Zoology is the branch of biology that includes the study of animals and animal life, including, but not limited to, the study of the structure, physiology and classification of animals. Zoology can be subdivided into two categories:
- Vertebrate zoology
- Invertebrate zoology
Vertebrate means to have a spinal column or backbone. Animals in this category include species of the phylum Chordata: fish, birds, reptiles, amphibians, and mammals. These are characterized by a segmented spinal cord and a distinct differentiated head. These animals possess internal skeletons which include the vertebral column which at least partially encloses the spinal cord and a skull which houses the brain. Vertebrates also usually possess two pairs of limbs and an outer covering (fur, skin, feathers).
Invertebrate means lacking a spinal column. Animals in this category include species of the following: protozoans, annelids, cnidarians, echinoderms, flatworms, nematodes, molluscs, and arthropods. Invertebrates make up 90% of the Earth's animals. They range in size from minute to extremely large. They possess soft bodies and external skeletons.