the shell consists. 



A transverse section through a valve reveals (i) an outer 

 horn-like layer of conchiolin secreted only by the extreme edge 

 of the mantle and hence always projecting slightly in advance of 

 the other layers; (2) a middle prismatic layer composed of 

 calcareous prisms placed at right angles to the shell surface ; 

 and (3) an inner pearly layer, composed of thin alternate layers 

 of calcareous matter and of conchiolin, parallel to the shell 

 surface. This presence of conchiolin throughout the shell 

 furnishes it with a sort of membranous framework sufficient 

 in amount to enable the shell to retain its shape after the re- 

 moval by acids of all calcareous matter. 



After the glands around the edge of the mantle have secreted 

 these layers in the manner described, other glands covering the 

 whole external surface 

 of the mantle line the 

 shell thus formed with 

 the inner pearly layer. 



When the seasons, 

 temperature or food 

 supply are unfavor- 

 able to rapid growth, 

 the mantle does not in- 

 crease in size. The 

 glands which secrete 

 the shelly matter con- 

 tinue active, however, 

 and their deposit forms 

 an extra thickness on 

 the shell already 

 formed, especially around its edge. With the return of con- 

 ditions favorable to the growth of the animal the mantle 



Fig. 91. — The quahog, Venus mercenaria Linne, 

 showing how age may be reckoned by counting 

 the more prominent growth lines. Three and a 

 half years old ; three inches long. At the same 

 time each year a notch was filed into the edge 

 of the shell. (Redrawn from Belding.) 



