is way he was surprised to see Colin
sitting on a bench near the Fisheries Building, absolutely engrossed in
a gray, paper-covered folio. Dr. Crafts recognized it as the Bulletin he
had given the lad early in the afternoon, and he laughed aloud at the
boyish impatience which had made it impossible for Colin even to wait
until he got the book home. The Deputy Commissioner had to speak twice
before he was heard.

"Well, Colin," he said, "are you learning it off by heart?"

The boy jumped up as soon as he saw his friend, fairly stuttering with
all the questions he wanted to ask.

"I've got to go home," the Deputy Commissioner said, when Colin stopped
to take breath; "and you've put queries enough to keep a staff of men
answering for a week! Didn't I tell you that there's a world of work to
be done over the mussel? But if you like to walk along, why, I'll tell
you anything I can."

"Thanks, ever so much," the boy said; "but what puzzles me in this
Bulletin is the mussel's marsupium, or pouch. Has a fresh-water mussel
really got a pouch like a kangaroo?"

The Deputy Commissioner pushed his hat back over his forehead.

"Colin," he said, "you have a knack of putting questions in the most
awkward fashion. I suppose, in a way, the answer is 'not quite,' because
in the kangaroo, the baby is almost completely formed when it is placed
in the pouch, while in the mussel, only the egg goes there. The word
'marsupium' was what threw you off. What really happens is that the egg
passes into this pouch or pocket in the gills, and is there fertilized
as the current of water flows in and out over the gills."

"And it stays there until it has a shell of its own, doesn't it?" asked
the boy.

"It does," was the reply.

"Well," said the eager questioner, "if it has a shell and is able to
look out for itself, why doesn't it? Yet the book says that it always
attacks a fish and lives as a parasite for a while."

"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 rap