204 
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 ter- 
minates in a small double fold, forming a cap, on the inside of which 
are fixed the long small stems of the two opercula, which become broad 
and flat towards their other extremity. These, when brought together, 
shut up the shell, and enclose the two contracted tubes within it : not 
one operculum corresponding to each tube, but in a transverse direc- 
tion. In the Teredo gigantea, the opercula are similarly situated, 
each shutting up one half of the bifurcation. 
The Teredo gigantea is found imbedded in a different substance from 
that in which the Teredo navalis is found, and may have many other 
characteristic differences ; although it appears, from comparing the 
shells in which they are incased, that they are formed of exactly the 
same materials. 
The Teredo gigantea, when arrived at its full growth, closes up the 
end of the shell. This, the Teredo navalis does also. In some of the 
specimens of Teredo gigantea the shell is just covered in, and that 
part close to the termination is extremely thin, but in others it is in- 
creased in thinkness twenty fold : in others, again, the shell has not only 
become thick, but the animal has receded from its first enclosure, and 
has formed a second three inches up the tube, and afterwards a third 
two inches on ; and has made the sides thicker and thicker, to diminish 
the canal in proportion to the diminution of its own size. 
These facts prove, that the Teredo gigantea, when arrived at its full 
growth, orwhenever prevented from increasing its length, closes up the 
endof its shell, and lives a longtimeafterwards, furnished with food from 
the sea -water it receives, like the actinice. The Teredo navalis closes 
up its shell in the samemanner : it must, therefore, afterthat period, be 
supplied with food entirely through the medium of sea-water*. 
Whilst treating of serpulas we found, that in those shells a similar pro- 
* Observations on the Shell of the Sea-worm, &c. by Everard Home, Esq. Philosophical 
Transactions, 1806. 
