Giant marine isopod
Here is the underside of a female roly poly. Her head is at the top. You can see her marsupial pouch taking up over half the length of her body. The yellow spot in the center is the opening for entering and exiting the pouch. The white rectangles under the pouch are her gills. As you can see, this one has empty places where more gills could be added. The drier the environment that the isopod lives in, the more gills it will have.
Holy Moly Roly Poly!!