48 
occupies an dev .ted position in the shell, projecting above its 
surface as much as half an inch. The heap of mud surrounding 
such worms is covered by a thin layer of shelly matter, and both 
the entrance and the outlet to the worm-tube stand up at right 
angles to the oyster-shell valve, so that the worm lives within the 
shell completely, and the ends of the tube have no connection 
with the outer water, except when the oyster is open. (See 
Plate G, fig. 5). Instances of this kind can only be explained 
by supposing that the worm and the mud have been enclosed by 
the shelly matter deposited by the oyster. 
There appear to be three well marked stages in the appearance 
of sections of the shell when viewed from the outside and looking 
into the ends of the tubes. (See Plate 6, figs. 7, 8, 9.) 
In the first stage we have the flattened solid part of the 
shell upon which the worm rests. Immediately over this is 
the thin layer formed by the oyster, which forms a semicircular 
outline (fig. 7). In this stage there are no grooves where the 
worm is in contact with the shell. In the second stage the 
basal surface is slightly grooved and the upper layer less of a 
semicircle, and somewhat flattened (fig. 8). In the third stage 
the grooves are so sunken in the basal surface that they appear 
somewhat like a keyhole, and consist of two openings, each 
forming three-fourths of a circle, with a space connecting them 
together. It is the appearance presented in the third stage 
that has led to the idea that the worm bores into the shell. 
At first sight such openings certainly look as if they had 
been bored ; but if the various stages are carefully examined, 
with due regard to the time the worm has been in the shell 
— which may be determined by the colour of the enclosed 
mud, the size of the worm, the thickness of the shelly de- 
posit, and the condition of the surface upon which the worm 
rests — the different phases presented may be traced easily, 
and the only way to get at the facts is to follow up what are 
evidently the early stages of the disease. In the first place the 
worm swims into the open shell, and settling down on the surface, 
near the margin, it at once collects a quantity of mud. The 
oyster, the moment it feels the presence of a foreign body, begins 
to deposit a layer of shelly matter, which determines or limits 
the extent of the muddy patch, according to the rapidity with 
which it is laid down and solidified. At this stage the worm 
rests on a smooth surface, and is covered over by a thin layer of 
shell. The oyster still continues to deposit shelly matter, and 
the growth at the edge tends to force the opening occupied by 
the worm further out. The body of the worm, resting on the 
shell, lias by reason of the constant movements in and out, a 
tendency to wear away the surface. 
Whether this is accomplished by strictly mechanical means, 
or by a corrosive acid, I am unable to say ; but the fact remains 
