PROCEEDINGS OF SECTION D. 243 
technically called the velum, later the foot, appears as a very 
long and mobile limb. 
The larve are then sufficiently advanced to commence an inde- 
pendent life, and are expelled into the sea through the exhalant 
siphon. Each is about the size of a millet grain, and is enclosed 
in a brown, globose shell. Their appearance at this stage is un- 
like the adult on all points, and is shown by Pl. X., Fig. 3 (after 
Quatrefages), the foot protruded from the parted valves, and the 
velum displayed half-furled. Two means of locomotion are at 
their disposal—swimming and crawling. In swimming, the 
velum is employed; it can be spread over half the shell or en- 
tirely retracted within it. By its means they dart through the 
water like a flash of light. In crawling, the long flexible foot 
is used, when climbing over a piece of timber ia search of a 
lodging their movements are comparable to those of a cater- 
pillar (4). It is not known by what sense they are enabled to 
select timber and to discriminate between it and other sub- 
“stances. Probably it is analogous to a sense of smell. 
In this free swimming stage, which from the economic point of 
view is the critical one, large numbers of the young perish without 
obtaining wood to bore. Sigerfoos remarks (op. cit., 1896, p. 
87) :—“ Though we have no direct evidence as to the time the 
shipworm larva is free-swimming, we may assume, I think, that 
it is at least a month, it may be two. During this time most of 
its energies are expended in locomotion, while after it has be- 
come attached it may devote all its energies to forming its bur- 
row and to secure food for itself, so that its rate of growth is 
very rapid. Coming in contact with the wood, the larva throws 
out a single long byssus thread for attachment, and never again 
_ leaves its place. The velum is lost within a few hours, and the 
transformation of the small bivalve into the shipworm is begun. 
“Almost as soon as the larva has settled it begins to clear 
away a place by means of the ventral edges of the valves of the 
shell. In this way a small pit is formed. But very soon rows 
of teeth are formed in succession on the anterior edge of the 
valves; the small knobs are formed on their umbonal and 
ventral regions, the ligament becomes functionless, and the two 
adductors become antagonistic to each other. The teeth are 
formed independently, and afterwards are cemented to the valves, 
pointed outwards and backwards. While the foot performs a 
cupping action, the posterior adductor contracts, the two valves 
swing on each other by means of the two pivots formed by the 
knobs, and the teeth are brought to bear on the wood, rasping 
away its surface. Twelve days after attachment the young has 
attained a length of an eighth of an inch.” 
(4) Moquin-Tandon. The World of the Sea. Engl. Transl., 5th ed., 1882, p. 193. 
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