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: Cholecystokinin - octapeptide ( CCK - 8 ) , which is a typical brain - gut peptide , exerts a wide range of biological activities on the central nervous system .
### Output Text:
<span class="chemical">Cholecystokinin - octapeptide</span> ( <span class="chemical">CCK - 8</span> ) , which is a typical brain - gut peptide , exerts a wide range of biological activities on the central nervous system . 