206 BULLETIN OF THE BUREAU OF FISHERIES. 
of the posterior region has formed a duplicature (fig. 8, 9), which projects like a 
collar over the base of the siphons. At the anterior portions of the sides of the 
space thus formed the epithelium of either side evaginates forward. In the 
pocket thus formed the handle of the pallet is secreted and projects backward 
into the “collar” space. The lining walls of the sides of the ‘‘collar” space secrete 
the funnel-shaped pieces which are cemented to the handle. New larger pieces 
are added in succession at the anterior end, and those first formed may be broken 
off. In species of Teredo the paddle part of the pallet is a solid piece and not 
divided into a series of pieces, as has just been described for Xylotrya. Strictly 
speaking, the segments of a pallet are semicircular when seen on end. When the 
two pallets are brought together in closing the outer opening of the cylindrical 
tube they thus form a truncated cone. 
MANTLE AND SIPHONS. 
In the adult ship-worm (fig. 5) the mantle forms a very long and very delicate 
tube, which stretches from the anterior edge of the shell to the base of the siphons. 
These latter are modifications of the mantle, as in other forms of lamellibranchs. 
The tube is open only at the anterior end, the pedal opening for the protrusion of 
the foot; and at the ends of the siphons, the inhalent and exhalent openings. It 
was formerly a much debated question how much of this tube should be considered 
body proper and how much siphons. In the light of present knowledge it is easy 
to see that the muscular collar marks off the end of the body and the beginning of 
the siphons. 
The mantle in ship-worms has undergone more differentiation, perhaps, than in 
any other lamellibranch. The anterior edge is thickened, as in other forms, and 
secretes the teeth, the edges of the valves, and the epidermis. The very delicate 
part underlying the shell, and stretching beyond it to the siphons, secretes the 
inner layers of the shell and the calcareous tube lining the burrow. Within the 
shell cavity induplicatures secrete the apophyses, and at the same time absorb 
part of them as they change shape and position. Also, other parts secrete the two 
pairs of knobs on the valves. The posterior edge of the shell is not marked by the 
thickened mantle edge as in other types, but the mantle forms a duplicature around 
the whole posterior edge of the shell, which stretches forward. On the dorsal 
side the whole umbonal region of the valves is covered by this duplicature, and to 
this special part Quatrefages gave the name of ‘‘cephalic hood” (c h, figs. 9, 26). To 
it he assigned the function of forming the burrow. It is somewhat, but not very 
muscular, and no such important work could be done by it. It probably serves 
as an elastic washer around the ‘‘head,’’ which prevents the fine particles of wood 
grated off by the shell from passing posteriorly to lodge between the mantle and 
calcareous tube. 
In the collar region it has been seen that the mantle forms the duplicature or 
collar, which projects posteriorly over the base of the siphons, and within the cavity 
of which the handles and paddles of the palettes are formed separately (fig. 9, 10). 
The siphons, specializations of the mantle, form two long tubes (fig. 5, 6, 9) whose 
walls are slightly fused together through half or more of their extent. The anal 
