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: Antagonists at serotonin type 6 ( 5 - HT ( 6 ) ) receptors show activity in models of learning and memory .
### Output Text:
Antagonists at <span class="chemical">serotonin</span> type 6 ( <span class="chemical">5 - HT</span> ( 6 ) ) receptors show activity in models of learning and memory . 