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: These data suggest distinct differences in the propensity to develop LIDs in monkeys with different rates of symptom progression or symptom durations prior to levodopa and demonstrate the value of these models for further studying the pathophysiology of LIDs .
### Output Text:
These data suggest distinct differences in the propensity to develop LIDs in monkeys with different rates of symptom progression or symptom durations prior to <span class="chemical">levodopa</span> and demonstrate the value of these models for further studying the pathophysiology of LIDs . 