Well, Grissom did mention that a rat's body was very flexible. As long as it could squeeze its head into a space, the rest of the body would follow. If we assume the dead guy wasn't damaged in any other place on his body and had no other, um, orifices to enter into, well then I can only think of two possible points of entry. One being the mouth. I'll refrain from spelling out the second one. Heh. I'm going with his mouth, then down his throat. As for actually surviving, we don't know how long the little guy was in there either. The body was bloated and gave him plenty of room to move around. I guess we can just leave the little rat's time spent inside the floater up to our imaginations. haha. and I just noticed mrb105 gave a similar explanation. So yeah, I would go with that.