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: Depression , impulsiveness , sleep , and memory in past and present polydrug users of 3 , 4 - methylenedioxymethamphetamine ( MDMA , ecstasy ) .
### Output Text:
Depression , impulsiveness , sleep , and memory in past and present polydrug users of <span class="chemical">3 , 4 - methylenedioxymethamphetamine</span> ( <span class="chemical">MDMA</span> , <span class="chemical">ecstasy</span> ) . 