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: The incidence and intensity of muscle fasciculations caused by suxamethonium were significantly greater in the control than in the alfentanil group .
### Output Text:
The incidence and intensity of muscle fasciculations caused by <span class="chemical">suxamethonium</span> were significantly greater in the control than in the <span class="chemical">alfentanil</span> group . 