The Sac- Back lives on the shores of Darwin IV's Amoebic Sea. This animal roams on its awkward, tripodal gait. This species gets its name from the male's sac that adorns its back. They show sexual dimorphism. The sac is used to store predigested food, which it later feeds to a group of females.

While the males lumber around on the surface, the females bury themselves, with only their tentacle and mouth showing. The male Sac- back travels from one group of females to the next, either mating with the females or feeding them the predigested food in the sac.