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 analgesic effect of the mu - agonist morphine was dose - dependently antagonized by naloxone and prevented by co - injection of pertussis toxin .
### Output Text:
The analgesic effect of the mu - agonist <span class="chemical">morphine</span> was dose - dependently antagonized by <span class="chemical">naloxone</span> and prevented by co - injection of pertussis toxin . 