Why are sponges different than any other animal?

Sponges are some of the earliest forms of animals. It lacks a gut (digestive cavity/system found in the majority of animals), a nervous system, and a circulatory system. Moreover, its structure, with a jelly-like mesohyl sandwiched by two thin walls of flattened cells, is unique. But what sets it apart from almost all other animals is the ability of some of its specialized cells to transform into other specialized cells (most other animals, like humans, only have unspecialized cells that differentiate into specialized cells which don't change functions).