Here is a illustration of how the Reanimator gets inside a fresh corpse and reanimates the body. A Reanimator uses its very powerful tail to break open and cut the rib cage, it then eats the organs until it makes room for itself to fit in the corpse, it slips inside just like a hermit crab and its shell. It then uses its tentacles to send pulses to the nerves and muscles in order to make the corpse move. Two tentacles connect with the arms, two to the legs, and the pair of tentacles that has eyes on it get forced through the head and to the eye sockets, pushing the corpse eyeballs to the side in order for it to see its surroundings from inside. The Reanimator will stay in a corpse until they outgrow it or if it gets too decomposed.