45 



most favourable spot by the current of water drawn in by the 

 oyster. If so, then we have the expLiuation why it is that 

 the anterior margin is more often infested than any otlier part of 

 the sheU. The worms appear to have the power of collecting a 

 large quantity of mud in a very short time. Some which I kept 

 in contiuement in moderately clear water added fully one-quarter 

 of an inch to the lengtli of their tubes in about an hour, and I 

 have frequently removed the projecting tubes at night, and in the 

 morning they had been repaired and projected beyond the edges 

 of the shell fully half an inch ; so that a vigorous young worm 

 on entering an oyster can soon accumulate a large quantity of 

 mud, which is immediately covered over by the oyster with a thin 

 layer of shelly matter, and if the oyster is healthy, the deposit 

 is laid down quickly, contining the worm with its patch of mud 

 to a very small space. On the other hand, if the oyster is 

 unhealthy and already infested, the shelly deposition is slower 

 and the worm collects a large patch of mud before the layer is 

 solidihed. Hence it is that the size of these accumulations of 

 mud get larger as the worms increase and the oyster gets weaker. 

 In some very severe cases the whole of the lower valves were 

 covered with freshly collected mud, and the oysters were reduced 

 to a mere skin, and utterly incapable of secreting any shelly 

 matter. The etfect of these blister-like structures, which increase 

 in number and size as the disease progresses, is to practically hll 

 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 vai-ying 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 of the mud was covered by a 

 thin pliable membrane. The mud surrounding the adult worm 

 is usually more compact and darker in tint, often inclining 

 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 overlying these putrefying patches are always discoloured 

 by yellowish spots. 



When the habits of the young worm are considered in con- 

 nection with the evidence derived from the examination of oysters, 

 in which the worm has just established itself, it points to the 

 conclusion that the larv:e 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 



