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: In addition , the patients received fentanyl and d - tubocurarine .
### Output Text:
In addition , the patients received <span class="chemical">fentanyl</span> and <span class="chemical">d - tubocurarine</span> . 