The Animal Kingdom is divided into two great subkingdoms, viz.:
I. Invertebra'ta. L. fr. in, not, + vertebratus, vertebrated - i. e., not vertebrated. These have no vertebra, backbone, or spinal column.
II. Vertebra'ta. L. fr. vertebratus, jointed, articulated - i. e., vertebrated. These have a backbone (spinal bone) surrounding, and thus protecting, the spinal cord, often called conjointly spinal column.