Why did early naturalists think that sponges were plants and not animals?

Early naturalists regarded sponges as plants due to their lack of obvious movement as well as there distinctive branching form. Sponges do not have any organs and their skeleton is fibrous. It wasn't until 1755 and later confirmed by observation in 1765, that changes in the diameter the openings to their central cavity were described, thus proving the animal nature of sponges.