I didn't think to snap a photo...

But yesterday out by my garden, "crawling" across the sidewalk was a slug-shaped mass of tiny slug like creatures. It was in the form of a slug, but was moving along with all the little tiny slugs (or whatever they were) crawling along themselves and making the whole thing move. Can anyone tell me what this was? I'd look it up on google image, but I have no idea what to ask for.

Is it (are they?) harmful to the garden? Are they helpful? If I took a stick and pushed through them, it would break the mass into two parts and then they would form again and try to get together. It was like a trail of ants, only bunched up in a big blob that very slowly moved along.

So weird.

Thanks!