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Key to Animalia
Single cell protists
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- 3: Specialized cells absent, though different types of cells may be present. Digestive cavity absent. Muscles, nerves, and organs absent. Parazoa.
- 4: Specialized cells present. Digestive cavity present. Muscles, nerves, and organs present in some form. True tissues organized into germ layers. Enterozoa.
- 5: Hollow body cavity present. Radially symmetric body. Aquatic. Colonial or solitary. Respiration and excretion accomplished by diffusion.
- 6: Body cavity not hollow. Radially or bilateral body symmetry. Aquatic, terrestrial or both. Mostly solitary.
- Cnidaria: Cnidocytes present; Medusa and polyp form; Incomplete digestive system.
- Ctenophora: Sticky prey-capturing colloblast cells present; Medusa form only; Complete digestive system.
- Chordate: Possesses a notochord, a hollow dorsal nerve cord, pharyngeal slits, or an endostyle, for at least some period of its life cycle.
- 7: Notochord, a hollow dorsal nerve cord, pharyngeal slits, and an endostyle absent during entire life cycle.
- Echinoderm: Adults have radial symmetry. Inhabitants of salt water.
- 8: Adults have bilateral symmetry. Inhabitants of fresh water, salt water, or dry land.
- Ecdysozoa: Exoskeleton shed during life cycle.
- 9: Exoskeleton absent, or not shed during life cycle.
Orthonectida, Rhombozoa, Acoelomorpha, Chaetognatha, Hemichordata, Xenoturbellida, Platyzoa, Lophotrochozoa