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: Therefore , in the present experiment , the effects of clonidine administration ( 0 . 5 mg / kg ) on cardiac rate and ultrasound production were examined in 2 - , 8 - , 15 - , and 20 - day - old rats .
### Output Text:
Therefore , in the present experiment , the effects of <span class="chemical">clonidine</span> administration ( 0 . 5 mg / kg ) on cardiac rate and ultrasound production were examined in 2 - , 8 - , 15 - , and 20 - day - old rats . 