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 effect of ritanserin ( 5 - HT2 antagonist ) on scopolamine ( muscarinic cholinergic antagonist ) - induced amnesia in Morris water maze ( MWM ) was investigated .
### Output Text:
The effect of <span class="chemical">ritanserin</span> ( 5 - HT2 antagonist ) on <span class="chemical">scopolamine</span> ( muscarinic cholinergic antagonist ) - induced amnesia in Morris water maze ( MWM ) was investigated . 