Taxonomy is the science of classification. In taxonomy, species are organized into groups based on their physical characteristics and evolution. Every kingdom of life branches into smaller groups, called phyla, and each phylum branches into classes, and so on, down to species. The animal kingdom is large and diverse. There are millions of animal species, and every year thousands more are discovered. The two main groups of animals that scientists use for classification are vertebrates and invertebrates. Vertebrates share the common characteristic of having a backbone. Vertebrates include mammals, reptiles, amphibians, fish and birds. Vertebrates make up one phylum of animals: Chordata. Most animals in the world (over 95%) are invertebrates. They don’t have a backbone. Several phyla of animals are invertebrates. Which of the following is true?

a. Invertebrates are an entire kingdom of living organisms.
b. More than one phylum of animals are invertebrates.
c. Invertebrates are a single phylum of animals.
d. Invertebrates are also called Chordata.