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: Learning and memory deficits in ecstasy users and their neural correlates during a face - learning task .
### Output Text:
Learning and memory deficits in <span class="chemical">ecstasy</span> users and their neural correlates during a face - learning task . 