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: Animals were tested for four consecutive days ( 4 trial / day ) in MWM during which the position of hidden platform was unchanged .
### Output Text:
Animals were tested for four consecutive days ( 4 trial / day ) in MWM during which the position of hidden platform was unchanged . 