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How are animals classified?
Introduction and Historical Background
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The largest invertebrate is the giant squid, Architeuthis dux, which averages 30 to 53 feet (9 to 16 meters) in length including its tentacles. It may reach a length of 69 feet (21 meters). These animals have the largest eyes, up to 10 inches (25 centimeters) in diameter, in the animal kingdom. It is believed that they generally live on or near the ocean bottom at a depth of 3,281 feet (1,000 meters), or slightly more than a half mile below the surface of the sea.
Animals belong to the kingdom Animalia. Most biologists divide the kingdom into two subkingdoms: 1) Parazoa (from the Greek para, meaning “alongside,” and zoa, meaning “animal”); and 2) Eumetazoa (from the Greek eu, meaning “true”, meta, meaning “later”, and zoa, meaning “animal”). The only existing animals classified as Parazoa are the sponges (phylum Porifera). Sponges are very different from other animals and function much like colonial, unicellular protoza even though they are multicellular. Cells of sponges can be versatile and change form and function and are not organized into tissues and organs. They also lack symmetry. All other animals have true tissues, are symmetrical, and are classified as eumetazoa.