Why were sponges once considered plants?

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Group of fallow deer (Dama dama) stags with large, palmate antlers standing alert in a green summer field. — Animal The fallow deer (*Dama dama*) has been introduced in many areas ever since the Neolithic Period.

Early naturalists historically classified sponges as plants primarily due to their frequent branching, tree-like forms and their lack of obvious movement. Because they appeared stationary and resembled vegetation, they were not immediately recognized as members of the animal kingdom.123

The animal nature of sponges was eventually confirmed in the 18th century—first described in 1755 and verified in 1765—after researchers observed their internal water currents and the movement of their pores. While sponges lack the organs and tissues found in more complex animals, their cellular structures, biological functions, and multicellular nature confirm they are indeed animals. Molecular data further supports this, indicating that sponges share a common ancestor with other animals.134