Animal Phyla

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Wayne's Word Zoological Trivia For November 2001

Major Phyla Of Animals

Usually Included In General Biology Courses

This survey of the animal kingdom is dedicated to Dr. Nancy M. Jessop
(1926-2001), esteemed colleague, zoologist extraordinaire and dear friend
with whom I spent many hours exploring tropical ecosystems.   [W.P.A.]

1.  Kingdom Monera
2.  Kingdom Protista
    [Protozoan Phyla]
3.  Kingdom Fungi
4.  Kingdom Plantae
5.  Kingdom Animalia

The Major Animal Phyla:

The following phyla of multicellular animals (called metazoans) are usually included in general biology courses. Remember that the botanical counterpart of a phylum is called a division. Unicellular animals (called protozoans) are usually placed in the kingdom Protista along with the divisions of unicellular and multicellular algae. True multicellular animals are typically without cellulose cell walls and photosynthetic pigments, and they form diploid embryos developing from a blastula stage. The blastula is a hollow, fluid-filled sphere bounded by a single layer of cells surrounding a central cavity or blastocoel. The blastula develops from a spherical cluster of cells called a morula. In addition, animals are capable of locomotion or body movement by means of contractile muscle tissue. In a cross sectional view, animal embryos are composed of an outer layer called ectoderm, a middle layer or mesoderm, and an internal layer or endoderm that surrounds the digestive cavity. Multicellular animals are heterotrophic by ingestion of food into a body cavity (coelom) that is completely lined by the mesoderm. Primitive phyla without a true coelom include the Porifera and Coelenterata (Cnidaria).

Animal phyla are classified according to certain criteria, including the type of coelom, symmetry, body plan, and presence of segmentation. Sponges (Porifera) have a primitive cellular level of organization and lack tissues and symmetry. They depend on a flow of water through the body to acquire food. Cnidaria and comb jellies (Ctenophora) have radial symmetry and a saclike body composed of two tissue layers derived from the germ layers ectoderm and endoderm. Cnidaria typically develop a polyp body plan (e.g. Hydra) or a medusa body plan (e.g. jellyfish), or they alternate between these two forms (e.g. Obelia). In species with both polyp and medusa, the polyp is the asexual form while the medusa contains the sex organs.

Flatworms (Platyhelminthes) and ribbon worms (Nemertea) have tissues and organs derived from a third germ layer called the mesoderm. They have the organ level of organization and are bilaterally symmetrical. Planaria are free-living predators, but flukes and tapeworms are adapted to a parasitic mode of life.

The flatworm (planarian) is an acoelomate because it does not have a coelom within its mesoderm layer. Its digesitive cavity is surrounded by the endodermal layer.

Roundworms (Nematoda) and rotifers (Rotifera) have a body cavity (coelom) where organs are found and that can serve as a hydrostatic skeleton. Their coelom is called a pseudocoelom because it is not completely lined by mesoderm. Roundworms are nonsegmented worms that include many common parasites.

The roundworm is a pseudocoelomate because it does not have a complete coelom within its mesoderm layer. In other words, its coelom is not completely lined by the mesoderm layer (blue).

All complex animals have a true coelom, including the mollusks, annelids, arthropods, echinoderms and chordates. They have a true coelom that is completely lined by the mesoderm layer. The internal organs in a true coelom are more complex, and they are held in place by mesentaries.

The earthworm (annelid) is a coelomate because it has a true coelom within its mesoderm layer. In other words, the coelom (white) is completely enclosed within the mesoderm layer (blue). Coelomates have more complex internal organs and a muscular gut (intestines) derived from the mesoderm.

Phyla with a true coelom and digestive system are divided into two groups, the protostomes and the deuterostomes. As the hollow sphere of cells known as a blastula develops, an invagination produces an opening called the blastopore. In protostomes, the blastopore becomes the mouth. In deuterostomes, the blastopore becomes the anus, and only later does a new opening form the mouth. The coelom also develops differently in the two groups. In protostomes, the mesoderm arises from cells located near the embryonic blastopore, and a splitting occurs that produces the coelom, called a schizocoelom. In deuterostomes, the coelom arises as a pair of mesodermal pouches from the wall of the primitive gut. The pouches enlarge until they meet and fuse, forming an enterocoelom. Protostomes include the phyla Mollusca, Annelida and Arthropoda. Deterostomes include the phyla Echinodermata, Hemichordata and Chordata.

Gastrulation in protostome and deuterostome embryos. In protostomes, the initial site of ingrowth or invagination (blastopore) becomes the mouth. In deuterostomes, the initial site of ingrowth or invagination (blastopore) becomes the anus.

See The Morula Stage Of A Starfish Embryo
See The Major Protista & Botanical Divsions
See The Major Phyla Classified As Protozoans

There are more than one million species of animals in at least 30 phyla, more species than all the other kingdoms combined. Only 21 phyla are outlined on this brief page. More than half of all animal species are insects (800,000 species), and beetles (300,000 species) comprise the largest order of insects. In fact, one fifth of all the 1.5 million species of plants and animals on earth are beetles. If all the species of plants and animals on earth were lined up at random, every 5th species would be a beetle!

See The Remarkable World Of Beetles

Multicellular animals have a dominant diploid life cycle (diplontic cycle) with the haploid generation reduced to the egg and sperm. Sexual reproduction may involve monoecious species in which members of the population are bisexual (hermaphroditic) with sperm-bearing testicles and egg-bearing ovaries on the same individual. Hermaphroditic species are capable of self fertilization but most species exchange sperm by cross fertilization between different individuals. Some of the different phyla of worms contain hermaphroditic species. The majority of animal species are truly dioecious with populations containing unisexual male and female individuals.

See The Diplontic Human Life Cycle

Fertilization in animals may be external or internal. In external fertilization, eggs and sperm are released outside the animal's body, usually into water where fertilization occurs. In internal fertilization the sperm are deposited during copulation into the female reproductive tract where fertilization subsequently takes place.

Multicellular animals have three variations in the site of embryo development and source of nutrition. Embryonic development occurs externally in oviparous animals regardless of the site of fertilization (e.g. many aquatic invertebrates, most insects, all birds). In ovovipivarous animals, fertilization and embryonic development occur internally, but the female does not contribute nutrients to the embryo during development (e.g. some insects, lizards and snakes). Fertilization and embryonic development also occur internally in viviparous animals; however, the female (mother) contributes nutrients to the embryo during development (e.g. most mammals). In placental mammals the developing embryo and fetus are attached to the mother's uterine wall by a placenta. In marsupial mammals, the tiny embryos crawl out of the mothers vagina and into a pouch (marsupium) containing teats. Nourished by the mother's milk, they complete their embryonic development within the marsupium.

There are several methods of sex determination in animals. In most dioecious mammals, the sex of the offspring is determined by the X-bearing and Y-bearing sperm of the father. Birds, some fishes, caddisflies and lepidopterans (moths and butterflies) have a similar method, except the female (hen) determines the sex of the offspring because she produces X-bearing and Y-bearing eggs. In this case the female has the unmatched pair of sex chromosomes, while the male has the matched pair. [Note: Some references use different letters to express the male and female chromosomes in domestic fowl.] The males of some insects (such as grasshoppers and true bugs) have X-bearing sperm and X-less sperm without an X chromsome. If the X-less sperm unites with an X-bearing egg, the resultant offspring will be a male with one X chromosome. Female offspring (XX) develop from X-bearing eggs that are fertilized by X-bearing sperm. In many hymenopterans (including honey bees and wasps), haploid (n) males develop parthenogenetically from unfertilized eggs, while diploid (2n) females develop from fertilized eggs. Haploid males have only one set of chromosomes and lack the genetic variability of diploid females. All of the genes of males are expressed, regardless of whether they are dominant or recessive traits.

Class Of
Site Of Embryo Development

Common reproductive patterns in vertebrates & insects.

44 autosomes +
    X & Y chromosomes    
44 autosomes +
    two X chromosomes    
  Domestic Fowl  
16 autosomes +
two X chromosomes
16 autosomes +
X & Y chromosomes
22 autosomes +
one X chromosome
22 autosomes +
two X chromosomes
Honey Bee
Drone (n=16)
Worker (2n=32)

Four methods of sex determination in animals.

A Brief List Of 21 Animal Phyla

A.  Invertebrate Phyla

Parazoa: Simple animals without a digestive cavity.

1. Phylum Porifera (Sponges)

Simple multicellular animals that spend their lives anchored to a rock or ocean bottom; they are mostly marine but some species live in fresh water; sponges have radial symmetry with a cylindrical, globose or irregular body containing an internal skeleton of minute spicules made of calcium carbonate, silica, or a fibrous collagen protein called spongin; the surface contains numerous pores connecting to canals and chambers lined by flagellated collar cells (choanocytes); sponges are filter-feeders, taking in microscopic plankton by miniature currents created by the choanocytes; sponges are divided into three classes: Class Calcarea (chalk sponges) with calcareous spicules; Class Hexactinellida (glass sponges) with siliceous spicules; and Class Demospongiae (horn sponges) with a skeleton of spongin or none; calcareous and siliceous sponge spicules often become components of tropical beach sand.

Two examples of marine sponges. The surface contains numerous pores connected to canals and chambers lined by flagellated cells called choanocytes. Sponges are filter-feeders, taking in microscopic plankton by miniature currents created by the choanocytes.

See The Spicules & Gemmules Of A Freshwater Sponge
See Three-Pronged Sponge Spicules In Tropical Beach Sand
Archeocyathida: Extinct Reef-Building Animals Similar To Sponges

Enterozoa: Animals with a digestive cavity or tract.

Radiata: Radial symmetry, lacking central nervous system.

2. Phylum Coelenterata (Cnidaria) Corals & Jellyfish

Marine and freshwater animals with radial symmetry; this phylum has two distinct body forms: a solitary or colonial polyp and a bell-shaped, free swimming medusa; both polyps and medusae are often fringed with stinging tentacles; the tentacles bear rows of "stinging cells" or cnidoblasts, each containing a "stinging organelle" known as a nematocyst; some coelenterates (Obelia) have both polyp and medusa stages in their diplontic life cycle; coelenterates are divided into three classes: Class Hydrozoa (hydroids), including the freshwater Hydra, Obelia and colonial Physalia (Portuguese man-of-war) with stinging tentacles and a bladderlike air float; Class Scyphozoa (jellyfishes); and Class Anthozoa (sea anemones and corals); in true corals, the individual tentacle-bearing polyp lives inside a calcareous chamber that it secretes; extensive, contiguous colonies of coral polyps form massive calcareous reefs in shallow tropical waters; coral reefs support entire animal communities that rely on the photosynthetic activity of unicellular zooxanthellae that live within the coral polyps; no other ecosystem other than the tropical rain forest rivals coral reefs in terms of complexity and productivity.

Zoology textbooks often include a fourth class of cnidarians known as the box jellyfishes (class Cubozoa). These jellysish have a characteristic squarish medusa. The class Cubozoa includes the deadly Australian sea wasp (Chironex fleckeri). If deadliest venom is measured by how long it takes a person to die, then this species is certainly one of the world's deadliest. Depending on the extent of envenomation, the sea wasp can inflict excruciating stings that may cause death within five minutes. Ingenious Australian swimmers and snorkelers have devised a complete nylon body suit made from two pairs of panty hose, one put on the usual way; and the other put on over the hands, arms, and torso, with a slit for the head. Curiously, the deadly nematocysts of Chironex do not sting through panty hose, but watch out for runs!

The toxin of jellyfish nematocysts is a complex mixture of proteins, enzymes, polypeptides and tetramine. Pain and local histamine release of some jellyfish stings are attributed to 5-hydroxytryptamine, one of the ingredients in the stinging trichomes of nettles. The mechanism causing cardiac and respiratory failure of sea wasp venom is unclear. It may be due to blockage of nerve impulses at the synaptic junctions, similar to the action of curare.

Jellyfish At The Monterey Bay Aquarium
Jellyfish At The Monterey Bay Aquarium
See Painful Botanical Encounters Of Nettles
Curare: Deadly Neurotoxin From A Tropical Vine

A marine stony coral showing the numerous calcareous chambers that were once occupied by tentacle-bearing polyps. Colonies of stony corals are important reef builders in warm, tropical waters. Reef corals may form (1) fringing reefs extending out to 0.4 kilometers from shore; (2) barrier reefs separated by a lagoon of considerable width and depth from a shore; and (3) atolls or circular reefs that encircle a lagoon of water and not enclosing an island.

A wide variety of marine invertebrates, including sponges, jellyfish, sea anemones, corals, gastropods and turbellarians harbor within them golden spherical cells termed zooxanthellae. The photosynthetic activity of these symbiotic algal cells is vital to the survival of the individual coral animals and to the entire reef ecosystem. The zooxanthellae include several species of unicellular algae in the order Zooxanthellales within the algal division Pyrrophyta (also spelled Pyrrhophyta). The term zoochlorellae refer to several species of symbiotic unicellular green algae of the division Chlorophyta. Along the Pacific coast of North America, zoochlorellae produce the greenish color in sea anemone tentacles.

A pristine tide pool along the Oregon coast of North America. A. Purple sea urchin, B. Blood Starfish, C. Coralline red algae, D. Six-rayed starfish, and E. Sea anemone. The tentacles of the sea anemone contain zoochlorellae, symbiotic green cells of the algal division Chlorophyta.

Small, brightly colored fish called damselfish or anemone fish have evolved a mutualistic symbiotic association with large sea anemones. A slimy, mucus coating on the fish's body in some way inhibits the discharge of nematocysts along the sea anemone's tentacles. These little fish gain protection from predators by swimming down among the tentacles. The anemone benefits from its symbiont's movements, which dislodge silt and attract other fish into the lethal thicket of tentacles.

A sea anemone and its symbiotic anemone fish.

See The Infamous "Air Fern"
Go To The Algae Divisions

3. Phylum Ctenophora (Comb Jellies)

Solitary marine animals with transparent biradially symmetrical bodies; externally there are eight plates of fused cilia that resemble long combs; the rows of ciliated comb plates are used for locomotion; most of the body is composed of a jellylike material called mesoglea; the digestive (gastrovascular) cavity has branched canals; unlike the coelenterates, comb jellies do not have nematocysts; an Atlantic coastal species (Mnemiopsis) exhibits bioluminescence, the emission of light by on organism or population of organisms. This phenomenon involves the oxidation of luciferin in the presence of ATP and the enzyme luciferase. Examples of bioluminesence include dinoflagellates causing "red tide," lightning "bugs" (beetles), glow worms (beetle larvae) and the deep sea angler fish.

Three comb jellies (ctenophores). Comb jellies resemble tiny hot air ballons (the size of a walnut or smaller) with eight rows of fused cilia (comb plates) extending down the sides. They propel themselves mouth first by the eight rows of comb plates. Ctenophores superficially resemble miniature medusae (phylum Cnidaria); however, most medusae arise asexually from a polyp generation and ctenophores have no polyp stage in their life cycle. Tentacles extending from the mouth contain "glue cells" or colloblasts containing spiral threads which snare small fish and crustaceans with a gluelike material. With the exception of one species, ctenophores do not have the stinging organelles (nematocysts) of jellyfish and sea anemones. During the day, ctenophores flash prismatically as their ciliary plates refract light; at night they are often bioluminescent, glowing like little lamps.

Live Comb Jellies At Monterey Bay Aquarium
Mysterious Bioluminescent Deep-Sea Angler Fish

Bilateria: Bilateral symmetry, with central nervous system.
Note: Phylum Echinodermata with radial symmetry.

Acoelomata: No body cavity (coelom).

4. Phylum Platyhelminthes (Flatworms)

Body flattened, leaf or ribbonlike, bilaterally symmetrical; digestive tract branched and without an anus, or absent in parasitic forms; this phylum is divided into three classes: Class Turbellaria (free-living flatworms); Class Trematoda (flukes); and Class Cestoda (tapeworms).

The life cycle of a human tapeworm:

1. A man eats a poorly cooked piece of beef containing the encysted bladderworm (tapeworm cyst) in the skeletal muscle tissue. The young tapeworm attaches to the man's intestinal wall by its smaller, anterior end called the scolex. The scolex anchors the the tapeworm to the host's intestinal wall by four suckers. Note: Some early diet pills actually contained a tapeworm cyst enclosed in a gelatin capsule, before they were banned by the Food & Drug Administration. You could take the pill and eat as much as you want without gaining weight! Of course, you were actually feeding a large tapeworm which could be very dangerous. The tapeworm grows by budding as the scolex produces flattened segments called proglottids. A human tapeworm may contain hundreds of proglottids and be several feet in length. Tapeworms compete with their host for nutrients. Large tapeworms may obstruct food passage, and if their scolex penetrates the intestinal lining, bacterial infection may result.

2. Tapeworms are hermaphroditic and each mature proglottid contains a complete male and female reproductive system. Self fertilization may occur, or cross-fertilization between proglottids of different worms may also occur. A single ripe proglottid may contain more than 100,000 eggs. The annual output of one worm is about 500 million eggs and the worm may continue this production for 20 years or more. Proglottids filled with embryonated eggs (containing embryos) break off and are discharged with human feces.

3. If human sewage is discharged on grazing land, the embryo-bearing, tapeworm eggs may be consumed by cattle while grazing. Within the cattle, the larva hatches into a six-hooked oncosphere stage which penetrates the gut wall and eventually ends up in striated muscle tissue where it encysts. Within the cyst, the larva develops into a bladderworm, a miniature tapeworm complete with scolex and attachment structures. If striated muscle tissue (poorly cooked beef) is eaten by another person, an encysted bladderworm may take up residency in the new person's intestine. There are literally hundreds of different species of tapeworms, and they infect cattle, sheep, dogs, cats, pigs, fish and many other animals.

4. Much more serious than infection with an encysted bladderworm is the ingestion of tapeworm eggs directly. If the eggs are swallowed, the oncosphere larvae migrate through the body, not only skeletal muscle, but also into the heart, brain and other organs. Larvae of the dog tapeworm (Dipylidium caninum) live inside the bodies of fleas and lice. If eggs from this dangerous tapeworm are ingested, the worm may invade vital organs such as the brain. The habit of kissing the lips and tongue of a dog is somewhat questionable, especially considering their habit of biting fleas and licking themselves and other dogs in anal-genital regions. Another story to "gross you out" involves an affectionate cat that liked to rub against my arms while petting her. One day I noticed a tiny flat object on my elbow that just happened to be a fresh proglottid from a tapeworm inside her intestine.

Human tapeworms may be several feet long. In fact, one reported case at a medical clinic in San Diego County is astonishing. A man noticed a long white ribbon in his feces and thought he had swallowed his pajama draw strings during the night. The white ribbon turned out to be a tapeworm!

Is this a (1) wet pajama draw string, (2) a long noodle, or (3) a human tapeworm? The answer is (3), a 20 inch (50 cm) human tapeworm. The small head or scolex from which the segments (progottids) arise is clearly visible in the photograph. The presence of a scolex is good evidence that the entire worm has been discharged from the host's intestine. Each proglottid contains a complete male and female reproductive system. In fact, one proglottid may contain literally thousands of eggs.

Magnified view (30X) of the human tapeworm shown in the above photograph showing the anterior end or scolex. The scolex bears four circular suckers which firmly anchor the tapeworm to the host's intestinal wall. The scolex produces proglottids by budding, which gradually enlarge as more segments are formed.

5. Phylum Nemertea (Ribbon Worms)

Free-living marine worms with a distinctive eversible proboscis consisting of a long, hollow tube; the unsegmented body is covered with cilia; like flatworms, arrow worms are acoelomates (without a coelom), but they do have a complete digestive system.

Pseudocoelomata: Coelom incomplete.

6. Phylum Rotifera (Rotifers)

Rotifers are common microscopic animals living in fresh water, particularly among plants and debris; the rotating cilia at the anterior end resemble the head of an electric shaver.

7. Phylum Gastrotricha (Gastrotrichs)

Microscopic aquatic animals similar to the rotifers, but with a more flattened body; they are planktonic organisms living among algae and debris.

8. Phylum Nematomorpha (Horsehair Worms)

The adults are very long, thin, hairlike worms that are free-living in water; the immature (larval) stage is usually parasitic on insects; people once thought that dark hairs from a horses tail at a watering hole miraculously came to life; these so-called "hairs" were actually horsehair worms.

9. Phylum Nematoda (Nematodes)

A large phylum of worms, often called round worms or thread worms; free living in soil or water, with many parasitic forms on plants and animals, including hookworms (Necator & Ancylostoma), pinworms (Enterobius), human ascaris Ascaris lumbricoides and trichinosis worm Trichinella; hookworm larvae burrow into the skin between the toes and eventualy migrate into the intestines; the disease trichinosis is caused by eating undercooked meat (especially pork) containing microscopic cysts of larval Trichinella spiralis; if ingested the larval cysts can eventually migrate into skeletal muscle tissue; elephantiasis is caused by minute filarial worms of the genus Wuchereria that invade lymphatic vessels; the filarial worms obstruct the flow of lyphatic fluids causing body extremities to swell to gargantuan proportions; microscopic filarial larvae causing elephantiasis are spread by blood-sucking female mosquitoes; other dreaded parasitic worms in the phylum include the Guinea worm (Dracunculus medinensis) and the giant kidney worm (Dioctophyma renale).

Root-Knot Nematode: A Serious Pest Of Horticultural & Field Crops

10. Phylum Acanthocephala (Spiny-Headed Worms)

Small round worms that are intestinal parasites of vertebrates, usually with a crustacean or insect as an alternate host in young stages of its life cycle; a proboscis at the anterior end is covered with hooks for attachment to the wall of the intestines.

Eucoelomata: Body with a true coelom.

11. Phylum Bryozoa (Bryozoans)

Bryozoans form coral-like colonies in the ocean and fresh water; they are often called "moss animals" because of their delicate branched colonies; each animal has a fringe of tentacles around its mouth, and occupies a separate chamber similar to the individual chambers of coral polyps; freshwater bryozoans produce disk-shaped, chitinous reproductive bodies called statoblasts which are sometimes very abundant is wolffia samples.

Green Wolffia Plants & Brown, Disk-Shaped Statoblasts

Protostomia: Blastopore forms mouth, schizocoelom present.

12. Phylum Tardigrada (Tardigrades)

Tardigrades belong to a remarkable phylum of minute multicellular animals.

They are adapted to extreme conditions, some of which are more severe than any earth environment. Does their origin defy natural selection?

Tardigrades are microscopic, water-dwelling, segmented animals with eight legs. Depending on the species, they range in size from 0.1 to 1.5 mm long. They are called "water bears," in reference to their stumpy legs tipped with claws, and especially their lumbering gait. More than 1,000 species have been described. They occur throughout the world in some of the most inhospitable places, from high peaks in the Himalayas (above 20,000 ft) to boiling water near heat vents on the ocean floor, and from the Arctic tundra to vast ice fields of Antarctica. During severe environmental conditions that would kill most creatures on earth, tardigrades roll up into little dehydrated balls called "tuns" where they survive for extended periods of time. This dormancy phenomenon is called cryptobiosis (or anabiosis). Their ability to survive in this desiccated state for long periods is largely dependent on high levels of the non-reducing sugar trehalose that protects their tissues and DNA. Tardigrade physiology has led to the discovery of "dry vaccines" that don't require refrigeration and thus can be delivered and stored at room temperature. Some species can survive temperatures near absolute zero (-273° C) where liquids and gasses freeze solid; some can survive temperatures up to 151° C (304° F); some can live without water for 10 years; some can survive 1,000 times more gamma radiation than other animals (tardigrades can withstand 570,000 rads of X-ray radiation while 500 rads would kill a human); some can withstand pressures up to 6,000 atmospheres (more than the deepest ocean trench), and some can actually live for a while in a vacuum. In fact, tardigrades were taken into earth orbit on the Russian robotic spacecraft FOTON-M3 and survived 10 days in space. Upon returning to earth and hydrated, many actually laid eggs that hatched normally!

Lateral (side) view of the exoskeleton of an aquatic tardigrade (Hypsibius sp.) containing 5 eggs. There are 4 pairs of stout, stumpy legs, each tipped with several slender claws. The name "water bear" refers to its deliberate "pawing" sort of locomotion. Thomas Huxley, English naturalist and good friend of Charles Darwin, gave tardigrades this name in 1869. Photographed with a Sony W-300 digital camera through an Olympus laboratory grade compound microscope (400x magnification).

Advocates of intelligent design believe that tardigrades defy evolution by natural selection because they are adapted to extreme conditions, some of which are more severe than any earth environment. Creationists maintain that natural selection can only select characteristics necessary for immediate survival. Consequently, evolution cannot be expected to "over-equip" species for a host of environments that they have never faced. Advocates of the panspermia hypothesis believe that tardigrades had an extraterrestrial origin, although DNA sequencing data indicate that the phylum Tardigrada (tardigrades) is a sister clade with the phylum Arthropoda (arthropods), and both phyla (Tardigrada + Arthropoda) form a sister clade with the phylum Onychophora (velvet worms).

A strong case to support the evolution of "over-equipped" species is "gene duplication" in diploid organisms that greatly increases genetic variability. One of the truly remarkable examples of gene duplication is the antibody mediated immune system of animals. Gene duplication is a plausible explanation for how organisms can produce antibodies against different antigens, even synthetic antigen proteins that animals have never been exposed to. Using this model, animals would not need separate genes for every antigen they will ever encounter. This mechanism goes way beyond the simple evolution of adaptations based on Darwin's theory of evolution by means of natural selection.

  See Mechanism For Immune Antibody Production  

An aquatic tardigrade of the genus Hypsibius. Its length is approximately 184 micrometers (microns), about the same length as the hair follicle mite Demodex brevis. It is much smaller than a grain of common table salt (NaCl). The image was enhanced with Photoshop to bring out detail of the claws. Magnification 400x.

See Demodex On The Table Of Cell Sizes  
  See Tardigrade Discovered On Owens Peak  

Tardigrade (Hypsibius):  10 MB .MPG File
Tardigrade (Hypsibius):  5 MB .M4V file

13. Phylum Brachiopoda (Brachiopods)

Primitive marine bivalve animals attached to rocks by a fleshy stalk; brachiopod shells are abundant in the fossil record, dating back to the Cambrian Period (500 million years ago). There are approximately 280 species of living brachiopods, only a fraction of the 30,000 described fossil species that flourished during the Paleozoic and Mesozoic eras. My college invertebrate paleontology textbook contains a lengthy chapter dedicated to the Brachiopoda, with literally hundreds of illustrations of extinct brachiopods.

Although they superficially resemble bivalve mollusks, they are really quite different. In brachiopods, the two valves enclose the body dorsally and ventrally instead of laterally as in mollusks. The bivalve shells may be hinged (articulate) or without hinges (inarticulate). In addition, the ventral valve is usually larger than the dorsal, and is usually attached to the substrate directly or by means of a cordlike stalk.

Left. Mesozoic rock from Arizona showing numerous brachiopods, evidence of a shallow sea along a continental shelf. Right. An articulate (hinged) brachiopod.

14. Phylum Mollusca (Mollusks)

Body soft with bilateral symmetry, often covered by a mantle that secretes a calcareous shell; usually with an anterior head and a ventral muscular foot for locomotion; divided into five classes: Class Amphineura (chitons); Class Scaphopoda (tooth shells); Class Gastropoda (univalve mollusks including snails & limpets); Class Pelecypoda (bivalve mollusks); and Class Cephalopoda (octopus & squid).

Gastropods shells (cowries and cone shells) from the south Pacific island of Moorea and Tetiaroa Atoll in French Polynesia. The lower right shell is from a land snail.

Assorted Mollusks & Echinoderms
The Poisonous Venom Of Cone Snails
Chitons & Limpets On A Rocky Seashore

The common garden snail (Helix aspersa). This snail was introduced into California in the 1850s by European immigrants for use as food. It turned out to be a poor substitute for the edible snail (escargot) served in French restaurants (Helix pomatia). It has since become well-established and a major nuisance in California gardens. Homeowners spend more money on eradicating this creature than all other invetebrate pests combined. The head has two pairs of retractile tentacles. The longer, upper tentacles each have an eye at the tip. The shorter, lower tentacles bear chemosensory organs (equivalent to taste and smell). The ventral side contains a mouth with a rasping tongue (radula) for feeding. This snail is hermaphroditic (bisexual) with both male and female sex organs. Although capable of self fertilization, the normal method is cross fertilization between two mating partners. The snail moves by the gliding action of its large muscular foot. Mucus secreted from slime glands on the foot provides a lubricant to facilitate movement, and leaves a silvery slime trail. Snail mucus is used in "snail cream" cosmetics. The cream is used to treat damaged skin and as a skin moisturizer.

15. Phylum Annelida (Segmented Worms)

Body elongate and composed of many segments, each segment with fine bristlelike setae for locomotion; divided into three classes: Class Polychaeta (clamworms); Class Oligochaeta (earthworms); and Class Hirudinea (leeches).

The formidable bloodworm (Glycera), a predaceous intertidal worm (phylum Annelida) with four fanglike jaws at the anterior end (red arrow). The hollow fangs inject poison from venom glands into its prey. The elongate body is composed of many segments, each segment with fine bristlelike setae for locomotion. This interesting worm lives in tubular gallaries constructed in muddy bottoms. Lying in wait within its tube, the worm can detect the surface movements of prey by changes in the water pressure. It slowly moves to the opening of its burrow and then seizes the prey, such as small crustaceans and other invertebrates, with its proboscis. The proboscis can be extended to about one fifth the length of its body. When retracted, the proboscis occupies approximately the first 20 body segments. During the capture of prey, the proboscis is everted with explosive force, and the four venom fangs emerge at the tip.

16. Phylum Sipunculoidea (Peanut Worms)

Body slender, gourd-shaped; anterior end retractile, with short hollow tentacles around the mouth.

Peanut worms (phylum Sipunculoidea) are named after their shape when when fully contracted, such as when they are handled. These interesting worms live in burrows under rocks of the intertidal zone. They are sometimes very abundant in tunnels they bore in coralline rocks, reportedly up to 700 per square meter in some Hawaiian reefs. The slender anterior end has a mouth surrounded by hollow tentacles.

17. Phylum Arthropoda (Arthropods)
 Index Of Arthropod Images On Wayne's Word 

Body composed of head, thorax and abdomen with three or more pairs of jointed legs; chitinous exoskeleton covering all body parts, molted at intervals; divided into seven major classes: Class Onychophora (walking worms); Class Crustacea (shrimp, crabs & barnacles); Class Insecta (insects); Class Chilopoda (centipedes); Class Diplopoda (millipedes); Class Arachnida (spiders & ticks); and Class Merostomata (horseshoe crabs).

See An Extinct Cambrian Trilobite From Utah

Ostracods Belong To The Arthropod Class Crustacea Order Ostracoda

These microscopic objects are not eggs. They are the dead, chitinous, bivalve carapaces of minute crustaceans called mussel or seed shrimps. When they were alive, a tiny shrimplike animal occupied each white case. They belong to the order Ostracoda which is considered a class in some textbooks. Another order of minute crustaceans (Conchostraca) are called clam shrimp; however, the bivalve carapace of an ostracod is more strikingly similar to a clam shell. Photo was taken along the shore of a desiccated vernal pool in the Santa Rosa Plateau of Riverside County, California.

Brine Shrimp Belong To The Arthropod Class Crustacea Order Anostraca

A thriving population of brine shrimp (Artemia monica) in Mono Lake, California. These minute crustaceans belong to the order Anostraca, along with the endangered fairy shrimp (Branchinecta). During the summer months, the shoreline of Mono Lake is colored pinkish-red by massive colonies of brine shrimp. The Phalarope (right) is swimming in circles as it feeds on the brine shrimp.

An Endangered Fairy Shrimp In The San Marcos Vernal Pools

A common southern California centipede (Scolopendra polymorpha).

The name centipede is derived from the Latin words "centi" (100) and "pede" (foot). Centipedes belong to the class Chilopoda in the phylum Arthropoda. Adult centipedes in North America may have fewer or more than 100 legs, one pair per body segment. Centipedes are the only animals with legs modified into fanglike "poison jaws" (toxicognaths) that inject poison for subduing and killing prey. Modified forelegs, called prehensors, are located under the head. Glands inside the prehensors release venom into ducts that lead into the fangs. The last pair of legs are longer than the others. Some species use these to subdue prey or as defensive pincers. The posterior body segment with its unique pair of legs superficially resembles the head. This "pseudohead" found in some species may serve to confuse potential predators by misdirecting their attacks to a less vital part of their body, thus leaving the head free to bite the attacker. A common question that students ask is "can centipedes bite?" I can personally verify that they indeed have powerful fangs and can inflict a painful bite, particularly larger species shown in the following two images.

Magnified view of the underside of a centipede (Scolopendra polymorpha) showing a pair of fanglike "poison jaws" or toxicognaths (red arrow). These are actually modified forelegs that inject venom.

A large southern California millipede (probably Hiltonius pulchrus), a member of the class Diplopoda in the phylum Arthropoda. Diplopoda means "double-footed," and refers to two pairs of legs on each body segment. The name millipede is derived from the Latin words "milli" (1000) and "pede" (foot). Actually, adult centipedes in North America may have up to 750 legs, typically two pairs per body segment. Unlike centipedes, most millipedes are harmless detritis feeders (detritivores) and feed on rotting vegetation.

The horseshoe crab (Limulus polyphemus) lives in shallow water along the Atlantic coast and the Gulf of Mexico. It belongs to the class Merostomata in the subphylum Chelicerata. The horseshoe crab is a living representative of the subclass Xiphosura, an line of aquatic chelicerates that dates back to ancient Cambrian seas 500 million years ago. The subphylum chelicerata is characterized by arthropods with a cephalothorax, no antennae and jaws called chelicerae. In addition to the horseshoe crab, this subphylum includes the large class Arachnida (spiders, scorpions, ticks and mites). Another large subphylum, the Mandibulata, is characterized by a head, thorax and abdomen, paired antennae and jaws called mandibles. It includes the class Crustacea (lobsters, crabs and barnacles), class Insecta (insects), class Chilopoda (centipedes) and class Diplopoda (millipedes).

A sun spider or solpugid (order Solpugida), an interesting member of the subphylum Chelicerata in the class Arachnida. Unlike true spiders (order Araneae), the solpugid has two pairs of formidable, scissorlike jaws called chelicerae (red arrow). The jaws are arranged vertically rather than the single pair of horizontal jaws in most spiders. The elongate pair of anterior appendages are pedipalps, each tipped with a suckers that are used for capturing prey and climbing smooth surfaces. Solpugids are extremely voracious carnivores, even attacking and killing other large, webless spiders, such as wolf spiders. With their twin pairs of huge, powerful jaws, solpugids literally crush and tear their prey to shreds.

Close-up view of the anterior end of a tarantula (Aphonopelma) showing two sharp, fangs (red arrow) extending from the jaws or chelicerae. This spider is a male because its pedipalps are tipped with a sharp-pointed, bulblike copulatory organ (purple arrow). During mating the male inserts sperm into an opening on the underside of the female using the pointed copulatory organs on his pedipalps. Several closely-spaced, orange eyes can be seen on the front of the head (cephalothorax). The tarantula kills its prey by crushing and injecting them with poison through its hollow fangs. Although it is a large spider, the tarantula is hunted and killed by a large black wasp with bright red wings known as the tarantula hawk (Peplis).

See A Pillbug (Order Isopda)
See A Tarantula Hawk Wasp
Southern California Arthropods
See Arthropod Images On T-Shirts
Argentine Ants: A Serious Invasion
See The Life Cycle of a Human Botfly
Beetles: The Largest Order Of Animals
Beware Of The Africanized Honey Bees
A Coconut Crab That Rafts On Coconuts
A Tick That Resembles The Castor Bean
Sexual Suicide In Males Of Some Animals
The Remarkable California Trapdoor Spider
The Amazing Fig Tree & Its Pollinator Wasp

Deuterostomia: Blastopore forms anus, enterocoelom present.

18. Phylum Chaetognatha (Arrow Worms)

Small, transparent marine worms with bristles or hooks around the mouth; they have paired fins on the main body trunk and a terminal tail fin for locomotion; although very small, they have well-developed digestive and sensory systems, and a true coelom.

Close-up view of several arrow worms (phylum Chaetognatha). These planktonic marine animals are only about 5-10 mm in length, about the size of duckweeds of the genus Wolffiella. The have a torpedo-shaped body with lateral and caudal (tail) fins that superficially resemble the feathers of an arrow. The head is covered by a rounded hood that is retracted when the arrow worm feeds. During feeding, the arrow worm captures small plantonic organisms, including copepods and other chaetognaths, with minute spines or bristles around the mouth.

19. Phylum Echinodermata (Echinoderms)

Body radially symmetrical, usually 5-parted around an oral-aboral axis; body wall with calcareous plates, often with external spines; coelom includes water vascular system and external tube feet for locomotion; divided into five classes: Class Crinoidea (sea lilies); Class Asteroidea (starfish); Class Ophiuroidea (brittle stars); Class Echinoidea (sea urchins & sand dollars) and Class Holothuroidea (sea cucumbers).

Two well-preserved, fossilized echinoderms. Left: A 40 million-year-old (Eocene Epoch) sea biscuit (Eupatagus ocalanus) from Ocala limestone (Marion County, Florida). Right: A 7 million-year-old (Pliocene Epoch) sand dollar (Millitella californicus) from Baja California, Mexico. Both of these organisms belong to the class Echinoidea along with sea urchins.

An Assorted Of Echinoderms & Mollusks
Fossil Sand Dollars In Anza-Borrego Desert

20. Phylum Hemichordata (Acorn Worms)

A small group of marine invertebrates that were once thought to have a short notochord, the supportive axial rod that characterizes the phylum Chordata; hemichordates do possess gill slits or pharygeal clefts that are also found in chordates.

21. Phylum Chordata (Chordates)

Having at some stage of development an axial rod-like notocord for support of the body; chordates also have a single dorsal tubular nerve cord, paired gill slits between the pharynx and exterior, and a tail behind the anus; subdivided into three major subphyla: Subphylum Urochordata (tunicates); Subphylum Cephalochordata (lancelets); and Subphylum Vertebrata (vertebrates).

B:  Vertebrates: Subphylum Vertebrata

With a cranium (skull) and spinal column composed of segmented vetebrae; skeleton composed of cartilage or true calcareous bones; divided into eight major classes: Class Agnatha (jawless fishes); Class Placoderms (armored fishes); Class Chondrichthyes (cartilaginous fishes including sharks and rays); Class Osteichthyes (bony fishes); Class Amphibia (amphibians); Class Reptilia (reptiles); Class Aves (birds); and Class Mammalia (mammals).

Garibaldi (Hypsypops rubicundus), a bright orange fish of the shallow waters off the coast of California. Like California's state flower (golden poppy) and state freshwater fish (golden trout), the garibaldi has been selected to be the state's official marine fish. [Photograph courtesy of Dave Randle]

    Class Osteichthyes (Bony Fishes)    
The Amazing Sea Dragon
See A Beautiful Blue Tang
See A Colorful Wimple Fish
The Very Poisonous Puffer Fish
Piranha From The Amazon River
The Strange Deep-Sea Angler Fish
Class Amphibia (Amphibians)
See Colorful Poison Dart Frogs
See Western Toad (Bufo boreas)
Class Reptilia (Reptiles)
Camouflaged Lizards & Snakes
Class Aves (Birds)
See A Colorful Male Wood Duck
Frigate Bird In The Galapagos Islands
California Bird With Largest Wingspan
Class Mammalia (Mammals)
See A Zebra in South Africa
Lions At The Wild Animal Park
See The Life Cycle of Human Botfly
See A Tick Embedded In Human Skin
See Chewing Louse That Lives On Birds

The Following Protists Are Often Included In Zoology Courses
They Are Placed In Phyla Rather Than Divisions By Zoologists:

    1.  Phylum Sporozoa (Parasitic Protozoans): e.g. malaria

    2.  Phylum Ciliophora (Ciliated Protozoans): e.g. paramecia

    3.  Phylum Rhizopoda (Amoeboid Protozoans): e.g. amoeba

    4.  Phylum Zoomastigophora (Flagellate Protozoans): e.g. trypanosomes

See Representative Images Of Protozoans


  1. Barnes, R.D. 1980. Invertebrate Zoology. W. B. Saunders Company, Philadelphia.

  2. Borror, D.J. and D.M. DeLong. 1964. An Introduction To The Study Of Insects. Holt, Rinehart & Winston, New York.

  3. Jessop, N.M. 1988. Zoology (Schaum's Outline Series). McGraw-Hill Book Company, New York.

  4. Margulis, L., K.V. Schwartz, and M. Dolan. 1994. The Illustrated Five Kingdoms: A Guide To The Diversity Of Life On Earth. HarperCollins College Publishers, New York.

  5. Storer, T.I. and R.L. Usinger. 1957. General Zoology. McGraw-Hill Book Company, Inc., New York.

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