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: These results suggest that PGs are involved in the mechanism ( s ) underlying fluorthyl - and PTZ - induced convulsions , but not picrotoxin - , electroshock - , or bicuculline - induced convulsions .
### Output Text:
These results suggest that <span class="chemical">PGs</span> are involved in the mechanism ( s ) underlying <span class="chemical">fluorthyl</span> - and <span class="chemical">PTZ</span> - induced convulsions , but not <span class="chemical">picrotoxin</span> - , electroshock - , or <span class="chemical">bicuculline</span> - induced convulsions . 