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: Antiarrhythmic plasma concentrations of cibenzoline on canine ventricular arrhythmias .
### Output Text:
Antiarrhythmic plasma concentrations of <span class="chemical">cibenzoline</span> on canine ventricular arrhythmias . 