"It doesn't attack a fish, Colin," the other answered; "it only fastens on to one. Besides which, although the mussel has a shell, it isn't able to look out for itself. There is a change of form while it is fastened to the fish."
"But doesn't it hurt the fish?"
"Not permanently. It causes a local sore or a cyst, like the tiniest kind of a blister, in the middle of which the larva of the mussel is safely curled up and stays there until fully developed. Then the cyst breaks, the mussel drops out, and the tiny wound heals rapidly. Even a small fish, four inches in length, can carry five hundred of these little creatures on its fins and in its gills without serious injury."
"Suppose it can't find a fish?"
"That's the end of the mussel, then! There is one kind of mussel that develops without going through the parasite stage, but it is not as common as the others. Curiously enough, the only way to raise the mussel artificially is by means of parasitism on the fish. As you read there, it is a simple matter to get these tiny creatures from
the 'pouch' of the mother mussel, put them in an aquarium with some fish, and keep the water stirred up. In a few minutes the larvæ will have fastened themselves on. It is wise to keep these fish in a hatchery for a month or so and then simply release them; when the mussels are ready they will drop off, and a new crop of mussels is on the way. By this means you can start them without much trouble in rivers and streams where there were none before, so that you see what chances there are for the development of the industry."
"Are all mussels equally good for making mother-of-pearl?"
"No," was the reply. "There are two chief commercial varieties, of different species, one larva having a hook on the shell, so that it can attach to fins or tail, the other being smaller and without hooks and making its way into the gills. But you'll go into all that when you get to Fairport, and even after you have worked at mussels all summer there will be a lot of problems you won't have touched. Don't forget now, the fifteenth."
"Never fear, Dr. Crafts," Colin answered; "I won't forget. I wish it were here now."
Time did not hang heavily on the boy's hands,