Why does not having a backbone make things so complex?
Invertebrates could be described as simple animals. And though some are very simple, the vastness of the invertebrate species makes this group of animals anything but simple. Characteristics, evolution, and classification of invertebrates will be discussed in the Invertebrates chapter.
Invertebrates describes eight invertebrate phyla with the greatest number of species. From the simple sponge to the echinodermata, this diverse group of organisms is anything but simple. Essentially, the only common characteristic among all the groups is the lack of a backbone. Many people may not even recognize the sponge as an animal. But it is. And how many people think a worm is an insect? But it is not. Add to this group all the different species of insects, and the diverseness of this group is easily apparent.