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 effects of intrathecal administration of prostaglandins on pain responses in conscious mice were evaluated by using hot plate and acetic acid writhing tests .
### Output Text:
The effects of intrathecal administration of <span class="chemical">prostaglandins</span> on pain responses in conscious mice were evaluated by using hot plate and <span class="chemical">acetic acid</span> writhing tests . 