I understand that topo-physiologically, the human body is a donut. i.e. not only are we a donut physically, but we are a donut as an organism. Our skin is an interface between the outside world and the inside of our bodies. Bacteria and other microbes have to get through our skin defense before they can infect us. But this barrier persists right though our GI tract as well, which is why we can eat dirt without getting infected. Physiologically, our intestines are considered outside the body. So, to my question: if we count the nostrils, we've got 4 orifices. (I wouldn't count orifices that lead to the interior of the body, such as urethra, or dead ends such as the ears or Eustacian tubes). So, are all blobs with 4 openings topologically equivalent? Or are there several flavours such as holes branching off into multiple holes and such?