Natural Perspective

The Animal Kingdom (Animalia)

The Animal Kingdom is at once the Kingdom most and least familiar to us. Almost all of the animals we commonly think of -- mammals, fish, and birds -- belong to a single subgroup within one of the 33 Phyla comprising the Animal Kingdom. On the other hand, over 100,000 species in some 25 animal phyla -- mostly small worms -- are so unfamiliar that they are virtually unknown to non-scientists. The same goes for several hundred thousand tiny insect-like species populating the Arthropoda phylum.

All told, around 800,000 species have been identified in the Animal Kingdom -- most of them in the Arthropod phylum. In fact, some scientists believe that if we were to identify all species in the tropical rain forests the ranks of Arthropoda would swell to over 10 million species!

Here are highlights of the larger members of the animal kingdom:

Spinal Cords (Chordata)

All animals having a spine, including fish, mammals, birds, reptiles, amphibians, sharks, and eels are grouped into Chordata.

Because these animals are so familiar to us, biologists have come up with elaborate classification schemes including subphyla, superclasses, infraorders, and the like.

The vast majority (including all the Classes listed above) fit into the subphylum Vertebrata -- those having a backbone. Subphyla Agnatha, jawless fish, includes certain eels such as the Lamprey. Cephalochordata and Tunicata round out the list of subphyla with fairly obscure creatures called Lancelets and Tunicates, respectively.

All told, this familiar phylum includes 45,000 species of which you are just one.

Joint-Legs (Arthropoda)

If your animal has jointed legs and no spine, you can find it in the Arthropoda phylum. This includes most, if not all, of the animals we commonly call "bugs" as well as the crustaceans. Scientists have described 500,000 species of arthropods and believe that up to 10,000,000 species are alive today.

The classes of this phylum include -- in the order pictured above -- the six-legged Insects (Insecta); the eight-legged Arachnids (Arachnida) including spiders (pictured), scorpions, and ticks; the hard-bodied Crustaceans (Crustacea) including crabs (pictured), shrimp, and barnacles; and Malacostraca, which includes the sowbug or pillbug.

Other Arthropod classes are: Merostomata -- home of the Horseshoe crab; Millipedes (Diplopoda); and Centipedes (Chilopoda)

Soft Bodies = Mollusks (Mollusca)

Mollusks are so named because of their soft bodies (Greek: mollis = soft). The soft bodies of many of the 110,000 Mollusk species are protected by a hard shell, however. Even those without this protection, such as slugs, still have vestigial traces of a shell.

The most common classes are: Polyplacophora characterized by a shell composed of 8 overlapping plates, such as the Lined Chiton (Tonicella lineata), above; Gastropoda commonly known as slugs and snails -- including the Banana Slug, above; Bivalvia or Bivalves, such as clams and mussels (above); and Cephalopoda, comprising octopus and squid, as well as the Nautilus.

Other classes are Monoplacophora, Scaphoda (tooth shells).

Spiny-skinned (Echinodermata)

These animals are radially symmetrical -- they have no distinct front and back, only a top and a bottom. The 6,000 species are usually found in tide pools along the seashore.

The classes are: Asteroidea -- starfish such as the Six-rayed Star above (Leptasterias hexactis); Ophiuroidea -- brittle stars; Echinoidea including sea urchins (pictured) and sand dollars; Holothuroidea -- sea cucumbers; and Crinoidea.

[Marine] Stingers (Cnidaria)

There are some 9,500 species of these water creatures, which are sometimes called Coelenterates.

These animals are radially symmetrical and have tentacles with stinging cells. The more familiar types include jelly fish, sea anemones (pictured), corals, medusas, and hydras.

• Sponges (Porifera, 10,000 species)
• Segmented Worms (Annelida, 8,800 species including the common earthworm and leeches),
• Lamp shells (Brachiopoda, 350 species)
• Comb Jellies (Ctenophora, 90 species)
• Nematodes (Nematoda, 80,000 species)
• Horsehair worms (Nematomorpha, 240 species)

Annotated Bibliography

Margulis, Lynn, Karlene Schwartz, Five Kingdoms: An Illustrated Guide to the Phyla of Life on Earth (2nd edition), W. H. Freeman and Company, New York, 1988

An overview of the highest levels of Taxonomy. I have chosen the authors' nomenclature where available. Names, however, are constantly changing in the field of Taxonomy, and no doubt many of these names are disputed or have changed since 1988.

Margulis, Lynn, Diversity of Life: The Five Kingdoms, Enslow Publishers, Inc., New Jersey, 1992

Although billed as a children's book, this book is quite appropriate for the adult amateur. Dr. Margulis strikes an excellent balance between detail and brevity in this fact-filled book.

Milani, Jean P., et. al. Biological Science: An Ecological Approach (6th edition), Kendall/Hunt Publishing Company, Iowa, 1987

A high school textbook that devotes several chapters to Taxonomy and the diversity of life on our planet. The Appendix titled: A Catalog of Living Things illustrates the phyla as well as many classes and families within the five kingdoms.