Identifying internal structure of dissected prawn
It is, indeed, the alimentary canal. It is actually this canal that it's important for one to remove when "de-veining" shrimp, rather than the vein itself, which lies immediately dorsal to the digestive tract.



In the future, I might suggest doing a little more research prior to asking the stack exchange, as frequent users of the site do like to see evidence of effort on the part of the user to find their own answers. A quick search for "alimentary canal prawn" or "arthropod anatomy" would likely have turned up your answer.