To understand how animals are related, scientists use a science called "taxonomy" to group different animals. The top level on the taxonomic tree for grouping living things is the kingdom. There are five basic kingdoms to which an animal can belong: animal, plant, protist, bacteria, or fungus. Sponges are in the animal kingdom.

Each kingdom is then divided into "phylums". After phylums, there are smaller divisions known as "classes". Underneath the classes, there are "orders". Orders contain "families", families contain "genuses", and genuses contain "species". Each organism, or living thing, in a phylum is alike, but as the groups get smaller, the organisms become more alike. Organisms in the same species are very similar because they have the most similar traits. Often, there may be smaller divisions of divisions such as sub-classes and sub- kingdoms. To remember the different types of groups, use this memorizing device. Each word in the sentence begins with the letter of a group name in the same order. It’s like the saying you may use to remember the names of the planets. (My very educated mother just served us...) Here’s the sentence:

Humans are only one species. However, sponges take up an entire phylum for several reasons. First, there are more than five thousand different types of sponges. Second, they are different from all other animals. Third, their history does not link them to other animals.

Usually, sponges are grouped into three classes known as the Demosponges, Calcarea, and Hexactinellida. When new tests are conducted, scientists may take a species or even an entire order out of one class and move it to another. There was once a fourth class of sponge (known as Sclerospongiae). However, it was dissolved, and the sponges within it were either moved to a different class or put into a new phylum. Some scientists even have a theory that the Hexactinellida should be a new phylum called "Symplasma".

Demosponges are the biggest group of sponges. Demosponges are the only group that has freshwater members. Only 5% of all sponges can live in freshwater. The other 95% live in salt water bodies such as oceans and seas. Most of the demosponges are more complicated than the other sponges. They are members of the Leucon grade, which means that they have many canals inside their body and their shape can change throughout the day to take in more water. By studying the fossil record, it appears that demosponges were not so common long ago as they are today. During the Cretaceous period, when there were lots and lots of sponges, demosponges were only half of all the sponges. But this could be so because demosponge spicules are made of silica. Spicules are tiny needles that grow from the inside to the outside of the sponge, and they provide protection and structure for the sponge. Since the demosponge spicules are made of a less strong material, when the the bodies decomposed, their spicules could have easily dissolved.

Calcareous sponges have spicules made of calcium carbonate. These spicules form rays and circles as they come out of the skin of the sponge. Their surfaces may be pastel colors as well as tan and black. If you were to see a calcareous sponge, it could be a pear or a purse shape. Also, it would look kind of bristly because of its spicules. There are over 100 different genuses of calcareous sponges. In the world, they usually live in tropical waters less than 1000 feet deep, but they have been found at far deeper depths.

Hexactinellids are very interesting sponges. Of all the sponges, they are thought to be the most beautiful. As is suggested by their name, hexactinellids have six points. They look a lot like stars. Many hexactinellids are called "glass sponges." Hexactinellids were the first group of sponges to develop, but as said before, many scientists do not believe that they are sponges at all. Oddly enough, they have electric receivers on their spicules which can conduct electricity!