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: This study was designed to evaluate the effects of pilocarpine and explore the underlying ionic mechanism , using both aconitine - induced rat and ouabain - induced guinea pig arrhythmia models .
### Output Text:
This study was designed to evaluate the effects of <span class="chemical">pilocarpine</span> and explore the underlying ionic mechanism , using both <span class="chemical">aconitine</span> - induced rat and <span class="chemical">ouabain</span> - induced guinea pig arrhythmia models . 