Major Phyla Of Animals

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.

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.

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.

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.

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.

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).

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.

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.

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.

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

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.

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).

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.

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.

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!

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.

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.

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).

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).

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

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).

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.

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).

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.

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).

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).

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