NATURAL HISTORY OF THE SHIP-WORMS. 199 
itself a constantly thickening calcareous tube which lines the whole burrow except 
the extreme anterior end, where the mantle is somewhat less delicate and where 
the tube fades out and the burrow is being constantly enlarged. 
When the young “worm” enters the wood, it penetrates vertically to the sur- 
face, cutting across the grain. It soon bends its course, however, so that within 2 
inches, usually, it takes a straight course in the direction of the grain. Individuals 
that enter the wood on end cut with the grain across the fibers from the start, so 
that their burrows are straight, unless they are turned from their course by obstruc- 
tions of some sort. In such a case the course is changed so that the burrows may 
become exceedingly tortuous. When the ship-worm is in danger of burrowing 
into the tubes of its fellows or into other spaces, if its course can be no longer changed, 
it contracts the anterior part of the body slightly, secretes a closed calcareous 
lining in front of it, and ceases to burrow further or to grow. Otherwise, appar- 
ently, it may grow indefinitely, and it is difficult to say how large specimens might 
become were there not adverse conditions to stop their growth. 
I believe that the calcareous lining of the burrow has been acquired primarily 
for the protection of the very delicate body from the rough surfaces of the wood, 
but it serves other purposes also. It prevents the diffusion of injurious substances 
into the burrow, and also prevents the intrusion of other creatures that live in the 
wood. Then, too, when the surrounding wood decays, or is eaten away by other 
animals inhabiting the wood, so as to endanger the life of the ship-worm, the tube 
may be so strengthened as to serve as the sole means of external protection. In 
this way the walls of the calcareous tubes, which, protected, are usually not over 
a quarter to a half millimeter thick, may become 2 mm. or more thick. This 
response to changed conditions on the outside is a very mysterious one and it is 
difficult to see by what means the animal recognizes its dangers. 
The peculiar mode of life of ship-worms has led to the development of the pair 
of pallets (fig. 20). These are protective structures peculiar to the ship-worms. 
They differ somewhat in the various species, but are essentially calcareous paddle- 
shaped structures, attached one on either side of the posterior end of the body at 
the point of origin of the siphons. In Xylotrya the expanded part of the paddle 
consists of a series of funnel-shaped calcareous structures set one within the other 
upon a cylindrical handle, while in Teredo it is composed of a single piece. The 
handle of the paddle is embedded in a forward evagination of the mantle and the 
expanded part projects freely behind, where, by means of a set of muscles, they 
may be protracted forcibly so as completely to close the outer end of the burrow 
against the intrusion of any enemy from the outside. Also, when the burrow 
extends upward and its opening is more or less exposed at low tide, as sometimes 
happens, the pallets may so hermetically close the external opening as to retain 
the water in the burrow and to prevent the collapse of the body of the ship-worm. 
The action of the pallets is illustrated in figures 35 and 36. When the animal is 
undisturbed and feeding, the pallets are drawn forward and the siphons are 
extended freely into the water, as shown in figure 35. When it is disturbed in any 
way the siphons are contracted very quickly and the pallets forced into the end of 
the tube, as shown in figure 36. 
