Subject: At what point in evolution was a circulatory system and heart developed?t

If you view evolution as progressive from the simplest organisms to more 
complex, at what point did the existence of a circulatory system and heart begin 
to be necessary. Was this related to the size of the organism? Was there a 
period of multicelled creatures capable of circulating their fluids without a 
heart? What animals today do not have circulatory systems? Jellyfish? In other 
words, what is the relationship between size, habitat ( aquatic or land) and the  
existence of a heart? I don't know whether to be looking in biology or 
evolution. I'd be happy for some direction. 

Re: At what point in evolution was a circulatory system and heart developed?t