NATURAL HISTORY OF THE SHIP-WORMS. 205 
of the valves causes a sudden transformation of the shell, which becomes very 
different from that of the typical bivalve. The initial stages in this change are 
shown in figure 14, which is an anterior view of the shell of a ship-worm which has 
been in the wood a day, more or less. After growing a small amount, the anterior 
border has cemented to it a row of teeth which have been secreted separately in 
small pockets in the epithelium of the anterior edge of the mantle. The first row 
of teeth, as well as those formed through life, are cemented to the shell so as to point 
almost straight outward and very slightly backward. The apophysis, present as 
a small rudiment in the larval shell, has grown out into the shell cavity, pushing the 
mantle before it; and, in this very rapidly attained stage, is almost as large com- 
paratively as in the adult. Meanwhile the ventral edge of the valve has grown 
rapidly, and there have been formed on the dorsal and ventral portions the two knobs 
upon which, in this and subsequent stages, the two valves swing during the mechan- 
ical process of boring. During these changes the hinge teeth present in the larval 
shell have disappeared, probably by absorption. The valves which, during larval 
life, have swung at the hinge so as to open and close the shell cavity on the 
ventral side, come to swing upon the knobs along a median transverse axis vertical 
to the main axis of the animal. The greater growth of the valves on their ventral 
edges causes them to gape before and behind for the protrusion of the foot in front, 
and the siphons, later the body, behind. 
Growth of the valves continues with great rapidity. The left valve of a speci- 
men 1 mm. Jong is shown in oblique view in figure 15. The chief features that have 
been introduced are as follows: The point of greatest growth is on the ventral 
edge. The lines of growth, and hence the rows of teeth, are wider apart on the 
dorsal half of the anterior border than on the ventral. In this way an angle is 
formed in the anterior edge, which soon (fig. 16) becomes a right angle. Meanwhile, 
the posterior border has grown rapidly and flares outward so as to give better 
purchase for the posterior adductor muscle during its contraction. Likewise, a 
much smaller portion of the dorsal anterior edge flares outward for the attach- 
ment of the anterior adductor (fig. 18). 
; There is little modification in form, structure, and relations of the shell after 
the stage shown in figure 16, which is a side view of the left valve from a specimen 
5 mm. long. As growth on the ventral edge takes place, the knob is constantly 
being added to toward the midline and absorbed on the side toward the con- 
cavity of the valve. And as growth at the posterior border takes place, the 
posterior adductor muscle is constantly moving backward. Jn the larval and 
subsequent stages, the whole shell, including the teeth, is covered externally by 
delicate epidermis. 
PALLETS. 
These structures are peculiar to the ship-worms and have been acquired for 
closing the outer ends of the burrow against intruders and for other purposes. The 
structure of one of these-is shown in figure 20, which represents the left pallet of a 
specimen 5 mm. long. It consists of a series of seven funnel-shaped structures 
which have been formed and cemented, in succession, to the handle. The forma- 
tion of the pallet is as follows: In specimens still less than 1 mm. long, the mantle 
