Weird but amazing animal. Hagfish are partway between fish and worms, with a spinal cord but no backbone. They have changed little in at least 300 million years. Hagfish largely scavenge, but have recently been found to hunt as well. When they come across a big carcass, they burrow into it and then eat it from the inside. Uniquely for a vertebrate, in addition to having a gut, they can absorb nutrients through their skin and gills. But feeding inside a decaying corpse, there will be little oxygen in the water and lots of toxic ammonia from the rotting flesh. The hagfish can cope with these adverse conditions, no problem. For hunting and defense, hagfish release a slime that gums up the gills of its predetor or victim, which suffocates it.