226 



of bod\ fluids among them for the purpose of moclifying the turgor of the foot in process 

 of extension or retraction. 



The muscles of the foot consist of a pair of anterior retractors, a pair, or rather a 

 series, of posterior retractors, and a pair of protractors. The anterior retractors, ex- 

 tending dorsally from the foot, ha\e their insertion in the anterior umbonal region 

 of the shell. The posterior retractors, extending as wide, semicircular bands from the 

 margins of the foot, are attached to the apophyses, which, extending \entrall\- from 

 beneath the dorsal knobs of the shell, almost encircle the foot. The protractor muscles, 

 wliich function in extending the foot by compressing the anterior portion of the body, 

 have their insertions near those of the anterior retractors. Their relatively small size 

 as compared with the wide bands of retractors, suggest that the foot is extended as 

 much by turgescence as by muscular action. 



In specimens of Teredo removed from their burrows and kept in open dishes in 

 the laboratory, the foot has sometimes been seen distended to a distance of two or 

 three millimeters beyond the margins of the shell, and extremely turgid, so as to be 

 almost transparent. Different areas of the foot appeared to be independently movable, 

 so that the organ could be compressed on one side and expanded on the other or rolled 

 about somewhat amorphously, and extended in an\- direction. All the movements 

 observed were such as could be produced by the combined action of turgor and con- 

 traction of the semicircular bands of retractor muscles. 



Post. Add. 



Posk.Add. 



AnhAdd. 



•Ant. Add. 



y^/z 



Fig. 82. Relative areas of attachment of the anterior and posterior adductor muscles of Teredo 

 navalis (left), and Mya arenaria (right). 



In order to determine if possible by direct observation the movements involved 

 in boring, the experiment was tried of opening a burrow carefully near the anterior 

 end and sealing over the opening a glass cover-slip, thus making a small window in 

 the burrow, through which the movements of the occupant could be observed with 

 the aid of a binocular microscope and a narrow shaft of light. Most of the animals 

 disturbed in this fashion would retract about a third of the length of the burrow, so 

 that they were entirely out of sight from the window, and after a few days' quiescence 

 they woidd bore off in another direction. But after a number of repetitions the ex- 

 periment finally proved successful. One specimen was found which carried on boring 

 operations directly in view of the small glass window, and the process was observed 

 in considerable detail. 



The boring is accomplished by rasping with the valves, which are held in position 

 by the combined action of the foot, attached to one wall of the burrow, and the dorsal 

 fold of the mantle, distended b\' turgor, pushing against the opposite wall. The typical 

 boring position is seen in figure 83 (r/. also figure 84, 1 and 2). 



At each stroke of the ^•al^•es the foot takes a new hold. The effective stroke of the 

 valves is the outward and backward one. This had been already assumed from the 



