112 THE WOKM DISEASE IN OYSTERS. 



were reduced to a mere skin, and ntterly incapable of secreting any shelly 

 matter. The effect of these blister-like structures, which increase in number 

 and size as the disease progresses, is to practically fill up the whole of the 

 lower valve and to bulge out the upper, so that there is no room left for the 

 oyster. 



In all cases the recently collected mud is of a light brown colour, and was 

 found to be the work of young worms varying in length from one-eighth to 

 half an inch, the patches of mud on the larger examples varying from one to 

 one and a half inch in length, by one-half to three-quarters of an inch wide. 

 In most of the examples mentioned the surface ot" the mud was covered by 

 a thin pliable membrane. The mud surrounding the adult worm is usually 

 more compact and darker in tint, often inclininir to slate colour ; whilst the 

 mud which the worms have left is frequently black. No doubt it is partly 

 due to the decomposition of this black mud that so many oysters die. The 

 parts of the oysters overylying these putrefying patches are always discoloured 

 by yellowish, spots. 



When the habits of the young worm are considered in connection with the 

 evidence derived from the examination of oysters, in which the worm has 

 just established itself, it points to the conclusion that the larvae simply swim 

 into the open shell ; and there is no evidence of any boring having taken 

 place from without from the fact that the place occupied by the worm is 

 quite smooth, and even in those cases in which the worm is full grown, the 

 surface is often devoid of any grooves. It is only in old-established cases 

 that grooves and tubular openings are found, and there they only exist on 

 the margins as a rule. The above remarks apply to the old or thick parts of 

 the valves ; the newer thin deposit over the mud, as before mentioned, 

 merely exhibits the irregularities of the surface over which it is laid whilst 

 in a soft pliable condition, and is usually without any trace of grooves, 

 except near the margin. Even these grooves, when examined with a lens, 

 show a mould of what was beneath, without exhibiting any signs of having 

 been bored. Another feature is the entire cavity occupied by the mud and 

 worm, which cannot be accounted for by the boring theory. If the worm 

 bores into the substance of the shell how are blister-like cavities formed ? 

 It is not reasonable to suppose that the w^orm has the power of raising 

 a rigid laver of shelly matter and forming a blister. To do this the layer 

 must be rendered pliable, otherwise there would be evidence of such raising 

 in the shape of cracks, &c. If the blisters are formed by the disintegration 

 of the shell, there ought to be some evidence on the inner surfaces ; but 

 there is nothing to show that disintegration had taken place. One surface is 

 comparatively smooth, and the other a perfect mould of the enclosed mud. 



Is is not more reasonable to suppose that the upper layer is deposited 

 over the mud whilst in a soft state, simply covering the mud and worms, than 

 to suppose that the worm bores into the shell and then forms the blister ? 

 If the blisters were formed by the disintegration of the shell, there ought to 

 be some variation in the thickness of the layer, inasmuch as the disintegration 

 would be unequal, and be most evident immediately over the worm. 8uch, 

 however, is not the case ; the deposited layer is pretty uniform in thickness 

 over each blister. 



The death of the oyster is brought about chiefly by the decomposition of 

 the mud after the death of the worms ; but no doubt the imperfect closing 

 of the valves has its effect. In all cases in which the worms are numerous, 

 the edges of the valves are defective, from the fact that the worms occupy 

 the edge, and that the shelly deposits are used in lining the shell. Oysters 

 that are infested with worms are much more sensitive than those which are 



