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: Effect of humoral modulators of morphine - induced increase in locomotor activity of mice .
### Output Text:
Effect of humoral modulators of <span class="chemical">morphine</span> - induced increase in locomotor activity of mice . 