What is "cellulase"?

[David]

I was of the impression that all animals need a 
bacteria in their system to be able to obtain any
nutrition from cellulose. A person in another ng
told me that is not true, that ruminants can and
do create their own enzyme called "cellulase"
which they use for that purpose, and that they 
do not need any bacteria to help them at all. 
Can anyone tell me what the truth is, and where 
to learn more about it?

Though I do respect ARAs and ethical veg*ns for doing
what they think is best, I do not believe either 
approach is a good solution to problems with  animal 
welfare issues.