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: Only phenytoin sodium displayed in vitro antiarrhythmic activity against strophanthidin - induced arrhythmias in guinea pig right atria .
### Output Text:
Only <span class="chemical">phenytoin sodium</span> displayed in vitro antiarrhythmic activity against <span class="chemical">strophanthidin</span> - induced arrhythmias in guinea pig right atria . 