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 clonidine , naphazoline and xylometazoline on analgesia induced by morphine , codeine , fentanyl and pentazocine , and on cataleptic effect of morphine , codine and fentanyl was studied in rats .
### Output Text:
The effect of <span class="chemical">clonidine</span> , <span class="chemical">naphazoline</span> and <span class="chemical">xylometazoline</span> on analgesia induced by <span class="chemical">morphine</span> , <span class="chemical">codeine</span> , <span class="chemical">fentanyl</span> and <span class="chemical">pentazocine</span> , and on cataleptic effect of <span class="chemical">morphine</span> , <span class="chemical">codine</span> and <span class="chemical">fentanyl</span> was studied in rats . 