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 hyperactivity induced by nomifensine in mice remained unaffected by fluvoxamine .
### Output Text:
The hyperactivity induced by <span class="chemical">nomifensine</span> in mice remained unaffected by <span class="chemical">fluvoxamine</span> . 