[quote=@Vilageidiotx]
If a Lobster got as big as a hippo, I don't think its circulatory system would continue to function. At least I am pretty sure that an open circulatory system wouldn't work at that size. If I am right, that means the Hippo could just sit back and watch as the Lobster suffered from organ failure. 
[/quote]

We're assuming a musculoskeletal system and related organ development that prevents a problem like that.