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 selective 5 - HT6 receptor antagonist Ro4368554 restores memory performance in cholinergic and serotonergic models of memory deficiency in the rat .
### Output Text:
The selective 5 - HT6 receptor antagonist <span class="chemical">Ro4368554</span> restores memory performance in cholinergic and serotonergic models of memory deficiency in the rat . 