48 



occupies an elzv .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 6, fig. 5). Instances of this kind can only be explained 

 by supposing that the worm and the mud have been enclosed by 

 the slielly matter deposited by the oyster. 



There appear to be three well marked stages in the appearance 

 of sections of the shell when viewed fi'om the outside and looking 

 into the ends of the tubes. (See Plate 6. figs. 7, 8, 9.) 



In the first stage we have the fiattened solid part of the 

 shell upon which the worm rests. Innnediately over this is 

 the thin layer formed by the oyster, wliich 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 fiattened (-Hg. 8). In the third stage 

 the grooves are so sunken in the basal surface that they appear 

 somewhat like a kf^yhole, 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 n)argin, 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 detennines or limits 

 the extent of the n.uddy 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 movements in and out, a 

 tendimcy 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 



