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: Gastrointestinal abnormalities in Parkinson ' s disease ( PD ) have been known for almost two centuries , but many aspects concerning their pathophysiology have not been completely clarified .
### Output Text:
Gastrointestinal abnormalities in Parkinson ' s disease ( PD ) have been known for almost two centuries , but many aspects concerning their pathophysiology have not been completely clarified . 