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: In the present experiment calcitonin decreased locomotor activity when locally injected into the same sites where it decreases food intake .
### Output Text:
In the present experiment <span class="chemical">calcitonin</span> decreased locomotor activity when locally injected into the same sites where it decreases food intake . 