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: Excessive stimulation of serotonin 5HT1A receptors causes a syndrome of serotonin excess that consists of shivering , muscle rigidity , salivation , confusion , agitation and hyperthermia .
### Output Text:
Excessive stimulation of <span class="chemical">serotonin</span> 5HT1A receptors causes a syndrome of <span class="chemical">serotonin</span> excess that consists of shivering , muscle rigidity , salivation , confusion , agitation and hyperthermia . 