THE ACEPHALOUS MOLLUSKS, 193 
sheath; it divides itself into two tubes, which the mollusk can 
lengthen or contract at will, One of the tubes carries the 
aerated water, which contains the oxygen necessary for the life 
of the creature, and likewise the organic particles which form 
its food; the other tube carries off the liquid which has been 
deprived both of its air and nutriment, together with all the refuse 
of digestion. All the internal organs of the teredo are placed 
one immediately behind the other, a position which is necessitated 
by the narrowness of the body, and the repeated elongations 
which it is required to undergo. When we think of the havoc 
these little worms are capable of producing, and how they are 
able in a short time completely to riddle with their holes even 
the hardest wood, we are at a loss to comprehend how, with their 
soft bodies, they can possibly do it. 
The teredo deposits greenish yellow eggs, which are spherical. 
As soon as the larva is born, it becomes covered with vibratory 
cils. It swims with facility, rises and falls in the water, seeking 
for wood into which it may penetrate. When it has found a 
piece of wood to suit its convenience, it walks up and down on 
its surface, like a caterpillar. From time to time it opens its 
little valves, as if practising for the mining operations it con- 
templates. At last it fixes upon a place, and makes an incision. 
In due time it has excavated a hole capable of containing half 
its body. The young teredo now covers itself with a coating 
of mucous matter, which, condensing by degrees, at last forms 
a resisting shell. In this covering two or three holes are left 
for the projection of the siphons. About the third day the tube 
has become solid—indeed, calcareous—and it is now the origin 
of the tube of the animal. We cannot see what is taking place 
inside, for this coat is opaque; but if we detach some of the 
young teredos from the wood, at this stage of their development, 
we shall find that there has been secreted a new shell, similar 
in all respects to that of the adult animal.’ It cannot be that 
this shell is the instrument wherewith the perforation is executed, 
for it is very fragile; and it would surely show some signs of wear 
and tear, which is never the case. Some naturalists suppose that 
the teredo secretes a corrosive fluid which acts on the wood ; but 
this theory cannot account for the smoothness of the walls of the 
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