portal comments

How to fetch Portal/System comments using Rest Api

In my scenario, Various users add comments to a task.
These comments can be viewed by Selecting the case and then view case details.
In addition there are System comments there as well.

Now I want to retrieve these comments and display them in a page.
Now this page is outside the portal.This is an Application page.

I believe that I need to use Web Rest API to retrieve these comments (Portal / system ) .

Kindly Suggest a method to do the following .