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: In the antinociceptive and antiamnesic dose range , ( + / - ) - PG - 9 did not impair mouse performance evaluated by the rota - rod test and Animex apparatus .
### Output Text:
In the antinociceptive and antiamnesic dose range , ( + / - ) - <span class="chemical">PG - 9</span> did not impair mouse performance evaluated by the rota - rod test and Animex apparatus . 