Final answer:
Animals, or Metazoa, are a diverse kingdom classified into five main clades, ranging from simple forms without specialized tissues to complex organisms with intricate body plans.
Step-by-step explanation:
What Really Defines an Animal?The definition of an animal encompasses a broad diversity of life forms within the animal kingdom, often referred to as Metazoa. There are currently recognized five monophyletic clades of animals, each representing various stages of complexity and specialization. At the base are the Parazoa (e.g., sponges) and Placozoa which do not have specialized tissues derived from germ layers. The other three clades – Cnidaria, Ctenophora, and the vast majority of animals within the Bilateria – possess specialized tissues and are collectively known as Eumetazoa ("true animals"). Variation in symmetry, presence of different germ layers, and distinction between coelomates, pseudocoelomates, protostomes, and deuterostomes further categorize the Eumetazoa.
Animal Kingdom: The animal kingdom is divided into five main clades: Parazoa, Placozoa, Cnidaria, Ctenophora, and Bilateria. Parazoa and Placozoa do not have specialized tissues, while Cnidaria, Ctenophora, and Bilateria do. Eumetazoa refers to the animals with specialized tissues, which is what we usually think of as animals. Myxozoa, mentioned in the question, is actually a group of parasites that are closely related to animals, but not considered animals themselves.