Levels of Organisation

Every member of Kingdom Animalia is a metazoan — a multicellular animal whose cells cannot exist independently and must divide labour among themselves. How tightly that labour is organised gives four recognisable levels.

At the cellular level, seen only in sponges (Phylum Porifera), cells are loosely aggregated rather than bound into tissues. Each cell type still specialises — the outer pinacocytes maintain the sponge's shape while the inner flagellated choanocytes drive water through the body — but they act individually rather than as an integrated tissue.

At the tissue level, seen first in diploblastic cnidarians such as Hydra, similar cells are bound together by nerve and sensory cells into a coordinated tissue that performs one function as a unit. This is the first real step toward a defined body plan.

At the organ level, first appearing in Phylum Platyhelminthes, different tissues combine to build a structure — an organ — dedicated to one specific job. …