The diversity of animal forms is truly amazing. Estimates offer the possibility of 10 to 30 million different species (forms) on this planet with many more awaiting discovery. Is there any commonality to this chaos of animal life? While no one criterion fits all types, several characteristics are common to all animals.
Cellular Characteristics
Multicellularity. All higher animals are multicellular beings. While the protozoa exhibit most animal characteristics, most of their species are single-celled. In regards to the animal characteristic of multicellularity, protozoa are most aptly described as animal-like.
Lack Cell Walls. Unlike algae and vascular plants, the cells of animals lack rigid outer cell walls. Instead, the outer boundary of animal cells is marked by a thin, flexible cell membrane.
