THE WORM DISEASE IN OYSTERS. 113 



free from them — at least those which I kept under observation were so. If 

 the vessel containing them were disturbed, the diseased oysters were the first 

 to close and the last to open. The sensitiveness will tend to deprive them 

 of a large quantity of food. In addition there are the worms placed in the 

 current which carries the food to the oyster, and which in bad cases may 

 number from twenty to thirty, each feeding on the food drawn from the 

 supply of the oyster. 



Evidence as to Horing^from an examination of the Shell. 



One frequent appearance of the interior of the valves tells very forcibly 

 against the boring theory. In many cases the worm occupies an elevated 

 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 4, 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 4, 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 semi-circular 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 semi- 

 circle, 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 tv/o 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 carf ully 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 deposit, and the condition 

 of the surface upon which the worm rests — the difterent 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, has by reason of the constant move- 

 ments in and out, a tendency to wear away the surface. 



Whether this is accomplished by strictly mechanical means, or by a corro- 

 sive acid, I am unable to say ; but the fact remains that it is worn away. If 

 the worm has been long in the shell, the grooves formed are deep, and the 

 longer they remain the deeper they become. "VVheu measured from the out- 



