43 



terior adductor oi' other Laniollibranchs , or witii both com- 

 bined. It was Grobben who established the homology of the 

 muscles w'len he discovered (5) the two small anterior ad- 

 ductor, which had been overlooked before. 



In the general transformation of the larva into the 

 "Ship-worm", the ligament, which in the larva opposes the 

 tv/o adductor muscles comes to serve only to keep the two 

 valves from separating from each other. And the two mus- 

 cles, which in the larva oppose the action of the liga- 

 ment, to close the sliell,come to cause the two valves to 

 svfing upon each other on the dorsal and ventral knobs of 

 the shell valves, during the process of boring. So the 

 tvio adductors become antagonistic to each other. 



In the newly-attached laiva, both adductors are pres- 

 ent, the posterior (a.jo. figs. 2,24) already considerably 

 larger than the anterior (a. a) . Both are attached in the 

 concavity of the shell valve and well towards the dorsal 

 side. In the general transformation, the adductor muscles 

 as the active mechanical agents in boring, undergo consid- 

 erable change. The posterior, as the one that really does 

 the work, becomes very large (figs. 7,9,10), and passes pos- 

 teriorly to be attached to the outwardly turned edges of 



