Guide to organismal biology

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Domain Archaea - methanogens, thermophilic, halophilic ex.
  Domain Bacteria – true bacteria, cyanobacteria (Nostoc, Anabaena, Oscillatoria)
  Domain Eukarya - everything else

Characteristics of Bacteria

Bacteria were traditionally viewed as a single kingdom, consisting of all the unicellular prokaryotes. The Kingdom Bacteria was later divided into two subkingdoms, the Archaebacteria and Eubacteria. We now realize that what we once called Archaebacteria are as distantly related to other bacteria as bacteria are to us. Archaea and Bacteria are now considered separate kingdoms, and a new taxonomic rank called Domain, a rank higher than Kingdom, was invented to emphasize the great difference between them. In the modern system, there are three domains, Archaea, Bacteria, and Eukarya. The first two domains each contain a single kingdom, the third domain contains four kingdoms, hence six kingdoms of living organisms.

Bacteria contain an amazing diversity of species, including several multicellular forms. The cyanobacteria are an especially important and interesting group. There are several thousand living species. For about 1900 million years (2500 mya to 600 mya) cyanobacteria dominated the earth’s ecosystems. (Nostoc, Anabaena, Oscillatoria). This group was formerly classified as a primitive type of algae, the “blue-green algae”, after their distinctive coloration. We now recognize them as a type of photosynthetic bacteria. Filamentous forms may have an enlarged structure called a heterocyst, in which nitrogen fixation takes place.

Only about half of the cyanobacteria actually show the strong blue-green color we associate with this group. They can actually come in many colors (red, yellow, purple, and brown). The red color of the Red Sea is due to the red pigment in the cyanobacteria Trichodesmium. Some of the earliest fossils we have are of large stacks of roughly circular plates called stromatolites. These are composed of enormous colonies of bacteria going back about 2.7 billion years ago in the fossil record. Paleontologists believe that these large formations of cyanobacteria were very important early habitats for a variety of ancient organisms.

Ecological, Evolutionary, and Economic Importance

Many bacteria are pathogenic, like those that cause syphilis, botulism, strep throat, tetanus, scarlet fever, meningitis, toxic shock, dysentery, and bubonic plague.

Cyanobacteria created our oxygen atmosphere, and account for most of the oxygen being added today.

Ironically, we also owe many of our most effective antibiotics to bacteria: streptomycin, aureomycin, and neomycin, to name a few.

Bacteria are the basis for most food chains. Most of the animals you will see in the next several weeks include bacteria in their diet. We use them to make cheese and yogurt.

Bacteria and fungi are the primary decomposers of dead organic matter, recycling materials on a planetary scale for other organisms to use.

Many bacteria, like Rhizobium, can perform nitrogen fixation, creating fertile soil for plants.

Introduction to Kingdom Protista

The Kingdom Protista includes an incredible diversity of different types of organisms, including algae, protozoans, and (perhaps) slime molds. No one even knows how many species there are, though estimates range between 65,000 to 200,000. (fr. Greek protos = first, ktistos = first established). All protists are eukaryotes, complex cells with nuclear membranes and organelles like mitochondria and chloroplasts. They can be either unicellular or multicellular, and in this group we find the first union of eukaryotic cells into a colonial organism, where various cell types perform certain tasks, communicate with one another, and together function like a multicellular organism.

Some protists are autotrophs, a photosynthetic group of phyla referred to as the algae. Autotrophs manufacture their own energy by photosynthesis (using light energy) or chemosynthesis (no light required). Algae use various combinations of the major chlorophyll pigments, chlorophyll a, b, and c, mixed with a wide array of other pigments that give some of them very distinctive colors. Some protists are heterotrophs, a group of phyla called the protozoa. Heterotrophs get their energy by consuming other organisms. Protists reproduce asexually by simple mitosis, and a few species are capable of conjugation (like bacteria). Many have very complex life cycles.

Protists are so small that they do not need any special organs to exchange gases or excrete wastes. They rely on simple diffusion, the passive movement of materials from an area of high concentration to an area of low concentration, to move gases and waste materials in and out of the cell. Diffusion results from the random motion of molecules (black and white marble analogy). This is a two-edged sword. They don’t need to invest energy in complex respiratory or excretory tissue. On the other hand, diffusion only works if you’re really small, so most protists are limited to being small single cells. Their small size is also due to the inability of cilia or flagella to provide enough energy to move a large cell through the water.

Protists lack the rigid cell walls of bacteria and archaea, relying for their shape and structure on a cytoskeleton, an internal framework of tiny filaments and microtubules, which gives them a greater variety of shapes. It also allows them to eat by phagocytosis - they engulf their food in their cell membrane, and pinch off a section of membrane to form a hollow space inside the cell. This hollow space, now enclosed by membranes, is called a vacuole or vessicle. Vacuoles are handy little structures. Protists also use them to store water, enzymes, and waste products. Paramecium and many other protists have a complex type called a contractile vacuole, which drains the cell of waste products and squirts them outside the cell.

All protists are aquatic. Many protists can move through the water by means of flagella, or cilia, or pseudopodia (= false feet). Cilia and flagella are tiny movable hairs. Motile cells usually have one or two long flagella, or numerous shorter cilia. The internal structure of cilia and flagella is basically the same. All of the characteristics that this group shares are primitive traits, a perilous thing to base any classification on, because convergent evolution may be responsible for these superficial similarities. So the concept of the Kingdom has been justly criticized as a “taxonomic grab bag” for a whole bunch of primitive organisms only distantly related to one another.

Protists are mainly defined by what they are not - they are not bacteria or fungi, they are not plants or animals. Protists gave rise to all higher plants and animals. But where did protists themselves come from? The earliest protists we can recognize in the fossil record date back to about 1.2 billion years ago. We are still uncertain how the various groups of protists are related to one another, though we have made great progress in recent years thanks to molecular tools. We assume they arose from certain groups of bacteria, but which groups and when are still investigating. Some are more closely related to animals (choanoflagellates) and some more closely related to plants (red and green algae). Different phyla of protists are so unlike one another they probably evolved independently from completely different ancestors. Lynn Margulis recognizes nearly 50 different phyla of protists. We will take a more conservative approach, and focus on several important phyla of protists.


Kingdom Protista

Protozoa = heterotrophic protists:

Phylum Euglenozoa - (Euglena)

Phylum Dinoflagellata - dinoflagellates

Phylum Apicomplexa – sporozoans (Plasmodium)

Phylum Ciliophora - (Paramecium, Blepharisma)

Phylum Amoebozoa - amoeboids (Amoeba)

Phylum Foraminifera - foraminiferans

Algae = autotrophic protists

Phylum Phaeophyta - brown algae (Fucus)

Phylum Bacillariophyta - diatoms

Phylum Rhodophyta - red algae (Polysiphonia)

Phylum Chlorophyta - green algae (Spirogyra, Volvox, Chlamydomonas)

Characteristics of Phyla

The protozoa:

Phylum Euglenozoa (800 sp.) - Euglena

Is it a plant, or is it an animal? It moves around like an animal, and sometimes eats particles of food, but a third of the euglenoids are also photosynthetic, a nice bright green pigment like a green algae (which it used to be called). This organism may actually have resulted from endosymbiosis, in which an ancestral form engulfed a green algal cell.

Phylum Dinoflagellata (3,000 sp., fr. Greek dinos = whirling, Latin flagellum = whip) - dinoflagellates, Ceratium

Dinoflagellates are named after their two flagella, which lie along grooves, one like a belt and one like a tail. Many species have a heavy armor of cellulose plates, often encrusted with silica. This species is very important both ecologically and economically. Some species form zooxanthellae, dinoflagellates which have lost their flagella and armor, and live as symbionts in the tissues of mollusks, sea anemones, jellyfish, and corals. These dinoflagellates are responsible for the enormous productivity of coral reefs. They also limit coral reefs to surviving in shallow waters, where sunlight can reach the dinoflagellates. Some dinoflagellate species often form algal blooms in coastal waters, building up enormous populations visible from a great distance. The amazingly potent toxins that about 20 species produce often poison shellfish, fish, and marine mammals, causing the deadly algal bloom known as red tide.

These are the organisms that can make Louisiana oysters a truly unforgettable experience!! One outbreak of red tide in 1987 killed half of the entire bottlnose dolphin population in the Western Atlantic.

Phylum Apicomplexa (3,900 sp.) – sporozoans (Plasmodium)

These of protozoans is non-motile, and parasitic. They have very complex life cycles, involving intermediate hosts such as the mosquito. They form small resistant spores, small infective bodies that are passed from one host to the next. Plasmodium, the parasite that causes malaria, is typical of this group. In more general terms, spores are haploid reproductive cells that can develop directly into adults.

Phylum Ciliophora (8,000 sp., fr. Latin cilium = eyelash, Greek phorein = to bear) – ciliates (Blepharisma, Paramecium)

These ciliates move by means of numerous small cilia. They are complex little critters, with lots of organelles and specialized structures. Many of them, like Paramecium, even have little toxic threads or darts that they can discharge to defend themselves. Typical ciliates you may see in lab include Paramecium and Blepharisma.

Phylum Amoebozoa (over 300 sp.) - amoeboids (Amoeba)

These organisms have a most unusual way of getting about. They extend part their body in a certain direction, forming a pseudopod or false foot, and then flow into that extension (cytoplasmic streaming). Many forms have a tiny shell made from organic or inorganic material. They eat other protozoans, algae, and even tiny critters like rotifers. Amoeba is a typical member of this phylum. Many amoeboids are parasites, such as the species Entamoeba histolytica, which causes amoebic dysentery. 10 million Americans are infected at any one time with some form of parasitic amoeba, and up to half of the population in tropical countries.

Phylum Foraminifera - foraminiferans

“Forams” can have fantastically sculptured shells, with prominent spines. They extend cytoplasmic “podia” out along these spines, which function in feeding and in swimming. Foraminiferans are so abundant in the fossil record, and have such distinctive shapes, that they are widely used by geologists as markers to identify different layers of rock. The Pyramids are constructed of limestone formed from the shells of billions of foraminiferans.

The algae:

Phylum Phaeophyta (1,500 species, fr. Greek phaios = brown) - Fucus

This phylum contains the brown algae, such as Fucus (rockweed), Sargassum, and the various species of kelp. Brown algae are the largest protists, and are nearly all marine. Kelp blades can stretch up to 100 meters long. Brown algae have thin blades with a central midrib or stipe. Like all algae, their blades are thin because they lack the complex conductive tissues of green plants (xylem and phloem), and must rely on simple diffusion, though some kelp have phloem-like conducting cells in the midrib. Kelp form the basis of entire ecosystems off the coast of California and in other cool waters. In the “Sargasso Sea”, an area of the Atlantic Ocean northeast of the Caribbean Islands, the brown algae Sargassum forms huge floating mats, said in older days to trap entire ships, holding them tight until the ship became a watery grave. Sargassum is also very common in the Gulf of Mexico.

Phylum Bacillariophyta - 11,500 sp., many more fossil sp., fr. Latin bacillus = little stick) - diatoms

Diatoms have a golden-brown pigment. Diatoms have odd little shells made of organic compounds impregnated with silica. The shells fit over the top of one another like a little box. Diatoms usually reform the lower shell after they divide. This means they become smaller and smaller, and when they become too small they leave their shells and fuse through sexual reproduction into a larger size and start over again. They are one of the most important organisms in both freshwater and marine food chains. Diatoms are so abundant that the photosynthesis of diatoms accounts for a large percentage of the oxygen added to the atmosphere each year from natural sources. Their dead shells form huge deposits, that are mined for commercial uses. Diatom shells are sold as diatomacious earth, and used in abrasives, talcs, and chalk. Various species of diatoms are also widely used as indicator species of clean or polluted water.

Phylum Rhodophyta (fr. Greek rhodos = red, 4,000 sp.) - Polysiphonia

Like brown algae, the red algae also contain complex forms, mostly marine, with elaborate life cycles. Chloroplasts in this group show pigments very similar to those found in cyanobacteria, and ancient red algae may have engulfed these cyanobacteria as endosymbionts. Red algae have many important commercial applications, such as the agar used for culture plates, and carrageenan, used as a thickening agent in the manufacture of ice cream, paint, lunch meats, cosmetics, beer and wine!

Phylum Chlorophyta (7,000 sp., fr. Greek chloros = yellow-green) - Volvox, Spirogyra, Chlamydomonas

Green algae are now considered the sister group to land plants, so we will look at them in more detail when we learn about primitive plants.

Economic, Ecological, and Evolutionary Importance

Algae and protozoa are important prey in food chains. Even humans eat algae.

Many protozoans are important disease causing organisms (malaria, toxoplasmoisis, amoebic dysentery)

Dinoflagellates cause billions of dollars in damage to the seafood industry, and are important symbionts in corals and other marine animals.

An extract of red algae is used to make paint, cosmetics, and ice cream.

Protozoans gave rise to all higher forms of animal life.

Bacteria first mastered the fine art of photosynthesis. Cyanobacteria established the oxygen atmosphere we breathe today. Diatoms are a primary source of the current atmospheric oxygen from photosynthesis.

Consider This

How does size affect basic processes like respiration, ingestion, or excretion?

What role did endosymbiosis play in the early evolution of cells?

Why is Kingdom Protista usually considered an “artificial” classification?

Why is it never a good idea to classify organisms together on the basis of primitive traits?

3 - Primitive Invertebrates

Introduction to Primitive Invertebrates

Today well examine several phyla that represent alternate pathways in early animal evolution. The sponges, in the Phylum Porifera, are so strange that they are placed in the Subkingdom Parazoa, which literally means “animals set aside”. Sponges are very primitive animals that lack true tissues and organs. All other animals belong to the Subkingdom Eumetazoa, or “true” animals. All eumetazoans have cells organized into tissues. Phylum Cnidaria contains a diverse group of radially symmetric animals called the Radiata, to distinguish them from all other animals, which are bilaterally symmetric (the Bilateria).

Most of the diversity of the animal kingdom consists of different kinds of aquatic worms. Today we will examine two groups that exemplify two of the three basic body plans found in higher animals. Flatworms are acoelomate. They lack a fluid-filled body cavity. Rotifers are pseudocoelomate. They have a fluid-filled body cavity that is formed in a different fashion from that of higher animals. A true coelom, as found in coelomate animals, is derived from tissues of the mesoderm, but a pseudocoelom is a remnant of the blastocoel, the hollow space inside the developing embryo. In contrast with the asymmetric sponges and the radial symmetry of the cnidarians, worms show bilateral symmetry. This type of symmetry is highly adaptive for animals in motion. Like protists and primitive plants, primitive invertebrates rely heavily on diffusion to move materials into, out of, and through their bodies.

Introduction to Sponges

Phylum Porifera - sponges (Grantia, Spongilla, Euplectella); >10,000 sp. (fr. L. porus= pores, and ferre =to bear)

Sponges probably share a common ancestor with other animals, but diverged early in the Paleozoic. They are a great example of a colonial organism, with many different cell types working together, each type specializing in some basic function. Special cells called amoebocytes, for example, wander through the sponge matrix like roaming amoeba, digesting and transporting nutrients, and carrying sperm cells to the eggs. Amoebocytes also secrete numerous small skeletal elements called spicules, which are scattered through the matrix of the sponge. Spicules can be made of silica or calcium, and come in a variety of shapes. Some sponges also rely for support on a network of protein fibers called spongin. Spicule shapes are used to classify sponges.

Sponges are sessile filter feeders on plankton and detritus. They feed by means of cells called choanocytes or collar cells. The collar acts as a sieve to filter out larger particles of food, which are drawn in by the beating flagellum, and move down the outside of the collar to the cell body where they are ingested. In this respect, sponges are like protozoa. They are limited to feeding on particles that are smaller than the feeding cell itself. These feeding cells closely resemble a type of protozoan called a choanoflagellate.

Sponges must maintain a constant flow of water through their bodies. This steady flow of water brings food and oxygen, while carrying away carbon dioxide, nitrogenous wastes (ammonia), particles of debris, and gametes. Water enters through the ostia, the many pores visible on the side of the sponge, flows through the incurrent canals to the radial canals to pass over the choanocytes or feeding cells, and exits through the osculum, the large exit hole on top of the sponge (sometimes more than one, pl.=oscula).

Simple sponges of the asconoid type have a small central cavity or spongocoel, where the choanocytes are located. The more complex syconoid sponges (like Grantia) have folded canals of feeding cells off the spongocoel. In the larger leuconoid sponges complex folding creates an enormous surface area of feeding cells, with the spongocoel reduced to a network of narrow excurrent canals with many oscula. The common bath sponge is a leuconoid sponge, as is Spongilla. Sponges are hermaphroditic, and reproduce by external fertilization, dumping clouds of gametes into the water. Asexual reproduction occurs by budding off a new sponge, or regenerating a new adult from a piece of the parent sponge (fragmentation), a process exploited by sponge divers to seed their sponge beds. Some can also form gemmules, small clusters of amoebocytes in a hard shell.


Kingdom Animalia

Subkingdom Parazoa

Phylum Porifera -Sponges (Grantia, Spongilla, Euplectella)

Economic, Ecological, and Evolutionary Importance

Both freshwater and saltwater sponges form the basis for the bath sponge industry.

Euplectella, the Venus Basket sponge, is a good example of mutualism. Why?

Consider This

What is the evolutionary link between sponges and the protozoa?

What poses a big problem for sessile organisms like sponges when it is time to reproduce?

How do the three sponge types represent a solution to the problem of increasing body size?

How does this solution relate to the pumping ability of the individual collar cells?

Why do the results (leuconoid sponges) come to resemble the interior of the human lung?

Why does being hermaphroditic make very good sense for sessile organisms like sponges?

Introduction to Cnidarians

Phylum Cnidaria - hydrozoans, jellyfish, corals, sea anemones; 9,100 sp. (fr. Gr. knide = nettle; formerly called Phylum Coelenterata)

Cnidarians are the most primitive "true" multicellular animals (Subkingdom Eumetazoa). They are radially symmetric, and can be either sessile or motile, and sometimes both (at different stages in their life cycles). They are mostly marine, though hydrozoans are abundant in freshwater. They are the simplest animals with true tissues (eumetazoans). They possess two of the three germ layers (embryonic tissues) that are typical of all higher animals, having an ectoderm (outer layer) and an endoderm (inner layer), but lacking a mesoderm (middle layer). This middle layer, which develops into muscle and bone in higher animals, is replaced by a layer of protein jelly called mesoglea, the "jelly in the middle". The endoderm layer in cnidarians is called the gastrodermis ("stomach skin"). Just as muscle and bone give us support, and leverage, mesoglea provides support for cnidarians. The water in their body cavity also acts as a hydrostatic skeleton, and some cnidarians (like corals) can also secrete an external shell for support.

Cnidarians are also the most primitive animals that digest their food in an internal body cavity, a simple blind pouch called a gastrovascular cavity or GVC for short. Food is stuffed into the GVC by the tentacles that fringe the mouth. Gland cells lining the GVC secrete digestive enzymes into the pouch to break up the food into particles small enough for the cells lining the GVC to absorb. Thus, unlike more primitive animals, they can eat things that are bigger than a single cell.

Cnidarians capture their food with special stinging cells called cnidocytes, which contain a coiled thread called a nematocyst. Contact with the cnidocytes releases the nematocysts at explosive speeds, with up to 140 atmospheres of osmotic pressure! Nematocysts may be simple whip-like threads that coil around the prey (Indiana Jones style), or more typically contain hooks or barbs, often tipped with a toxin to paralyze the prey. Once the cnidocytes are pressurized, they require only simple physical contact to trigger them. So a dead jellyfish can sting you just as badly as a living one! Salt or sand is needed to remove stinging tentacles safely-never use fresh water or alcohol.

Cnidarians evolved the first true muscle and nerve cells. They have a primitive nerve net, with no central nervous system. Primitive senses include mechanical and chemical receptors, and (in the medusae) primitive eyespots and balance organs (statocysts). Cnidarians are typically dimorphic, existing as either a sessile polyp or as a motile medusa, which in many ways is like a polyp turned upside down. Many species alternate between the two forms, with the medusa serving as the sexual stage. The sessile polyp buds off tiny medusae from its upper surface. Many cnidarians are hermaphroditic.

Phylum Ctenophora - fr. Gr. cten = comb, phoros = to bear)

These strange creatures used to be classified with the Cnidarians, but later research revealed that the resemblance between comb jellies and true jellyfish was only superficial. For example, they usually lack cnidocytes, catch their prey with sticky cells (coloblasts) that line their tentacles, and are the largest organisms to use cilia for locomotion. In life they are among the most beautiful organisms on Earth (look for them at the downtown aquarium, or in Lake Pontchartrain).

Characteristics of Classes

Class Hydrozoa - Hydra, Obelia, Physalia, "fire corals"; 3,100 sp. (fr. Gr. Hydra [the immortal mythical monster])

Hydrozoans are mostly polyps, although many alternate between polyp and medusa, with the polyp form dominant in the life cycle. Hydrozoans frequently contain symbiotic algae, so are generally limited to shallow water. In sessile forms, the GVC's may be interconnected. Hydra is immortal (hence its name, from Greek mythology). New cells arise near the top, then gradually shift to the bottom where they die and fall off. Hydrozoans can be solitary, like Hydra, or colonial, like Obelia. In colonial forms, polyps specialize as feeding or reproductive polyps. Physalia, the "Portuguese Man Of War", is a colony in which feeding and reproductive polyps are carried along by a medusa that forms the "bell" or float for the colony.

Class Scyphozoa - true jellyfish (Aurelia); 200 sp. (fr. Gr. skyphos = cup)

In scyphozoans, the medusa form is dominant, the polyp occurs only as a small larval stage. The medusa makes gametes to form a zygote, which develops into a planula larva, which settles down for a brief existence as a polyp before budding off new medusae. The planula larva is also part of the life cycle of the other cnidarian taxa, and is also found in the Phylum Ctenophora (comb jellies)

The long tentacles that hang down from the mouth are covered with stinging cells, and push captured prey into the mouth. They eat a variety of crustaceans, and some feed on fish. Many jellyfish also have tentacles along the outer edge of the umbrella (bell). The umbrella itself can be contracted to move the animal in pulses through the water. Jellyfish are mostly water, up to 99% in freshwater forms.

Class Anthozoa - corals, sea anemones, sea fans; 6,200 sp. (fr. Gr. anthos = flower, zoa = animal)

Anthozoans are the most advanced form of cnidarians. They occur only as polyps, and the polyp body is much more complex than that of the hydrozoans. The GVC is typically divided into six chambers, providing a large surface area for digestion. Most have symbiotic dinoflagellates, so they are restricted to shallow waters, usually down to about 60 meters. Because anthozoans are mainly suspension feeders, they can be easily smothered and starved by muddy water. So nearshore and offshore development of any kind can kill large stretches of coral reefs. Stony corals are colonial anthozoans that form coral reefs by secreting a skeleton of CaCO3 (calcium carbonate). All the polyps in the colony (reef) are joined by an external layer of tissue. Coral reefs are among the most productive and complex ecosystems on the planet. Sea anemones are very large solitary polyps that feed on invertebrates and small fish. A few species are powerful enough to be toxic to humans.

Class Cubozoa - sea wasps; 20 sp.

These tiny jellyfish are important mainly because they are among the deadliest animals on Earth. Their sting is so potent that many divers have been killed by them. They are a particular problem off the northern and eastern coast of Australia, where two of the deadliest species are found.


Subkingdom Eumetazoa


Phylum Cnidaria

Class Hydrozoa - Hydra, Obelia, Physalia (Man of War)

Class Scyphozoa - true jellyfish (Aurelia, Cassiopeia)

Class Anthozoa - corals, sea anemones

Class Cubozoa - sea wasps

Economic, Ecological, and Evolutionary Importance

Coral reefs form one of the most diverse and important ecosystems in the world.

Hydrozoans are an important link in the freshwater food chain.

Consider This

Why is the medusa usually the sexual stage in the life cycle?

What is the fundamental limitation of a body cavity with a single external opening?

Why is the evolution of the cnidocyte so adaptive for a sessile animal like Hydra?

Why does a sessile animal need a motile larval stage?

Introduction to Flatworms

Phylum Platyhelminthes - flatworms, flukes and tapeworms; 18,500 sp. (fr. Gr. platys = flat, helminth = worm, ).

Flatworms are spiralian animals. Like molluscs and annelids, they grow by simply getting larger, not by molting (as do nematodes and arthropods). Most members of this clade also follow a pattern of spiral cleavage as embryos (see the chapter on How to be an Organism). Flatworms and rotifers are members of the clade Platyzoa, along with several other groups of invertebrates. Platyzoans are mostly acoelomate flat worms that get about by beating tiny cilia. Many platyzoans (like rotifers) also have complex mouth parts.

Flatworms are highly cephalized. Cephalization is a characteristic of all bilaterally symmetric animals. Like cnidarians, flatworms digest their food in a gastrovascular cavity, a simple cavity with a single opening. They are dorsoventrally flattened (back to belly). Because they are so flat, diffusion is sufficient for respiration, and flatworms lack respiratory and circulatory systems. They have a primitive nervous system, and a type of primitive excretory organ called a protonephridia, a simple tube ending in special flagellated cells called flame cells or flame bulbs. Flatworms are the most primitive organisms in which we find all three germ layers: ectoderm, mesoderm, and endoderm. Such animals are called triploblastic. Flatworms, nematodes and rotifers are protostomes, the first opening in the ball of embryonic cells becomes the mouth.

Flatworms are both free living and parasitic. The free-living forms, like the turbellarians, eat insects, crustaceans, other worms, and various protists and bacteria. A few species even capture prey by stabbing it with a sharpened penis, which they stick out through the mouth. A novel method of getting supper, and one you should definitely not try at home!

Parasitic forms, like flukes and tapeworms, clearly illustrate the basic strategy of being a parasite - if you don't need it, get rid of it. Parasites in this phylum are highly modified, and lack the obvious cephalization of Planaria and the other free-living genera from which they are descended. The evolutionary origins of flatworms are still unknown.

Flatworm phylogeny is a real mess! The acoelomate body plan thought to unite the various groups of flatworms led us down a blind alley. We assumed that this shared trait marked them as a monophyletic group. More recent studies revealed that the traditional three classes are paraphyletic, or even polyphyletic, and we are still sorting out the changes. It seems clear that at least some flatworms are basal to the other bilateral animals (basal means ocupying a position lower down on the “tree of life”, closer to the “root” ). The other flatworms are more closely to the annelids and molluscs.

Characteristics of Classes

Class Turbellaria - flatworms (Planaria; fr. L. turbella = turbulence); 3,000 sp.

Flatworms are commonly found in marine and freshwater habitats, moving along the undersides of underwater rocks, leaves or sticks. Feeding flatworms evert a long pharynx out of their mouths. This tube leads directly into the digestive tract. The intestine is a simple sac with one opening. Two large branches run down the length of the body. Side branches of this gut cavity reach almost all of the clusters of cells in the flatworm's body.

They also show the typical arrangement of a series of circular muscles surrounding a series of longitudinal muscles. Movement is aided by a carpet of cilia along the epidermis (usually the ventral surface) that gives them a smooth gliding motion. The turbulence caused by the beating cilia is visible as a swirling of tiny nearby particles, giving the Class its scientific name Turbellaria, which means whirlpool.

Flatworms reproduce asexually by transverse fission, dividing cross-wise into small buds that develop into complete adults, or by reciprocal copulation with internal fertilization. They excrete ammonia wastes by diffusion, and water and other wastes through special cells called flame cells, named from the flickering of the tiny cilia that drive fluids through the complex network of excretory tubes that crisscross the body. They have two lateral nerve cords and a rudimentary brain, really a cerebral ganglia. A ganglion (-ia) is just a large concentration of highly interconnected nerve cells, the nervous system equivalent of a telephone junction box. In addition to their auricles and eyespots (see below), flatworms have primitive balance organs called statocysts, which consist of a cup of cells with pressure sensitive hairs and small grains of material that can roll around to tell the animal which way is up.

Class Trematoda - flukes (Chlonorchis, Schistosoma)

There are over 11,000 species of trematode flukes. The digenean flukes are endoparasites on all classes of vertebrates, while the monogenean flukes are ectoparasites of aquatic vertebrates (mostly fishes). Although trematodes are generally similar to turbellarians, they are highly modified as parasites. Flukes have one or two large suckers to attach themselves to their hosts. Their extra tough epithelial tissue (cuticle) resists being digested by the enzymes they encounter in the bellies of their hosts.

Like many parasites, they have evolved intricate life cycles, involving multiple hosts. The Chinese liver fluke (Chlonorchis sinensis) needs a fish and a snail as intermediate hosts to complete its life cycle inside the human liver. 20 million east Asians are infected with this parasite, which can cause severe jaundice and even liver cancer. One of the deadliest flukes is the tropical blood fluke Schistosoma. In many tropical countries, worms are introduced into irrigated fields because human feces are used as fertilizer. Schistosoma uses snails as intermediate hosts. After leaving the snail, the worm enters the skin of a farmer wading through the fields. Schistosomiasis is widespread in tropical areas, and causes severe anemia and dysentery. The weakened victims often die of secondary infections. Worldwide, about 200 million people are infected with these dangerous flukes.

Class Monogenea – flukes; 1,000 sp.
Monogeneans are a small group of aquatic ectoparasites on fish. Unlike trematodes, they have relatively simple life cycles, without multiple intermediate hosts. They use a variety of complex anterior hooks, spines, suckers and clamps to attach to the skin, fins, and gills of fish.

Class Cestoda - tapeworms (Taenia, Dipylidium); 3,400 sp. (fr. L cestus = belt, Gr. oda = resembling)

Tapeworms represent the logical extreme of the parasite's evolutionary strategy. They have no mouth, and no gastrovascular cavity of any kind. They have no respiratory system, relying on diffusion. They absorb what they need directly from the intestinal fluids of their hosts. Most tapeworms are very specific with regard to the hosts they can infect.

They have a highly modified head end, called a scolex, with numerous small barbs at the top to aid in attaching to the intestinal wall. The rest of the tapeworm is a ruthlessly efficient machine with a single purpose - make more tapeworms. Behind the scolex are up to 2,000 identical segments called proglottids. These "segments" are designed to break off and serve as sacs full of mature eggs. When you look at these segments under the microscope, the only visible structures are the complete hermaphroditic reproductive systems in each and every segment. And tapeworms, unlike many hermaphroditic species, are usually self-fertilizing.

As you follow down the length of the worm, the more mature proglottids gradually fill with fertilized eggs, until the eggs blot out all other visible detail. Each of these reproductive sacs can generate around 100,000 eggs when mature. That means a single tapeworm can produce over 600 million tapeworm eggs a year! The shed proglottids look like tiny sesame seeds or grains of rice. These shed proglottids are often picked up during the hosts' grooming. The beef tapeworm, which can reach up to 30 feet long, is shed in cattle feces. When the cow pies dry and turn to powder, they are scattered over the grass, which is eaten by other cows who are then infected.


Subkingdom Eumetazoa





Phylum Platyhelminthes - flatworms

Class Turbellaria - flatworms (Planaria)

Class Trematoda - flukes (Chlonorchis, Schistosoma)

Class Cestoda - tapeworms (Taenia, Dipylidium)

Economic, Ecological, and Evolutionary Importance

Parasitic flatworms include the Chinese liver fluke, tapeworms, and Schistosoma, (schistosomiasis is a debilitating tropical disease).

Consider This

What features of flatworms show the typical evolutionary strategy of a bilaterally symmetric animal?

How do parasitic forms contrast with free-living flatworms?

How do these differences reflect the basic strategy of being a parasite?

How is being very flat an "end run" around the problem of increasing body size?

Why do flatworms have bilateral symmetry and a definite head end?

Introduction to the Pseudocoelomates

A large group of ten or more phyla of small aquatic worms, traditionally called the Pseudocoelomata or Phylum Aschelminthes, have long been lumped together on the basis of their general body plan. All were presumed to be pseudocoelomates, having a fluid-filled body cavity derived in a different way than a "true" coelom. This turned out to be a gross oversimplification of a complex evolutionary past.

Phylum Rotifera - rotifers, "wheel animals" (Philodina); 2,000 sp.

(fr. Latin rota = wheel, ferre = to bear)

Rotifers are very widespread aquatic animals, very common in freshwater, marine, and interstitial habitats (small spaces between grains of sand). We usually overlook them because they are so small, about 0.04 to 2 mm in size, not much larger than a big protozoan. They are very abundant, with about 1,000 rotifers in a typical liter of freshwater habitat. Rotifers are pseudocoelomate, with a complete digestive tract, and a muscular pharynx or mastax, which they use to grind their food. They feed by means of a crown of cilia called a corona, which beat together to draw water over the mouth. This tuft of cilia gives them their common name "wheel animals". Rotifers have a primitive eye cup, like the flatworm, and other primitive senses tied into a rudimentary brain. They can be either sessile suspension feeders, filtering out tiny protozoans and algae, and bits of detritus, or raptorial, animals that actively pursue their tiny prey. A few species are parasitic. Some rotifers reproduce sexually, and have separate sexes. Most are

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