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: We extend past research by examining overnight memory consolidation among regular ecstasy users ( n = 12 ) and drug naive healthy controls ( n = 26 ) .
### Output Text:
We extend past research by examining overnight memory consolidation among regular <span class="chemical">ecstasy</span> users ( n = 12 ) and drug naive healthy controls ( n = 26 ) . 