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: The results of these studies demonstrated that CS - mediated protection is not selective for a particular species , organ system or toxic chemical .
### Output Text:
The results of these studies demonstrated that <span class="chemical">CS</span> - mediated protection is not selective for a particular species , organ system or toxic chemical . 