Let's break down a few of these and see what you are actually letting your dog eat.
* Poultry by-product meal = beaks and feet
Here's a link to a first-hand account of what it's like inside a rendering plant: http://www.poisonedpets.com/a-rare-look-inside-a-rendering-plant/
* Animal digest = digest is produced by the chemical or enzymatic hydrolysis of clean animal tissue that has not undergone decomposition (thanks once again, Wikipedia). There have been arguments made by certain pet food companies that the word "digest" is a process, not a thing, but another expert has described animal digest as a "cooked-down broth which can be made from unspecified parts of unspecified animals." Again, this stuff comes from the rendering plant, so you are getting the 4-D animals: dead, diseased, disabled, or dying before slaughter.
Here's a link you should read on this subject: http://truthaboutpetfood.com/excuse-me-myth/
* Corn gluten meal = while the phrasing isn't exactly correct, it is still nothing more than a meat substitute that dogs and cats can build an intolerance too over time.
Actual gluten is what's leftover from certain grains such as wheat, barley, rye and other wheat-type cereal grains. All the starchy carbohydrates (the actual good stuff) has been washed out of it and you are left with gluten. While corn isn't a grain, the word "gluten" best describes what's leftover.
Big-name companies add corn gluten as a protein substitute. It's cheaper than meat and therefore cheaper to manufacture. There is NO substitute for meat. NONE! Thanks to Dog Food Advisor (http://www.dogfoodadvisor.com/choosing-dog-food/dog-food-gluten/) for this: