Animals

Members of the animal kingdom, which are most familiar to us, can vary greatly. Sponges, mammals, worms, birds, and bugs are all animals, though they appear quite different from each other. Besides being eukaryotes, all animals are multi-cellular, mobile, and ingestive-feeding heterotrophs.

Animals are all multicellular, which means that they are comprised of many cells, usually billions. Also, these cells are all interdependent, which means, with some exceptions, that these cells are not capable of existing separated from the whole organism. These cells are animals are usually highly differentiated, which means that each type of cell has a specific function and has characteristics that allow it to fulfill that function. In most cases, those cells are incapable of shifting functions.

Animals are also mobile, which means that at some stage of their lives, all animals are able to move under their own power. This is a major characteristic for animals for it is an adaptation that is unique to animals. Plants and fungi are immobile. If the conditions of their environment become hostile, then they will have to cope or die. Most animals are capable of leaving their environment and search for more suitable conditions.

Animals are also ingestive heterotrophs, which means that they are unable to produce their own food and they digest that food inside some form of body cavity. Animals typically have some structure that serves as a mouth into which they can put food and some form of stomach in which they can digest that food. Variations in the digestive tracts of animals are a major key characteristic.

The primary exception to the characteristics of animals is Phylum Porifera. Sponges are not multi-cellular in the sense that we think. Though sponges are composed of millions, if not billions, of cells, they are not necessarily interdependent. Cells that have been removed from the whole organism can often survive. Also, there are some cells called amoebocytes which are undifferentiated and are capable of becoming any cells necessary to the organism. Sponges are also not truly ingestive heterotrophs in that they do not have a mouth or stomach. Despite these exceptions, biologists have decided that sponges are complex enough to be considered members of Kingdom Animalia.