Animals: Kingdom Animalia

What is an animal?

It might seem like an easy task to distinguish an animal from a plant or an animal from a fungus.  Elephants, chimps and fish are animals, but they don't look all that similar.  So what makes something an animal?  Scientists use a lot of criteria to classify something as an animal and its almost impossible to give a list of characters that all animals share.  Instead, we've come up with a list that with few exceptions, most animals share.

#1 - All animals are eukaryotes.

The term eukaryote is used to define animals that have complex cells (membrane bound cells that have nuclei and organelles). Every living thing can be classified as either prokaryotic or eukaryotic.  Prokaryotes are things like bacteria.  They have no nucleus and their DNA is often in a loop. Plants and fungi sharek the animal distinction of being eukaryotic.

#2 - All animals are heterotrophs.

Unlike plants, which can make their own food (they are autotrophs), animals must ingest their food. This is how they get the nutrients they need to survive.

#3 - Animals are multicellular.

All animals have more than one cell.  With the exception of sponges, all animals also have bodies that are differentiated into tissues that do different functions.

#4 - Most animals are capable of movement.

Not all animals move, but in general animals can move.  Corals and sponges for example remain in a fixed place throughout most of their lives.

#5 - Most animals undergo sexual reproduction

The vast majority of animals are capable of sexually reproducing via eggs and sperm.  Some animals can also undergo asexual reproduction whereby they essentially produce clones of themselves. Several reptiles can do this as well as corals and other cnidarians. 

#6- Most animals are diploid

With few exceptions, animals are almost entirely diploid.  A diploid animal contains two full sets of the cell's genetic material.  Certain cells, like our sex cells (sperm and egg) can be haploid, but the other cells in our bodies are almost always diploid.

#7 - Animals do not possess rigid cell walls

Unlike plants and some bacteria that have cell walls, animals do not.  The cell membrane of animal cells is rarely rigid and never looks like that of a plant cell wall.