227 



direction of the points of the denticles on the shell, and from the extraordinary de- 

 velopment of the posterior adductor muscle. As the backward stroke of the \'alves is 

 completed, the foot is relaxed and ils m.iri^ins spread out so tar that they o\erlap the 

 edges of the shell. Then, by a sudden contraction, the margins of this structure are 

 drawn in antl the valves brought forward into position for a new stroke. Then follows 

 the slow, labored contraction of the large posterior adductor, causing the forward 

 edges of the valves to spread apart and rasp the w-ood by their outward thrust. That 

 the valves do actively scrape the wood on this stroke is indicated by the fact that the>- 

 were observed frequently to slip, the backward margins being drawn together with 

 a jerk instead of the usual slow, steady pull. 



The boring movements occurred rhythmically, from 8 to 12 times a minute. It 

 was not possible to measure accurately the length of the stroke, but experiments on 

 other individuals have shown that the maxinmm stroke possible allows the forward 

 edge of each valve to move through an arc of from 20 to 30 degrees. 



The anterior tip of the burrow is mined out by the anterior lobe of the shell; the 

 movement of the shell is necessarily in a direction longitudinal to the ridges of this 

 area, so that their serrate edges act upon the wood as so many small saws. It will 

 be recalled that the serrations on the ridges of the anterior area are extremely fine 

 (fig. 80) as compared with the denticulations of the anterior median, and hence better 

 fitted to act as the advance boring edges. Also the shape and position of the anterior 

 lobe especially adapt it to working in the extreme tip of the cupped end of the burrow. 

 The marks of the work of the anterior lobe of the shell are often plainly evident on the 

 wood (figs. 84 and 85). 



Fig. 83. Boring position of Teredo in end of burrow. 



While the ridges of the anterior lobe are working saw-fashion in the tip of the 

 burrow, the coarser wedge-shaped teeth of the anterior median area are at the same 

 time working rasp-like, enlarging the diameter of the burrow and advancing the peri- 

 pheral portion of its cupped extremity. Thus, w-hile the tools might be compared 

 to saw and rasp, their work is in effect that of a drill and reamer. The marks of the 

 work of the anterior median area are rarely to be distinguished clearly, owing, no doubt, 

 to a tendency of the coarse, close-set denticulations of this area to abrade and tear the 

 macerated surface of the wood rather than to leave a clean-cut impression, as do the 

 finely serrate ridges of the anterior portion of the shell. 



The disposal of the rasped-off particles of wood it was not possible to observe, 

 because of their minute size. There is, however, every reason to believe that it is the 

 function of the cilia of the periphery of the foot to sweep these particles up and into 

 the range of the cilia of the oesophagus. Apparently all of the rasped-off w'ood passes 

 through the digestive tract, which is continually found to contain wood particles in 

 considerable quantities. 



It was observed in studying the particles of wood in the digestive tract that these 

 are of two sorts. Large particles stand out conspicuously, from 0.30 to 0.40 mm. in 



