Dichotomous Key/Animalia

Key to Animalia

• 1: Single-celled organism.
• 2: Multi-cellular organism.

1

incomplete

Single cell protists

2

• 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.

3: Parazoa

• Placozoa: Simple tissues present.
• Porifera: Simple tissues absent.

4: Eumetazoa

• 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.

5

• Cnidaria: Cnidocytes present; Medusa and polyp form; Incomplete digestive system.
• Ctenophora: Sticky prey-capturing colloblast cells present; Medusa form only; Complete digestive system.

6

• 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.

7

• Echinoderm: Adults have radial symmetry. Inhabitants of salt water.
• 8: Adults have bilateral symmetry. Inhabitants of fresh water, salt water, or dry land.

8

• Ecdysozoa: Exoskeleton shed during life cycle.
• 9: Exoskeleton absent, or not shed during life cycle.

9

incomplete

Orthonectida, Rhombozoa, Acoelomorpha, Chaetognatha, Hemichordata, Xenoturbellida, Platyzoa, Lophotrochozoa