44 



the sliell (figs. 15-17) so as to civ-^ it u ttL-r purchase 

 during its contraction. Tho antaripr, whosn only work is 

 to bring tiio shell valves back to tlieir original position, 

 after contraction of the posterior, is comparatively very 

 small (figs, 7,9,10, a_.a. ) , and moves forvrards from the 

 position in the larva, to be attached to the anterior, 

 outwardly turned edges of the shell valves. 



In minute structure , all of the muscle fibres of 

 both adductors are apparently striated, due to a more or 

 less regular deposit of granular material on their surface. 

 This structure seems to su.pport the viev/- held of the tv/o 

 parts of the addiictor in forms like Pecton. One part is 

 tendinous and is supposed to prevent the shell valves from 

 separating too far. The otlier part is for active adduc- 

 tion of the valves. In Teredo, where it is not necessary 

 to oppose the action of a hinge ligament, all parts of 

 both muscles are of the same character as that part in 

 Pecten w'nich is supposed to serve for active adduction. 



The pedal muscles in the larva are tb.ose typical of 

 Lamellibranchs with a foot. A pair of protractors of the 

 foot are attached in the anterior umbonal region of the 

 shell valves; and a pair of retractors, in the posterior 



