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: METHODS : The changes in pain thresholds were determined using mechanical stimuli - - the modification of the classic paw withdrawal test described by Randall - Selitto .
### Output Text:
METHODS : The changes in pain thresholds were determined using mechanical stimuli - - the modification of the classic paw withdrawal test described by Randall - Selitto . 