In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: The exact mechanism by which diphenylhydantoin exerts its toxic effects is not known .
### Output Text:
The exact mechanism by which <span class="chemical">diphenylhydantoin</span> exerts its toxic effects is not known . 