Fossil or geologic formation?

[GreatHoatzin]

I’ve been going through my drawer and I rediscovered these guys from many years ago. I have no idea where they are from or what they are (except that I picked them up from somewhere on vacation when I was young). I don’t know what to make of the smooth hole going down the centres. I’m interested in your thoughts on this.

[webmasterj]

Mosty "mud tubes" I believe or sediment left with holes caused by creatures or water.  Some can appear to be almost "prefect" and manmade but often naturally occur. 

[Fossildude19]

Could also be root casts.

[Anomaly]

i believe these are all rocks but picture #4 could be... a crinoid???? i need a different angle, probably not a crinoid tho