222 



CHAPTER XV 



THE BORING HABIT 



The method by which the smooth walled, perfectly rounded burrows of Teredo 

 are excavated has excited considerable interest over a long period of time. As early 

 as the first century A. D. the Roman naturalist Pliny the Elder remarked upon it. 

 His conjecture that boring is accomplished by means of the shell appears to have 

 gone unchallenged until 1733, when the Dutch investigator Sellius objected that the 

 shell of Teredo does not appear adequate to the task of boring, especially in hard 

 woods, such as oak. He believed the boring to be accomplished by a kind of suction 

 with the foot, aided by the action of the water which is taken into and forced out of 

 the burrow. 



The opinion of Sellius precipitated a discussion which has been carried on 

 intermittently for nearly two hundred years. While a majority of writers have favored 

 the \\e\\ that boring is carried on by movements of the shell, not a few have insisted 

 that the burrows are excavated by a slow but continuous wearing away of the wood 

 by friction and suction with the foot. Proponents of the latter theory have stated 

 that the shells are inadequate to boring, that they do not show signs of wear as would 

 be expected if their function were that of rasping wood, that certain forms such as the 

 lim.pets are known to excavate depressions in rocks by means of the foot, and that the 

 walls of the burrow of Teredo are too smoothly polished to have been rasped me- 

 chanically. It is only recently, in this investigation (Miller, 1924a), that the question 

 has been definitely decided, observations of animals in opened b\irrows having shown 

 that boring actually is accomplished by the shell, as Pliny had long ago conjectured. 



The Method of Boring 



Corresponding to its unusual function, the shell of Teredo has been specialized 

 in a very remarkable way. Although the organism is anatomically quite closely related 

 to the common soft shell clam, the early naturalists are hardly to be blamed for 

 placing it in an entirely different group. The shell of the adult Teredo has been reduced, 

 relative to the size of the animal, until it covers only a small portion of the anterior 

 part of the body, the much elongated visceral and branchial portion protruding 

 posteriorly, and having as its protection the smooth, nacre-lined walls of the tapering 

 burrow. The shell itself is an irregularly shaped, sub-globular structure, consisting 

 of two valves, united dorsally by a reduced ligament. The valves gape widely in 

 front (fig. 80) for the protrusion of the foot, and somewhat less widely behind for the 

 backward extension of the long, slender body. 



The terminology used in discussion of the shell has been explained in an earlier 

 chapter and illustrated by the diagram, figure 68, to which reference should again 

 be made. 



The ridges of both the anterior and anterior median areas are equipped with 

 specialized denticles. Those of the anterior lobe are merely fine serrations along the 

 summit of each ridge. The denticles of the anterior median area, however, are rather 

 intricately sculptured, wedge-shaped structures, the points of which are directed 

 outward and backward. They are on the whole regularly spaced, and are much larger 

 and coarser than the denticulations of the anterior lobe. The latter act as the advance 

 boring edges, working in the extreme end of the burrow, while the coarser denticles 

 of the anterior median area work peripherally with a reaming action, enlarging the 



