7A 



c. The Shell and Palettes. 



The Shell.- The nov/ly-attachcd "Ship-worm" larva por.:;os- 

 ses a typical bivalve shell. The valves are equal and uni- 

 ted dorsally by a well-developed hinge apparatus. The 

 shell in side view is v/ider than long; the transversal 

 diameter is about equal to the longitudinal. The ri.^ht 

 valve (fig. 11) bears three equal hinge teeth; the left, 

 two. Dorsal to the teeth is an external hinge ligament. 

 In either valve, the apophyses of l-ater stages is present 

 as a rudiment. Up to this time grov/'th has taken place 

 along concentric lines. From this time on, rapid, very 

 iinequal growth in different parts of the valves causes a 

 sudden transforniiat ion of the shell, which becomes very 

 different from that of the typical bivalve. The initial 

 stages in this change are shown in fig. 14, v^hich is an 

 anterior view of the shell of a "Ship-worm" which has been 

 in the '.vood a day or less. After growing a small amoixnt , 

 the anterior border has cemented to it a row of teeth 

 which have been secreted separately in small pockets in 

 the epithelium of the anterior edge of the mantle. The 

 first row of teeth, as well as those formed through life. 



