READING IN CLASS


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  1. System of metabolism in flat, round and annelids. Metabolism of Flatworms.
  2. Both free-living and parasitic platyhelminths utilize oxygen when it is available. Most of the parasitic platyhelminths studied have a predominantly anaerobic metabolism (i.e., not dependent upon oxygen). This is true even in species found in habitats—such as the bloodstream—where oxygen is normally available.

    Parasitic platyhelminths are made up of the usual tissue constituents—protein, carbohydrates, and lipids—but, compared to other invertebrates, the proportions differ somewhat; i.e., the carbohydrate content tends to be relatively high and the protein content relatively low. In larval and adult cestodes, carbohydrate occurs chiefly as animal starch, or glycogen, which acts as the main source of energy for species in low oxygen habitats. The level of glycogen, like other chemical constituents, can fluctuate considerably, depending on the diet or feeding habits of the host. In some species, more than 40 percent of the worm’s dried weight is glycogen.

    Because carbohydrate metabolism is important in parasitic flatworms, a substantial amount of carbohydrate must be present in the host diet to assure normal growth of the parasite. Hence the growth rate of the rat tapeworm (Hymenolepis diminuta) is a good indicator of the quantity of carbohydrate ingested by the rat. Experiments have shown that most parasitic worms have the capability of utilizing only certain types of carbohydrate. All tapeworms that have been studied thus far utilize the sugar glucose. Many tapeworms can also utilize galactose, but only a few can utilize maltose or sucrose.

    An unusual constituent of both trematodes and cestodes is a round or oval structure called a calcareous corpuscle; large numbers of them occur in the tissues of both adults and larvae. Their function has not yet been established, but it is believed that they may act as reserves for such substances as calcium, magnesium, and phosphorus.

    The chief proteins in cestodes and trematodes are keratin and sclerotin. Keratin forms the hooks and part of the protective layers of the cestode egg and the cyst wall of certain immature stages of trematodes. Sclerotin occurs in both cestode and trematode eggshells, especially in those that have larval stages associated with aquatic environments.

    Platyhelminth eggs hatch in response to a variety of different stimuli in different hosts. Most trematode eggs require oxygen in order to form the first larval stages and light in order to hatch. Light is thought to stimulate the release of an enzyme that attacks a cement holding the lid (operculum) of the egg in place. A similar mechanism probably operates in cestodes (largely of the order Pseudophyllidea) whose life cycles involve aquatic intermediate hosts or definitive hosts, such as birds or fish.

    In many cestodes, especially those belonging to the order Cyclophyllidea, the eggs hatch only when they are ingested by the host. When the host is an insect, hatching sometimes is apparently purely a mechanical process, the shell being broken by the insect’s mouthparts. In vertebrate intermediate hosts, destruction of the shell depends largely on the action of the host’s enzymes. Activation of the embryo within the shell and its subsequent release depend on other factors, including the amount ofcarbon dioxide present, in addition to the host’s enzymes. Factors involving a vertebrate host are also important in establishing trematode or cestode infections after encysted or encapsulated larval stages have been ingested. Under the influence of the same factors, tapeworm larvae are stimulated to evaginate their heads (i.e., turn them inside out, so to speak), a process that makes possible their attachment to the gut lining.

    Metabolism of Nemathelminthes.

    Metabolism of Annelida. Oligochaeta.

    Gas exchange in amost all oligochaetes, aquatic and terrestrial, takes place by the difi of gases through the general body integi which in the larger species contains a capillary! work within the outer epidermal layer.

    True gills occur in only a few oligochaetes>| cies of the aquatic genera Dero and Aulophorus have a circle of finger-like gills at the posterior of the body. A tubificid, Branchiura, has filamentous gills located dorsally and ventrally in the posterior quarter of the body.

    The larger oligochaetes usually have hemoglobin dissolved in the plasma. The hemoglobin in Lumbricus transports 15 to 20 percent of oxygen utilized under ordinary burrow condition| where the partial pressure of oxygen is ah same as that in the atmosphere above ( When the partial pressure drops, the hemoglobin compensates by increasing its carrying capari| (Weber, 1978).

    Many aquatic oligochaetes tolerate relative low oxygen levels and, for a short period, eve complete lack of oxygen. Members of the far Tubificidae, which live in stagnant mud and 1 bottoms, are notable examples. There are members of this family, such as Tubifex tubiex, from long exposure to ordinary oxygen tens Tubiex ventilates in stagnant water by щ its posterior end out of the mud and waving about.

    Class Hirudinea.

    The glossiphoniids and piscicolids (rhynchobdellids) have retained the blood-vascular system of oli¬gochaetes, but the coelomic sinuses act as a supplemental circulatory system. In the other leech orders the ancestral circulatory system has disappeared, and the coelomic sinuses and fluid have been converted to a blood-vascular system. The hemocoelomic fluid is propelled by the con¬tractions of the lateral longitudinal channels.

    Gills are found only in the Piscicolidae, the gen¬eral body surface providing for gas exchange in other leeches. The piscicolid gills are lateral leaflike or branching outgrowths of the body wall.

    Respiratory pigment (extracellular hemoglobin) is found only in the gnathobdellid and pharyngo-bdellid leeches and is responsible for about half of the oxygen transport.

  3. The structure of the digestive system of flat, round and annelids.Type Flatworms.
  4. The mouth leads into a muscular pharynx that pumps into the digestive tract the cells and cell fragments, mucus, tissue fluids, or blood of the host on which the parasite feeds. The pharynx passes into a short esophagus and one or, more commonly, two blind intestinal ceca that extend posteriorly along the length of the body. The physiology of nutrition is still incom¬pletely understood, but secretive and absorptive cells have been reported, so digestion is apparently extracellular in part.

    The blind-ending intestine of trematodes consists of a simple sac with an anterior or midventral mouth or a two-branched gut with an anterior mouth; an anus is usually lacking, but a few species have one or two anal pores. Between the mouth and the intestine are often a pharynx and an esophagus receiving secretions from glands therein. The intestine proper, lined with digestive and absorptive cells, is surrounded by a thin layer of muscles that effect peristalsis; i.e., they contract in a wavelike fashion, forcing material down the length of the intestine. In many larger flukes lateral intestinal branches, or diverticula, bring food close to all internal tissues. Undigested residue passes back out of the mouth.

    Cestodes have no digestive tract; they absorb nutrients from the host across the body wall. Most other flatworms, however, have conspicuous digestive systems.The digestive system of turbellarians typically consists of mouth, pharynx, and intestine. In the order Acoela, however, only a mouth is present; food passes directly from the mouth into the parenchyma, to be absorbed by the mesenchymal cells.

    Introduction:

    During this week of our animal diversity survey, we will study three worm phyla. All of the phyla of worms that we will examine - the annelids, the nematodes, and the platyhelminthes - contain species that are parasites of humans (not to mention other animals and plants). You may already be familiar with some of these creatures: you are likely to encounter leeches (an annelid) simply from wading in a steam or pond, and if you ever had a dog or cat, you probably took it to the vet at least once to be treated for worms (such as roundworms and whipworms, both nematodes, and tapeworms, a platyhelminth).

    The parasitic worms that you will examine are for the most part eating and reproducing machines. Consequently, when studying the parasitic worms, take a good look at their digestive and reproductive systems, and then compare them to the digestive and reproductive systems of free-living worms (e.g., earthworms).

    Phylum Platyhelminthes

    The phylum Platyhelminthes (platy, flat; helminth, worm) includes a diversity of marine, freshwater, and terrestrial worms, plus two rather important parasitic groups: the flukes and the tapeworms. Like cnidarians (= hydras, jellyfish, and corals), flatworms have a rather simple body plan and share some features with them. They also have a few morphological advancements over cnidarians. Some characteristics of flatworms are:

    1. They are triploblastic, as all three primary germ layers (e.g., ectoderm, endoderm and a middle tissue layer, the mesoderm) form during embryonic development. As a result, flatworms have well-developed, mesodermal-derived muscle layers. However, they are acoelomate, lacking a true body cavity.
    2. Flatworms lack organs for transporting oxygen to body tissues. As a consequence, each of their cells must be near the body surface for gas exchange to take place, resulting in a flattened body plan.
    3. Flatworms are bilaterally symmetrical.
    4. The digestive system of flatworms, if present, consists of a single opening that serves as both the mouth and anus. This opening, the mouth, leads into a branched gastrovascular cavity. Both digestion and absorption of nutrients occur in the gastrovascular cavity, obviating the need for a well-developed circulatory system.

    The phylum is divided into four classes:

    Class Turbellaria, free-living marine, freshwater, and terrestrial flatworms. Class Trematoda, parasitic internal flukes

    Class Cestoda, parasitic tapeworms

    Class Monogenea, parasitic external flukes

    Specimens of Platyhelminthes

    We will examine live speciemens (Dugesia) and microscope slides (Dugesia, Clonorchis, Taenia) representative of free-living and parasitic platyhelminthes.

    Dugesia, Class Turbellaria, live specimen.

    Obtain a live Dugesia flatworm by sucking it up from the side or bottom of a glass jar using a medicine dropper. Place the specimen in a small Petri dish, making sure that it is completely covered by pond water, and examine it under your dissecting scope.

    Dugesia is a common turbellarian (= planarian) that resides in freshwater steams and ponds. Note your animal's shape, pigmentation, and mode of locomotion. Dugesia, as well as most free-living flatworms, move over surfaces by means of cilia on their ventral surface. Note the pigmented eye spots, or ocelli, located on the triangular "head" of the animal. These eye spots are sensitive only to light and dark, and are unable to resolve images. On either side of the eye spots are lateral lobes which serve as chemosensory organs. Cover your culture dish (top and sides) with a piece of aluminum foil, and place the dish on a dark background with a microscope light shining on it. After 5 to 10 minutes, remove the foil and observe where your animal is relative to the light. Is your animal positively or negatively attracted to light (= phototactic)? How might this behavior be adaptive for the animal in its natural environment?

    Dugesia feeds by extruding its pharynx from a ventrally-located pharyngeal cavity. The mouth of the pharynx opens into the gastrovascular cavity, which has many branches (diverticula) to facilitate digestion. Place a small piece of food into the culture dish and observe the response of your specimen. If you are lucky, you may be able to see Dugesia extrude its pharynx and suck up food particles like a mini vacuum cleaner (Figure 1).

    Figure 1: Planarian flatworm, Dugesia, feeding (from Pechenik 1991, Biology of the Invertebrates).

    Dugesia, microscope slide (Figure 2).

    Observe a prepared whole mount of Dugesia under low power of your compound microscope. You should see the eye spots and the diverticula of the gut. Also look for the "brain", nerve cord, and excretory system. Figure 2: Dugesia, whole mount (from Stamps, Phillips & Crowe. The laboratory: a place to do science, 3rd ed.)

    Clonorchis, Class Trematoda, preserved specimen and microscope slide (Figure 3).

    Clonorchis sinensis, the human liver fluke, is a parasitic trematode found in the bile ducts of humans. Like most parasitic worms, the life cycle of C. sinensis is extremely complex and involves several hosts. The adult worm sheds eggs into the bile ducts of its human host, which eventually reach the small intestine and are passed with feces. If the eggs are ingested by the proper species of aquatic snail, they hatch into larvae that then progress through a series of asexual stages, culminating in an infective larval stage known as cercariae. The cercariae are ciliated, and have a tail for swimming. They pass out of the snail, and then briefly swim about in the water until they encounter a fish. Then the cercariae penetrate the muscles of the fish, lose their tails, and remain encysted until the fish is eaten by the definitive (= final host). These encysted larvae are freed in the human small intestine after consumption of improperly prepared fish. The immature flukes migrate through the bile duct and its tributaries throughout the liver, where they develop into adult worms. If untreated, an infection by Clonorchis can lead to enlargement and cirrhosis of the liver.

    Observe a prepared whole mount of Clonorchis under low power of your compound microscope. Unlike flatworms, flukes have a protective cuticle covering their bodies (why?). Note the anterior oral sucker around its mouth, for attachment to host tissues. A muscular pharynx and esophagus lead to a two-branched intestine (= gastrovascular cavity). Slightly posterior to the branching point of the intestine is the ventral sucker, or acetabulum, that also serves to attach the organism to its host's tissues. A small excretory pore is located at the posterior end.

    The remaining conspicuous organs are reproductive structures. The large, branched organs located in the posterior of the organism are the two testes. A vas deferens connects each testis to a single, median seminal vesicle (not easily seen) that stores sperm and transports it to a genital pore located anterior to the acetabulum.

    The mid-section of the fluke contains the female reproductive structures. An enormous uterus occupies much of the central region of the worm, and stores eggs. On either side lateral to the uterus are yolk glands that secrete yolk for egg formation via yolk ducts (not visible). A small, lobed ovary can be seen posterior to the uterus, and behind that is a sac-like seminal receptacle for storing sperm received during copulation.

    Taenia, Class Cestoda, preserved specimen and microscope slide (Figure 5).

    Observe a prepared slide of Taenia under low power of your compound microscope. Your specimen, Taenia pisiformis, is a tapeworm of carnivores (notably, dogs), and closely resembles T. solium and T. saginata, common parasites of humans contracted by eating poorly prepared beef or pork, respectively. Tapeworms share many features with flukes, including an outer cuticle, attachment structures, expansive reproductive organs, and complex life cycles involving intermediate hosts. Unlike flukes, however, tapeworms lack a mouth and gastrovascular cavity, a consequence of their life in vertebrate organs of high nutritional activity (i.e., the small intestine). Bathed by food in their host's intestine, they absorb predigested nutrients across their body surface via diffusion and possibly, active transport.

    The body of a tapeworm is divided into four main regions. A small scolex ("head") bears suckers and an elevated rostellum with curved hooks; the suckers are used for attachment to the host's organs. Immediately posterior to the scolex is a "neck" that produces many proglottids ("segments") by asexual budding. Each proglottid is potentially a complete reproductive unit containing by male and female reproductive organs (i.e., each is hermaphroditic). Why might hermaphroditism be especially advantageous for an internal parasite?

    The second region consists of small, immature proglottids nearest to the neck and scolex.

    The third region, or mid-section, consists of mature proglottids, each with well-developed male and female reproductive organs. These proglottids engage in internal, cross fertilization. In a mature proglottid, locate the lateral genital pore that contains both a thin, tubular vagina and a stouter vas deferens. Trace the vagina posteriorly and note that it passes between two ovaries and terminates at a shell gland anterior to a yolk gland. Eggs are fertilized and "yolked" before passing anteriorly into a sac-like uterus. The male reproductive system consists of numerous small, round testes, each with a tiny tubule that connects to a single vas deferens, which transports sperm to the genital pore.

    The fourth and posterior region of the tapeworm consists of gravid (= "pregnant") proglottids. In gravid proglottids, most of the gonads are atrophied, leaving only an enlarged uterus packed with eggs. These gravid proglottids eventually break off from the body of the adult worm, and pass out of the digestive tract in the host's feces. When a small mammal, such as a rabbit, ingests a proglottid or eggs, the eggs hatch into larvae that then bore through the intestinal wall and then move through the circulatory system where they eventually become encysted in muscle tissue. When the rabbit is eaten by a dog, the encysted larvae are released, and develop into adult worms. As can be seen from the specimen on display, tapeworms can be quite large: T. solium, a parasite of the human intestine, can reach a length of 10 feet!

    Phylum Nematoda

    Nematodes are probably the most abundant and ubiquitous animals on earth, having invaded virtually every habitat. Most of the approximately 10,000 species of nematodes are free-living, but many are parasites of animals, including humans. Trichinella spiralis, for example, is contracted by eating insufficiently cooked pork. The adult worms develop in the human intestine, releasing larvae which move through the lymphatic system, eventually ending up in muscle tissues where they encyst. Other nasty nematode parasites of humans include Necator americanus (hookworms) and Wuchereria, which results in elephantiasis. Nematodes also are parasites of plants and can cause enormous crop damage; as a result, some large universities have departments of plant pathology devoted to the study of plant pathogenic nematodes.

    Noteworthy characteristics of nematodes are:

    1. they are triploblastic.
    2. they have a pseudocoelom, a cavity incompletely lined by mesodermally-derived tissue.
    3. the fluid-filled pseudocoelom functions as a hydrostatic skeleton.
    4. they have a complete, one-way digestive tract, having both a mouth and an anus.
    5. they have a non-living, protective cuticle covering their bodies.

    Specimens of Nematodes

    We will examine preserved specimens of Ascaris lumbricoides, commonly known as the roundworm, an intestinal parasite of humans. Humans contract Ascaris by ingesting eggs from the soil. Once ingested, the eggs hatch, releasing larvae. The larvae bore through the small intestine and migrate via the venous and lymphatic systems to the lungs. There the larvae continue to grow, and pass through several larval stages. After a few weeks, the larvae are coughed-up, literally, and then swallowed, where they develop into mature adults in the small intestine.

    Ascaris, external morphology (Figure 6)

    Examine preserved specimens of male and female ascarids. The male is smaller, and has a curved, posterior end for grasping the female during copulation. These differences in size and morphology are examples of sexual dimorphisms. Why do you think sexes of Ascaris differ in size?

    Ascaris, internal morphology (Figure 6)

    Obtain an Ascaris worm from your laboratory TA. Female Ascaris are somewhat easier to dissect, because their larger size makes it easier to find and identify various organs. However, you should examine both a dissected male and female worm, so ask around in lab to find a dissected worm of the opposite sex. Determine the dorsal surface by locating the anus, which is on the ventral side. Then, place the animal in a dissecting pan, pinning it at both the head and tail ends, dorsal side up. Using fine scissors or a scalpel, carefully cut along the midline of the dorsal surface to expose the internal organs. Pin the body wall back so that organs are exposed, and submerge your animal in water so that its internal organs float freely.

    Note the body cavity, which is a false coelom (pseudocoel). How does this pseudocoel differ from a true coelom? The two, faint lateral stripes are lateral lines that bear excretory canals which empty into an excretory pore, located anteriorly on the ventral surface (not visible). Other, fainter longitudinal streaks are bundles of longitudinal muscle, formed from embryonic mesoderm. There are no circular muscles. Given the absence of a hard, bony skeleton and circular muscle, how do you think a nematode moves?

    The straight, tubular digestive system for the most part is undifferentiated (why?) and consists of a mouth, pharynx, intestine, and anus.

    The most conspicuous organs in the pseudocoel are the tubular reproductive organs. Nematodes are very prolific, and females of some species may shed thousands of eggs daily. Carefully uncoil the reproductive organs, which are Y-shaped. The vagina is located at the base of the Y, and the two arms are the uteri. Each uterus connects to an oviduct, which in turn connects to an ovary. The uterus, oviduct, and ovary are continuous and have no obvious demarcations between them, although the uterus tends to be slightly larger in diameter.

    Phylum Annelida

    The phylum Annelida includes approximately 15,000 marine, freshwater, terrestrial, and parasitic species. It is the archetypal 'wormy' phyla, with the majority of forms possessing a long, thin shape. The long shape is attained in annelids by metameric segmentation, a linear repetition of body parts and organs. Segmentation has enabled annelids to become particularly adept at a particular type of locomotion, burrowing. In addition to segments, other annelid features include:

    1. A triploblastic, bilaterally-symmetric body plan with a true coelom; that is, their body cavity is completely lined by mesodermally-derived tissue (the peritoneum).
    2. The fluid-filled coelom functions as a hydrostatic skeleton.
    3. A closed circulatory system with dorsal and ventral blood vessels, with one to many "hearts"; often with hemoglobin as a respiratory pigment.
    4. A complete, one-way, digestive tract, with a separate mouth and anus.

    The phylum is divided into three classes, two of which are characterized by tiny bristles (setae) in their body walls:

    Class Polychaeta (= many setae), marine species such as sandworms that usually possess fleshy, lateral extensions (parapodia) from their body wall.

    Class Oligochaeta (= few setae), freshwater and terrestrial species (e.g., earthworms).

    Class Hirudinea, leeches, which lack setae and move in an inch-worm fashion using anterior and posterior suckers, or swim via undulations.

    Earthworm dissection: Obtain a preserved specimen of the earthworm (Lumbricus) for dissection. Identify the dorsal and ventral surfaces. Make an incision on the dorsal surface from the prostomium (mouth) to the middle of the body. Carefully cut and pin back the skin to expose the internal anatomy. Use Figure 8 to identify the structures listed below, and consider the basic function of each structure as you examine it.

    You should be able to identify the following structures on a dissected earthworm:

    prostomium
    crop
    gizzard
    pharynx
    intestine
    esophagus

    Hirudo, preserved specimen (demonstration), ectoparasite

    Observe the specimens of Hirudo, a leech representative of the class Hirudinea. Leeches probably evolved from oligochaetes, and are the most specialized of annelids. Some leeches are predaceous, but most are external parasites of other animals, and have several adaptations for a parasitic lifestyle. Their body is dorso-ventrally flattened, and the first and last segments are modified to form suckers. Why is a flat shape useful for ectoparasites? Can you think of any arthropod parasites that have flat shapes? Except in primitive species, the internal segmentation has been lost. Consequently, the movement of leeches differs somewhat from other annelids, and depends on the use of suckers for attachment to the substrate. Given what you know about locomotion in earthworms, how do you think a leech uses its suckers to move about? The mouth of the leech has toothed jaws, which it uses to make an incision in its host to feed on its blood. An anticoagulant, hirudin, secreted into the wound keeps the host's blood flowing. What arthropods might benefit from having such an anticoagulant? Like oligochaetes, leeches are hermaphroditic, bearing both male and female reproductive organs.

    DIGESTIVE SYSTEM (FOOD PROCESSING)

    Movement, coordination and sensing things all require energy, so organisms need ways to capture and process energy from their environment. In some organisms (plants), this is a matter of directly harnessing the sun's energy to convert carbon to glucose (photosynthesis), but in many other organisms, locating and immobilizing prey to obtain energy is a complex process. Once animals have located and/or subdued their food, that food needs to get broken down into smaller bits, nutrients like glucose need to get absorbed into the body system, and waste products must be eliminated. This requires some sort of digestive system.

    You probably know the story in mammals (like you) - you break apart the food with your teeth and enzymes (amylase) found in your saliva, then you swallow the smaller bits, and enzymes in your stomach and small intestine break it down further. Although substances like alcohol and caffeine can be absorbed across the stomach lining (hence the buzz when consumed on an empty stomach), the site of most absorption of nutrients occurs in the small intestine. The surface of the small intestine is lined with villi, projections into the lumen of the intestine that dramatically increase the surface area for digestion. The small intestine has three specialized regions: 1) the duodenum, where most digestion occurs, 2) the jejunum, and 3) the ileum, where (with the jejunum) 90% of the absorption of nutrients occurs. From the small intestine, waste passes into the colon where water and ions are absorbed into the body, and undigested wastes are eliminated through the rectum and anus.

    So, what happens in other organisms that are not you? How do invertebrates go about the processes of acquiring food, where do they digest it, and where do they absorb it? Knowing what they need to do, think about what types of materials each organism consumes, and try to locate the likely places that these processes occur. Refer back to Figure 8 and to your dissected earthworm to answer these questions for the free-living earthworm.

    Animal Diversity and Organ Systems Lab I: Worms

    The questions that follow will more specifically guide your analyses. The list of questions is not exhaustive, but should direct your attention to specific key points of the worm systems that you will see today. Some questions will require that you synthesize information from lectures, your book and the lab.

    Questions on Worm Phylogeny:

    1. What are the major features of the platyhelminthes?
    2. What are the majr taxonomic divisions in the phylum Platyhelminthes?
    3. What are the major features of the nematodes?
    4. What are the major features of the annelids?
    5. What are the major taxonomic divisions in the phylum Annelida?
    6. If comparing two organisms, what characteristics do they share because of homology (history)? What do they share because of convergent evolution?
    7. How are parasitic worms similar and different from their free-living relatives? What structures have they lost? What structures/organs are expanded?
    8. Are there features that are common between the ecto- and endoparasites that you observed? What are the differences between these types of parasites?
    9. Why is a flat shape useful for ectoparasites? Can you think of any arthropod parasites that have flat shapes?