7 
a calcareous shell of extreme delicacy and pearl-like whiteness. 
The composition of this lining has been found to contain no phos- 
phate of lime, but to consist of 97 parts of carbonate of lime and 
three parts of animal matter. When the animal has attained its 
maturity it closes up the inner end of this tubular shell, but the 
external orifice remains open for the supply of nutriment through 
the siphon. The instinct of self-preservation is further displayed 
by the Teredo when the wood which it has tunnelled becomes 
decayed, as in the case of piers, piles, or wrecks, so that the 
animal is in danger of exposure. The delicate shell which was 
deposited as a lining to the burrow, and which was of the thinnest 
possible material, is gradually but rapidly thickened by successive 
layers of calcareous matter on the inside, until it becomes exactly 
like the substantial tube constructed by Serpula contortuplicata ; 
and I have in my collection portions of an old wreck in which the 
tubes (which contained living animals when taken from the water) 
are quite exposed by the disintegration of the wood in which they 
had originally been imbedded. 
Like most other molluscs, the embryo Teredo begins life as a 
swimming animal, furnished with a shell from which are protruded 
ciliated lobes. By means of this apparatus it is enabled to move 
through the water with great activity. It rapidly passes through 
several changes of form, and is ultimately found furnished with a 
foot by which it can crawl. Having now fastened upon a piece of 
submerged timber, whether it be the hull of a new vessel, the 
remains of an old wreck, or a solid pile supporting a pier or 
forming an embankment, it begins to drill a hole which is rapidly 
extended far into the substance of the timber, and when a piece of 
wood is attacked by a number of these marauders simultaneously 
it is rapidly destroyed. 
The hole by which the creature obtains an entrance is no 
larger than would be made by a fine pin, but, as the burrow is 
extended, its diameter increases with the increasing size of the 
body of the Teredo which is at first developed rapidly, but, after 
attaining a diameter of about a quarter of an inch, there is little 
or no increase in this direction, but the animal still continues to 
grow in length, and is frequently found as much as two feet long. 
Ordinarily the length of a mature Teredo is about 12 or 14 inches. 
The danger resulting from the inroads of this destructive 
creature is enormously increased by the fact that, beyond the 
minute opening through which it entered there is no external indi- 
cation of the mischief which is being wrought within; a remark- 
able instinct guiding the Teredo to avoid penetrating the surface of 
the timber, and (which is still more extraordinary) to steer clear of 
neighbouring tunnels. The habit of the Teredo is, generally, to 
strike into the grain of the wood and follow its course. Thus a 
plank or a balk of timber may appear perfectly sound, and yet be, 
