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 anti - dementia effects of s - limonene and s - perillyl alcohol were observed using the passive avoidance test ( PA ) and the open field habituation test ( OFH ) .
### Output Text:
The anti - dementia effects of <span class="chemical">s - limonene</span> and <span class="chemical">s - perillyl alcohol</span> were observed using the passive avoidance test ( PA ) and the open field habituation test ( OFH ) . 