282 Mr. Home on the Teredo Gigantea 
of this thickened part, there is an adhesion to the cylindrical 
shell, which is the only part of the animal connected with it. 
There is a depression in the shell pointing out this spot. The 
double fold of the outward covering, that forms the cup, con- 
tains the sphincter muscle, which closes the orifice by bringing 
the opercula together. 
The general covering is composed of two membranes, the 
outer the strongest, and made up of circular fibres, the inner 
much finer, having no fibrous structure. On the back of the 
animal, this covering is firmly connected to the parts under- 
neath, and is there strongest. On the belly it forms a cavity, 
and is thinner. It is every where sufficiently transparent, to 
shew the different viscera through it. 
In examining the internal structure of this worm, the dis- 
section was begun by dividing this covering, and exposing its 
cavity ; into which there are two natural openings : one, that 
of the largest of the tubes above described, by which it receives 
water from the sea : the other a transverse slit under the 
union of the boring shells, ~ of an inch long, opening into 
the space before the mouth. The smaller tube has no com- 
munication with this cavity, nor is there any between this 
cavity and that of the belly ; the viscera having a proper 
covering of their own : but the breathing organs, which are 
attached on the posterior surface of this cavity, have their 
fringed edge, loose, and exposed to the influence of the salt 
water ; so that the larger tube is constantly applying salt water 
to them, and conveying it to the animal's mouth, through the 
aperture for that purpose. 
The abdominal viscera with the head occupy about one 
third of the animal’s length : the breathing organs another. 
