Gut flora
The traditional view is that no metazoan phylum can break down cellulose by producing the enzyme cellulase. Instead, herbivores contain, in their gut, microorganisms which produce cellulase. This is important because cellulose is the most common organic compound on Earth. About 33% of all plant matter is cellulose (the cellulose content of cotton is 90% and that of wood is 40–50%). Recently, evidence has emerged that some animals do produce their own cellulase. The question is not yet quite settled.