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: We compared the systemic toxicity of bupivacaine , levobupivacaine , and ropivacaine in anesthetized rats .
### Output Text:
We compared the systemic toxicity of <span class="chemical">bupivacaine</span> , <span class="chemical">levobupivacaine</span> , and <span class="chemical">ropivacaine</span> in anesthetized rats . 