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 effect of humoral modulators on the morphine - induced increase in locomotor activity of mice was studied .
### Output Text:
The effect of humoral modulators on the <span class="chemical">morphine</span> - induced increase in locomotor activity of mice was studied . 