Brain teaser conceived by my professor

[tamtam402]

My professor is a brain teasers fanatic and he told us his favorite one, which is one he made himself. He says only 4 persons have gotten it so far, and he's been telling it to all his classes for over 10 years. No one got it right this year =(

I am usually long, but I can be short
I can breath, but it is not essential to me
I never sleep
I am often counted, but rarely noticed
I cannot see my brothers and sisters

 

[Antiphon]

Earthworm?

 

[Jimmy Snyder]

Spoiler

Bacterium