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 pooled statistical analysis for ventricular septal ( VSD ) and midline ( MD ) defects was performed for rat fetuses exposed to piroxicam , selective and non - selective COX - 2 inhibitor based on present and historic data .
### Output Text:
The pooled statistical analysis for ventricular septal ( VSD ) and midline ( MD ) defects was performed for rat fetuses exposed to <span class="chemical">piroxicam</span> , selective and non - selective COX - 2 inhibitor based on present and historic data . 