208 BULLETIN OF THE BUREAU OF FISHERIES. 
new vesicles are formed as outgrowths from those already present. What the func- 
tion of the gland is has not been determined, but its position indicates that it may 
be the secretion of some material noxious to enemies that may get into the end of the 
ship-worm burrow. 
MUSCULAR SYSTEM. 
Early in this century it was a much debated question whether the muscle then 
known in the shipworms was homologous with the anterior or the posterior adductor 
of other lamellibranchs, or with both combined. It was Grobben who established 
the homology of the muscles when he discovered (i888) the small anterior adductor, 
which had been overlooked previously. 
In the general transformation of the larva into the ship-worm the ligament, 
which in the larva opposes the two adductor muscles and opens the shell, comes to 
serve only to keep the two valves from separating from each other. And the two 
muscles which together, in the larva, oppose the action of the ligament and close 
the shell come to cause the two valves to swing upon each other on the dorsal and 
ventral knobs of the shell valves during the process of bormg. So the two adductors 
become antagonistic to each other. 
In the newly attached larva both adductor muscles are present, the posterior 
(ap, fig. 7, 22,) already considerably larger than the anterior (aa). Both are 
attached to the valves within the concavity of the shell and well toward the dorsal 
side. In the general transformation these muscles, as the active mechanical agents 
in excavating the burrow, undergo considerable change. The posterior adductor, 
as the one that really does the work, becomes very large (fig. 6, 9, 10) and passes 
posteriorly to be attached to the outwardly turned edges of the shell (fig. 15-17), so 
as to*give it better purchase during its contraction. The anterior adductor muscle, 
whose only work is to bring the shell valves back to their original position, after 
contraction of the posterior adductor, is comparatively very small (fig. 6, 9, 10, aa), 
and moves forward from its original position in the larva to be attached to the out- 
wardly turned antericr edges of the shell valves. 
In minute structure all of the muscle fibers of both adductors are apparently 
striated, due to a more or less regular deposit of granular material on their surface. 
This structure seems to support the view as to the function of the two parts of 
the adductor in forms like Pecten, where one part is tendinous and is supposed to 
prevent the shell valves from separating too far. The other part, composed of 
striated fibers which contract quickly, is for active adduction of the valves. In 
ship-worms, 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 which is sup- 
posed to serve for active adduction. 
The pedal muscles in the larva are those typical of lamellibranchs with a foot. 
A pair of anterior retractors and a pair of protractors of the foot are attached in the 
anterior umbonal region of the shell valves; and a pair of posterior retractors, in the 
posterior umbonal region, anterior to the attachment of the posterior adductor 
muscle. With the remarkable growth of the apophyses of the shell, the posterior 
retractor muscles suddenly lose their old attachment in the umbonal region, to 
become attached to the apophyses through almost the whole length of the latter. 
