Which type of organism is the simplest?

A. Sponges

B. Chordates

C. Vertebrates

D. Deuterostomes

A. Sponges

They are multicellular organisms that have bodies full of pores and channels allowing water to circulate through them, consisting of jelly-like Mesohly sandwiched between two thin layers of cells. Sponges have unspecialized cells that can transform into other types and that often migrate between the main cell layers and the mesohly in the process. Sponges do not have nervous, digestive or circulatory systems. Instead, most rely on maintaining a constant water flow through their bodies to obtain food and oxygen and to remove wastes.