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: According to the literature pallidotomy improves the " on " symptoms of PD , such as dyskinesias , as well as the " off " symptoms , such as rigidity , bradykinesia , and on - off fluctuations .
### Output Text:
According to the literature pallidotomy improves the " on " symptoms of PD , such as dyskinesias , as well as the " off " symptoms , such as rigidity , bradykinesia , and on - off fluctuations . 