| A | B |
|---|
| Invertebrates | animal without a backbone |
| Exoskeletons | hard or tough outer covering of many invertebrates that provides support, protects body tissues, prevents water loss, and protects the organism from predation |
| Endoskeletons | internal skeleton that protects internal organs, provides support for the organism’s body, and can provide an internal brace for muscles to pull against |
| Vertebrate | animal with an endoskeleton and a backbone |
| Hermaphrodites | animal that produces both sperm and cells |
| Zygote | fertilized egg formed when a sperm cell penetrates an egg |
| Internal fertilization | type of fertilization that occurs when sperm and egg combine inside an animal’s body |
| External fertilization | type of fertilization that occurs when sperm and egg combine outside an animal’s body |
| Blastula | fluid-filled balls of cells formed by mitotic cell division of the embryo |
| Gastrula | two-cell-layer sac, with an opening at one end that forms from the blastula during embryonic development |
| Endoderm | inner layer of cells in the gastrula that develops into digestive organs and the digestive tract lining |
| Ectoderm | outer layer of cells in the gastrula that develops into nervous tissue and skin |
| Mesoderm | layer of cells between the endoderm and the ectoderm that can become muscle tissue and tissue of the circulatory, respiratory, and excretory systems |
| Symmetry | balance or similarity in body structures of organisms |
| Radial symmetry | body plan that can be divided along any plane, through a central axis, into roughly equal halves |
| Bilateral symmetry | body plan that can be divided into mirror images along only one plane through the central axis |
| Posterior | tail end of an animal with bilateral symmetry |
| Anterior | head end of an animal with bilateral symmetry |
| Cephalization | tendency to concentrate sensory organs and nervous tissue at an animal’s anterior end |
| Dorsal | backside of an animal with bilateral symmetry |
| Ventral | underside or belly of an animal with bilateral symmetry |
| Coelom | fluid-filled body cavity completely surrounded by mesoderm |
| Pseudocoelom | fluid-filled body cavity between the mesoderm and the endoderm |
| Acoelomates | animal with a solid body that lacks a fluid-filled body |
| Protostomes | coelomate animal whose mouth develops from the opening in the gastrula |
| Deuterostomes | coelomate animal whose anus develops from the opening in the gastrula |
