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 effects of morphine ( 0 . 5 to 4 mg . / kg . i . v . ) on CP - induced behavioral modifications were tested administered alone and after naloxone ( 1 mg . / kg . s . c . ) .
### Output Text:
The effects of <span class="chemical">morphine</span> ( 0 . 5 to 4 mg . / kg . i . v . ) on <span class="chemical">CP</span> - induced behavioral modifications were tested administered alone and after <span class="chemical">naloxone</span> ( 1 mg . / kg . s . c . ) . 