I can't get Thesaurus results in Pages 5.5?

Every time I try to right click and look up a word, I am able to access dictionary definitions, wikipedia, sometimes even Maps or iTunes listings, but never Thesaurus. Is this a feature I need to turn on, or does it not exist, or some other explanation? I am running Pages 5.5.2 on Yosemite (10.10.1).

Thanks in advance.