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: Dual effects of melatonin on barbiturate - induced narcosis in rats .
### Output Text:
Dual effects of <span class="chemical">melatonin</span> on <span class="chemical">barbiturate</span> - induced narcosis in rats . 