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: Urotensin - II ( U - II ) receptors are widely distributed in the central nervous system .
### Output Text:
<span class="chemical">Urotensin - II</span> ( <span class="chemical">U - II</span> ) receptors are widely distributed in the central nervous system . 