230 



length, and from 0.02 to 0.08 mm. across. On the other hand, minute particles 0.02 

 mm. or less in length and of about the same breadth, occur numerously. While many 

 particles of intermediate size indeed occur, one is able to differentiate readily the two 

 classes of particles; elongate, fibrous ones and fine, granular ones. The conclusion 

 immediately suggests itself that the latter are produced by the fine serrations of the 

 anterior lobe of the shell, -while the former are rasped off by the coarser denticulations 

 of the anterior median area. 



The position assumed by Teredo in boring off at a right angle to its former direc- 

 tion of progress is seen in figure 86. One would hardly have supposed that such an 

 awkward position is assumed by the animal in changing the course of its burrow, 

 were this not actually a matter of observation. Apparently the valves function 

 without difficulty under such circumstances. This explains how Teredo is able to 

 make the abrupt changes in the course of its burrow which are so often found, es- 

 pecialh- in heavily riddled piling, where such changes of direction are necessary in 

 order to avoid breaking through into neighboring burrows. 



Fig. 86. Boring position of Teredo beginning a side passage at a right angle to former course of 

 burrow. 



The commonly made statement that one teredo will never bore into or cross 

 the burrow of another is not, strictly speaking, true. Occasional instances have 

 been found in which one burrow passes directly through another. Such cases are rare, 

 however, and it is probable that the first animal was dead before the second entered 

 its burrow, as otherwise it would doubtless ha\e been able to protect itself by a 

 thickened wall of nacre. In heavily riddled timbers the teredos will sometimes adopt 

 imusual expedients, such as to cross a crack of considerable magnitude in order to 

 find new wood to attack. One instance was obser\-ed in an aquarium where a teredo 

 had bored completely through a piece of wood, so that its shell and the anterior part 

 of the body protruded into the water. This animal was doubtless abnormal. 



The foot appeared, from observation, to be an organ of sense and a means of 

 limited locomotion about the narrow confines of the burrow. The animal under 

 observation turned from left to right and back again a number of times, as though 

 exploring the wall of the burrow with the foot, before it made any boring movements. 

 Probably thus in some way Teredo becomes aware of the proximity of neighboring 

 burrows, so as to turn aside ana avoid them. In executing these turning mo\ements, 

 the foot several times passed directly across the glass, so that the manner in which 

 it functioned could be well studied. The organ was first flattened against the surface 

 of the glass, and its margins spread out so that they extended beyond the edges of 

 the shell. Then, apparently by contraction of the retractor muscles, the margins 

 were rather suddenly drawn in and the central disc of the foot slightly cupped, ob- 

 viously constituting a sucker. After each contraction the foot sought a new attach- 

 ment, nKA'ing laterally about 0.5 nmi. and hitching the shell along to a new position. 

 Thus the turning of the shell was really accomplished by a sort of crawling around 

 the burrow with the foot. 



