242 PROCEEDINGS OF SECTION D. 
shield the animal from predacious intruders. If the siphons fail 
to maintain free communication with the water, the animal must 
at once be choked and perish. 
A part of the skin of the shipworm, technically called the 
mantle, secretes a shelly tube, which lines the burrow. This tube 
is better developed and thicker in some instances than in others. 
Occasionally it seals over the termination of the burrow, con- 
verting the whole into a blind sack; thus conclusively showing 
that the object of the Cobra in boring is to seek shelter, not 
food. The shelly tube is shown in the split log (Plate IX. Ng 
perforated by Vausitoria edaz, as standing out from the mass. 
Some allied bivalves which do not bore in “wood, but burrow in 
sand or mud, construct a similar tube. Hence it has been 
argued by Wright that the habit of constructing a shelly tube 
is older than the habit of boring in timber. These tubes serve 
to protect the molluscs from the attacks of the Sphaeroma, 
which, though it lays them bare, cannot pierce them. When so 
exposed, they are always thickened from within. 
PROPAGATION. 
The breeding of the shipworm has been watched by several 
careful and able observers, of whom De Quatrefages (Ann. Sci. 
Nat. (3) Zool. XI., 1849, pp. 202-226, Pl. IX), Hatschek (Arb. 
Zool. Inst. Universitat Wien. Tihy 1880, pp. 1-44), and Sigerfoos 
(John Hopkins’ University Circular XIV. , 1895, and XV. "1896), 
are the most important. 
In all the species hitherto examined, the sexes are separate, 
ova and spermatozoa not occurring in the same individual. The 
females appear to preponderate over the males. Quatrefages 
remarked of 7’. fatalis (op. cit., p. 35) that out of 100 examples 
which he dissected, not more than five or six proved males. 
On the other hand, Sigerfoos found the sexes in 7’. norvegica of 
about equal proportions (op. cit. XIV., p. 78). 
The course of development usual in marine bivalves is fol- 
lowed. When expelled from the glands, the ova are about one 
five-hundredth of an inch in diameter, and the spermatozoa one- 
thousandth of an inch in length ; the latter possess great vitality, 
and are active and potent for many hours after emission. 
Though we have no direct evidence, it seems safe to conclude 
that the spermatozoa, which are shed in great quantities, are 
inhaled by the female through her siphon, and impregnate the 
ova in the gills. The knots, which are a conspicuous feature in 
the body of a shipworm, are composed of masses of developing 
ova lodged in the branchize. On the fourth day of its existence 
the larva acquires a membraneous shell, the next stage is marked 
by the acquisition of a ciliated membrane, the swimming organ, 
