224 



diameter of the burrow. Thus the external sculpture of the shell is adapted in detail to 

 its peculiar function of boring in wood. 



The two valves are in contact with each other dorsally and ventrally b>' two 

 specialized knobs. The anterior and posterior adductor muscles (fig. 81), opposing 

 each other, enable the shells to rock back and forth upon these two points of contact, 

 executing a movement about the dorso-ventral axis, instead of the simple opening 

 and closing movements of which alone most bivalves are capable. Thus a relatively 

 slight modification of the shell enables Teredo, without change of musculature, to 

 execute an entirely new movement. The valves of the shell are able to move back 

 and forth with the alternate contractions of the anterior and posterior adductor 

 muscles, somewhat like the jaws of a clam-shell dredge. The edges of the valves, 

 of course, are never brought together, but gape widely in front to permit the pro- 

 trusion of the foot even when the anterior adductor is tightly contracted. The move- 

 ment of the valves is limited, the edge of each one being able to describe an arc of 

 from twenty to thirty degrees, varying somewhat with the individual specimen. 



With this understanding of the method of movement of the shell, the reason 

 for the peculiar de\elopment of the auricle becomes apparent; its purpose is to serve 

 as an attachment for the posterior adductor muscle, which is thus able to function 

 more efficiently because of the increased leverage obtained by means of this backward 

 extension of the shell. The margin of the auricle is usually slightly flexed outward, 

 so that its edges do not cut sharply into the viscera when the backward margins of 

 the valves approach each other on contraction of the posterior adductor. 



The posterior adductor is a large, powerful muscle, which has its attachment over 

 nearly the entire inner surface of the auricle. Its structure is exceedingly firm and 

 compact, fitting it to exert a strong tension on the shell. In sagittal section (fig. 81) 

 the area of the posterior adductor is nine times that of the anterior adductor. Near 

 its attachment to the shell the posterior adductor becomes expanded, so that in cross 

 section its area may be as much as thirteen times that of the anterior muscle. As the 

 length of the posterior adductor in the contracted state is about three times that of 

 the anterior adductor, its volume is accordingly at least thirty times that of the 

 latter. The coarse, granular nature of the posterior muscle and, in the living animal, 

 its distinctly reddish color, indicate that it is adapted to perform powerful con- 

 tractions. In general, muscles which perform repeated and strong contractions are 

 darker in color than other muscles, a typical example of this being the vertebrate heart. 



The great development of the posterior adductor of Teredo relative to the size 

 of the shell is seen from figure 82, in which the areas of attachment of the shell muscles 

 of Teredo are compared with those of Mya. 



The foot of Teredo, although not, as some have supposed, the actual tool of boring, 

 nevertheless plays an important role in the boring process. This organ, in a contracted 

 state, as seen in figure 80, is a subcircular disc, the periphery of which is thrown into 

 folds and is raised somewhat above the plane of the central area. A median notch 

 in this wrinkled periphery, dorsal and ventral, marks the division between the muscle 

 insertions of the right and left sides. 



In sagittal section (fig. 81 ), several differences are to be noted between the central 

 disc of the foot and this peripheral area. The former is relatively smooth, non-ciliated, 

 non-glandular; the latter is folded and wrinkled, strongly ciliated, and characterized 

 by numerous close-set, deeply staining gland cells. The interior of the foot, with the 

 exception of the muscular portion discussed below, is of an open, spongy texture, 

 made up of loosely arranged connective tissue cells which permit an easy movement 



