What are the 'sandbags' on a turtle

I’ve read recipes for cooking turtle and some mention ‘removing the sandbags’ when cleaning
Note: Historic and Unusual Recipes…Just for fun !!! —- How to Cook a Terrapin (or Turtle)
and
Chowhound

WHat, anatomically, are the sandbags? and can anyone point to an anatomical figure showing where and what they are?

That is probably the best explanation of how to clean a turtle. The ‘sandbags’ are what the site tells you to avoid puncturing - they look like sandbags but are full of nastiness.