The dog taking a hand again

[karitaru]

Hello.
Reading Mark Twain's "The man who corrupted Hadlyberg," I stopped at the following paragraphs:

The Saddler. “Mr. Chairman, we’ve got ONE clean man left, anyway, out of the late aristocracy; and he needs money, and deserves it. I move that you appoint Jack Halliday to get up there and auction off that sack of gilt twenty-dollar pieces, and give the result to the right man—the man whom Hadleyburg delights to honour—Edward Richards.”

This was received with great enthusiasm, the dog taking a hand again; the saddler started the bids at a dollar, the Brixton folk and Barnum’s representative fought hard for it, the people cheered every jump that the bids made, the excitement climbed moment by moment higher and higher, the bidders got on their mettle and grew steadily more and more daring, more and more determined, the jumps went from a dollar up to five, then to ten, then to twenty, then fifty, then to a hundred, then—

I am sure this is an idiom, but I just can't find it online, and I can't figure out from context what it means exactly.

For the full text of the story, http://www.gutenberg.org/files/3251/3251-h/3251-h.htm
It's a bit long, and I wouldn't ask you to read the whole thing, but if someone spots a quick meaning, I would really appreciate it.

 

[GoesStation]

Has a dog featured in the story before this incident?

 

[karitaru]

Has a dog featured in the story before this incident?

Not a single one.

 

[Roman55]

The dog is mentioned earlier on in the story. It was woken from a sleep and added to the pandemonium of the moment with some loud barking of its own. This is another instance of it joining in, and not the last if you read to the end.

 

[karitaru]

The dog is mentioned earlier on in the story. It was woken from a sleep and added to the pandemonium of the moment with some loud barking of its own. This is another instance of it joining in, and not the last if you read to the end.

Oh, man. I totally forgot about that dog.
Thank you very much.
I was sure it was an idiom.