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: Emerging evidence indicates that voltage - dependent Na ( + ) channels have pivotal roles in the cardiotoxicity of aconitine .
### Output Text:
Emerging evidence indicates that voltage - dependent <span class="chemical">Na</span> ( + ) channels have pivotal roles in the cardiotoxicity of <span class="chemical">aconitine</span> . 