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 mechanisms underlying the activity of aniracetam and its ' therapeutic window ' are unknown .
### Output Text:
The mechanisms underlying the activity of <span class="chemical">aniracetam</span> and its ' therapeutic window ' are unknown . 