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 H2 - histamine receptor antagonists ranitidine and cimetidine were compared for their abilities to control gastric acid hypersecretion on a short - and long - term basis in 22 patients with gastric acid hypersecretory states .
### Output Text:
The H2 - <span class="chemical">histamine</span> receptor antagonists <span class="chemical">ranitidine</span> and <span class="chemical">cimetidine</span> were compared for their abilities to control gastric acid hypersecretion on a short - and long - term basis in 22 patients with gastric acid hypersecretory states . 