and Teredo Navahs. 
281 
From the middle of the exposed part of the head, projects 
a kind of proboscis : which in the living animal has a vermi- 
cular motion : its extremity is covered by a cuticle of a convex 
form, not unlike the cornea of the eye. When this is removed, 
the cavity immediately under it is found to contain a hard 
brown-coloured gelatinous substance, of the form of a Flo- 
rence flask, with the large end upwards. As this proboscis 
has no orifice in it, there is reason to believe that it adheres 
to the wood, acting as a centre bit, while the animal is at 
work with the shells ; and by this means the canal in the 
wood is so perfectly cylindrical. 
The mouth of the animal is nearly concealed by the pro- 
jection of the proboscis, but when exposed is a very distinct 
round orifice ; between the proboscis and the large digastric 
muscle. 
The body of the worm is inclosed in one general covering, 
extending from the base of the boring shells, with which 
it is firmly connected, to the root of the two small tubes, 
which appear out of the wood. It terminates in a small double 
fold, forming a cup, on the inside of which are fixed the long 
small stems of two opercula, which become broad and flat 
towards their other extremity. These, when brought toge- 
ther, shut up the shell, and inclose the two contracted tubes 
within it : not one operculum corresponding to each tube, but 
in a transverse direction. In the teredo gigantea, the opercula 
are similarly situated, each shutting up one half of the bifur- 
cation. 
At the base of tins cup the general covering is thick, and 
ligamentous, for about £ of an inch in length, where the 
stems of the opercula are connected with it ; and at one spot 
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