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: Despite the absence of rigidity , animals that received ketanserin ( greater than 0 . 31 mg / kg i . p . ) followed by alfentanil were motionless , flaccid , and less responsive to external stimuli than were animals receiving alfentanil alone .
### Output Text:
Despite the absence of rigidity , animals that received <span class="chemical">ketanserin</span> ( greater than 0 . 31 mg / kg i . p . ) followed by <span class="chemical">alfentanil</span> were motionless , flaccid , and less responsive to external stimuli than were animals receiving <span class="chemical">alfentanil</span> alone . 