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: ( P < 0 . 05 ) , suggesting the partial involvement of the opioidergic system in the synergism observed .
### Output Text:
( P < 0 . 05 ) , suggesting the partial involvement of the opioidergic system in the synergism observed . 