288 Mr. Home on the Teredo Gigantea 
blood. They turn round in their shell, with which the body 
has no attachment, and with which their covering only has a 
slight connection, at one particular spot, to prevent the ex- 
ternal tubes from being disturbed. This motion of the animal 
is for the purpose of boring. 
Their most striking peculiarities are, having three external 
openings instead of two : the stomach being unusually large, 
and the breathing organs having an uncommon conformation. 
As the teredo gigantea bores in mud, on which it cannot be 
supposed to subsist, or even to receive any part of its nutri- 
ment from it, a question arises whether the teredo navalis (an 
animal of a much smaller size ) receives its support from the 
wood it destroys, or is wholly supplied with food from the 
sea. 
The following observations make the last opinion by much 
the most probable. The animal having red blood, and very 
perfect organs, necessarily requires a great deal of nourish- 
ment for the purposes of growth, and to supply the waste 
constantly going on : but if the aggregate of shell and animal 
substance is taken, it will be found equal in bulk, and greater 
in specific gravity, than the wood displaced in making the 
hole : hence it is obvious that the quantity of wood, it has 
taken into its body, is wholly insufficient for its formation and 
subsequent support. It must therefore have other means of 
subsistence. When once it is established that the worm can 
be supported, independantly of the wood, which is eaten, and 
can afterwards subsist, when the communication between it 
and the wood is cut off, it creates a doubt respecting the wood 
forming any part of its aliment, and makes it probable that 
