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: Although some studies in experimental animals indicate clear pharmacological interactions between them , no studies have shown a specific interaction on anxiety responses .
### Output Text:
Although some studies in experimental animals indicate clear pharmacological interactions between them , no studies have shown a specific interaction on anxiety responses . 