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”. All other animals belong to the Subkingdom Eumetazoa (“true” animals). Phylum Cnidaria contains a diverse group of radially symmetric animals, sometimes referred to as Radiata, to distinguish them from all other animals, which are bilaterally symmetric (Bilateria).
Most of the diversity of the animal kingdom consists of different kinds of aquatic worms. Today we will examine two groups of worms that exemplify two of the three basic body plans found in higher animals. Flatworms are acoelomate. They lack a fluid-filled body cavity. Rotifers and larger roundworms 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 eucoelomate 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, all of the worms in today's lab 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.
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)
Examine the sponges on display. Identify the choanocytes, spicules, ostia, osculum, radial canals, and spongocoel.
Observe the freshwater sponge Spongilla. Sponges are among the simplest forms of animal life. See if you can spot any gemmules.
Examine slides of spicules. Notice the regular geometric shapes, used by taxonomists to classify sponges.
Examine cs and ls slides of Grantia, a syconoid sponge. Identify the choanocytes (collar cells) and The collar cells are quite small, and will require a high power lens. Notice the many spicules. Can you see the radial canals?
Both freshwater and saltwater sponges form the basis for the bath sponge industry.
Euplectella, the Venus Basket sponge, is a good example of commensalism. Why?
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?
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. 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 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.
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.
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 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.
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.
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.
Subkingdom Eumetazoa
Phylum Cnidaria (Radiata)
Class Hydrozoa - Hydra, Obelia, Physalia (Man of War)
Class Scyphozoa - true jellyfish (Aurelia, Cassiopeia)
Class Anthozoa - corals, sea anemones
Class Cubozoa - sea wasps
Examine the preserved specimens and slides of each of the three classes. Hydrozoans are mostly polyps. Be sure to scope out the living Hydra. What is the largest prey that these little critters could theoretically eat?
Examine slides of Hydra, both cs and wm (whole mount, i.e. the entire critter) slides - Look for the cnidocytes (which appear as round bumps along the tentacles), mouth, and tentacles. Are there any buds visible on the whole mount slides?
Examine slides of Obelia hydroid colony. Observe the feeding polyps and reproductive polyps. Obelia has a medusa stage which produces the gametes for sexual reproduction. Note that the GVC's of the various polyps are interconnected. What other famous (and dangerous) colonial hydrozoan is on display?
Examine slides of the Planula larvae. What are the stages in the life cycle of a typical cnidarian like Aurelia?
Examine slides of the Aurelia Ephyra, an early stage in the jellyfish life cycle. Look for the mouth and the tentacles. Can you see the four gastric pouches which extend from the mouth and stomach? The horseshoe shapes inside the gastric pouches are the gonads. A complex system of canals connects these pouches with a ring canal that runs along the outer edge of the umbrella. Food and seawater are circulated throughout the umbrella.
Observe the other Anthozoans on display. In life, the skeletons of corals and sea fans would be covered by hundreds of tiny polyps.
Observe the preserved specimens of comb jellies. Comb jellies (Phylum Ctenophora; fr. Gr. cten = comb, phoros = to bear) 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, and catch their prey with sticky cells that line their tentacles. In life they are among the most beautiful organisms on Earth (look for them at the aquarium).
Coral reefs form one of the most diverse and important ecosystems in the world.
Hydrozoans are an important link in the freshwater food chain.
Why is the medusa usually the sexual stage in the life cycle?
What is the fundamental limitation imposed by a body cavity with only a single external opening?
Why makes the evolution of the cnidocyte so adaptive for a sessile animal like Hydra?
Why does a sessile animal need a motile larval stage?
Phylum Platyhelminthes - flatworms, flukes and tapeworms; 18,500 sp. (fr. Gr. platys = flat, helminth = worm, ).
Flatworms are highly cephalized; they are the first animal we look at in lab that actually looks back. 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.
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 longtitudinal 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.
There are over 12,000 species of 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 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 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.
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 or excretory or nervous systems, 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. 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.
Phylum Platyhelminthes - flatworms
Class Turbellaria - flatworms (Planaria)
Class Trematoda - flukes (Chlonorchis, Schistosoma)
Class Cestoda - tapeworms (Taenia, Dipylidium)
Observe the flatworms under the dissecting scope. Watch their smooth forward motion, a result of the interplay between circular muscles and longtitudinal muscles. Note that they are bilaterally symmetric and highly cephalized. Identify the ear-like auricles, which are not ears, but are sensitive to both mechanical and chemical stimuli, and the eyespots, shallow cups of light-sensitive pigmented cells. When the worm turns on its side you may also see the protruding pharynx with which it vacuums up its food.
Examine slides of Planaria wm slides, look for auricles, eyespots, pharynx, GVC
Examine cs slides of Planaria. Look for ventral nerve cords, branches of GVC, ciliated epidermis, note the absence of a fluid-filled body cavity (acoelomate condition).
Examine slides of Taenia. Look for the scolex, and the proglottids. Find the prominent uterus, vagina, and ovaries, and the vas deferens, which gathers sperm from the scattered testes. Scan the length of the worm and note the numerous eggs in the posterior segments.
Examine slides of Chlonorchis sinensis, the Chinese liver fluke. Observe the oral sucker and mouth, and the two long forks of the intestine (GVC) on either side of the prominent uterus. Can you see the testes near the excretory pore? This pore collects wastes from the flame cells.
Parasitic flatworms include the Chinese liver fluke, tapeworms, and Schistosoma, (schistosomiasis is a debilitating tropical disease).
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?
A large group of ten or more phyla of small aquatic worms, called the Pseudocoelomata or Phylum Aschelminthes, have been traditionally 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 may have been a gross oversimplification of a complex evolutionary past, and these phyla do not seem to be truly related to one another. Many now appear to be acoelomate (gastrotrichs and most nematodes) rather than pseudocoelomate (larger nematodes like Ascaris, and rotifers). Today they are treated as separate phyla, with the Nematoda being the most diverse and important group.
Nematodes are incredibly diverse, with over 12,000 named species, living in all aquatic habitats, including the water film that surrounds particles of soil (interstitial habitat). They are critically important ecologically, as major recyclers of organic matter in the soil, and in aeration of the soil. Nematodes feed mainly on the abundant soil bacteria and fungi, as well as other small animals, including other nematodes. They are economically important, because the parasitic forms are major pests of agricultural crops, causing an estimated $5 billion in damage each year! They are round, bilaterally symmetric and pseudocoelomate, with a toughened cuticle, an outer layer that protects the parasitic forms against digestive enzymes. The basic body shape seems to be an adaptation for living in interstitial habitats, the small spaces between grains of sand. Nematode worms lack circular muscles. They only have longtitudinal muscles, and thus appear to thrash about aimlessly. This type of motion appears rudderless when we see them free floating in water or vinegar (Tubatrix), but works very well in their usual interstitial habitat, where there are plenty of packed grains of soil to push against and wriggle through.
They are mainly aquatic. Even the terrestrial forms are basically aquatic, living in the thin film of water that usually coats grains of soil. Males are often smaller than females, and have a copulatory hook at the posterior end with which they can hold open the genital pore of the female. Nematodes excrete ammonia by diffusion, sometimes in conjunction with special excretory cells that are peculiar to this phylum. They have a rudimentary nervous system, with a nerve ring serving as a brain, nerve cords that run the length of the body, and numerous bristles and other structures for mechanical and chemical senses.
Some of the nastier parasitic forms deserve special mention. Nematodes of the genus Trichinella form cysts in pork, which can lead to a deadly case of trichinosis. When the larval worms begin to tunnel through the body, as many as 500 million at a time, the resulting physical trauma can be fatal, and survivors are often left with permanent muscle damage. Hookworms and pinworms are common nematode parasites found in small children, or in anyone walking barefoot over infected soil. Filarial worms are a serious pest in many tropical countries, and cause the grotesque swelling called elephantiasis. Ascaris, the intestinal roundworm, is a common human parasite. A single female holds up to 30 million eggs, and can lay up to 200,000 eggs a day. The eggs are spread from dried feces contaminating the soil. One out of six people worldwide are infected with intestinal roundworms (yuck!).
(fr. Latin rota = wheel, ferre = to bear)
Rotifers are very widespread aquatic animals, very common in freshwater habitats. 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. They only reproduce sexually, and have separate sexes. Most are parthenogenetic, unfertilized eggs can develop directly into female adults. They copulate by means of "hypodermic injection". These strange little animals are a very important link in the food chain in aquatic environments. They may have evolved from flatworms, because they share many basic features, such as flame cells, a similar pharynx, and numerous cilia.
Phylum Nematoda - roundworms (Ascaris, Tubatrix [vinegar eels])
Phylum Rotifera - rotifers (Philodina)
Make a wet mount slide of the nematode Tubatrix, the vinegar eel, using Protoslo. Observe its characteristic movement. Why does it seem to twitch aimlessly?
Examine cs slides of Ascaris Look for the intestine, ventral and dorsal nerve cords, muscle cells, and gonads. Is this a male or a female worm? Look for the appropriate sex organs.
Observe the dissected Ascaris. Look for the mouth, anus, and intestine. Is this a male or female worm? Does it have a copulatory hook? Ascaris is sexually dimorphic. Note the pseudocoelom, defined by the lack of membranes enclosing the body cavity.
Make a wet mount slide of rotifers, using Protoslo. Watch for the swirling currents generated by the corona as it feeds. Observe the grinding pharynx or mastax as it periodically pulls in and crunches up the food carried in by the ciliated corona.
Nematodes cause billions of dollars in crop damage every year.
Nematodes are important in soil aeration, as global recyclers of bacteria and fungi, and as food for other animals.
Many nematode parasites are medically important, such as Ascaris, the intestinal roundworm, Trichinella, hookworms, pinworms, and filarial worms.
How does the nematode's lack of circular muscles help it move through the grains of soil and other particles in its natural habitat?
The Berkeley server has a good introduction to flatworms, with several interesting links: