THE WORM DISEASE IN OYSTERS. Ill 



were those of the JPolydora ciliata. They were found on the sides of the 

 membranous tubes of the worm, in little transparent sacs, each cluster 

 somewhat less in diameter than the body of the worm, and each sac containing 

 between fifty and sixty eggs. I placed some of these egg-sacs in a test tube 

 and kept them for six days, during which time most of the eggs hatched out. 

 On examining a cluster under the microscope, I observed the newly -hatched 

 larvre swimming about in the egg-bag, and by means of a dissecting needle, 

 I ruptured the wall and allowed them to escape. They swim very rapidly by 

 means of the oral and anal wreathes of cilia? and the tufts of long stiff setse, 

 whicli they only used occasionally. They appear to jump or skip when the 

 bristles are brought into plav, and are consequently very difficult to follow 

 under the microscope. At birth the body is about twice as long as broad, 

 and consists of six segments. The antennae are represented by small rounded 

 lobes, the eyes are four in number, two near the mesial line, and two a little 

 further forward and widely separated. On each side of the body there are 

 a series of bristles, on the first segment there are about nine on each side, on 

 the remaining segments the bristles diminish in size and number posteriorly. 



For the first six days the larvsB swim about vigourously, after which they 

 begin to settle down, and appear to be in search of some suitable place to 

 commence life in earnest. At this stage it is very difficult to transfer them 

 from one vessel to another by means of the dipping tube, from the fact that 

 the moment they feel the current of water, they suddenly fix themselves on 

 the sides of the tube, and no amount of shaking will move them. They hold 

 on to the glass by the head with leech-like tenacity, whilst the rest of the 

 body moves up and down with the water. The only way to get them on to 

 a glass slide is to wait until they settle down to the bottom of the tube. 

 This peculiar habit of being able to fix on an object suddenly, when caught 

 in a current of water, is a very important factor in ^enabling the worm to 

 select any spot it chooses for its abode. If the place first chosen is not 

 convenient, it can move to another more favourably situated, even in the 

 face of a strong current of water. 



From what I have seen of the habits of the young worm in its free swim- 

 ming state, and also of those already mentioned as having just settled down, 

 I am of opinion that the young worm simply swims into the open oyster, and 

 fixes itself by its head on'the margin of the shell. If the position is suitable 

 it immediately begins to construct a tube and collect a large quantity of 

 mud. It may possibly be guided to the most favourable spot by the current 

 of water drawn in by the oyster. If so, then we have the explanation why it 

 is that the anterior margin is more often infested than any other part of the 

 shell. The wopms appear to have the power of collecting a large quantity of 

 mud in a very short time. Some which I kept in confinement in moderately 

 clear w^ater added fully one-quarter of an inch to the length of their tubes 

 in about an hour, and I have frequently removed the projecting tubes 

 at night, and in the morning they have 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, confining 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 solidified. 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 



