LAB 3. PHYLUM PLATYHELMINTHES

Transcription

1 LAB 3. PHYLUM PLATYHELMINTHES 1. Overview The Flatworms (= platy - helminthes) are bilaterally symmetrical, acoelomate, dorsoventrally flattened organisms. Phylogenetically, they are the first animal phylum to possess complete organ systems. Most of the approximately 30, 000 species are parasitic and as such are widely distributed worldwide. Almost all invertebrates (especially molluscs) and vertebrates are host to at least one species of platyhelminth. By far, most research on this group deals with the pathogenic trematodes (e.g. Schistosoma) and cestodes (e.g. Echinococcus) of humans and their domesticated animals. We will look at representatives of the three classes of Platyhelminth - the Turbellarians, the Trematodes and the Cestodes. The focus of the lab will be on life-cycle diversity, functional morphology and biodiversity of these 3 taxa. 2. Essential components of the lab observe motility, behaviour and functional morphology of live Planaria spp. observe living and/or frozen material of all life-cycle stages of a representative trematode and cestode understand functional morphology of a representative specimen from each class of Platyhelminth 3. Classification Class Turbellaria Planaria sp. (live demo) Bdellaura sp. (slide mount) Class Trematoda Subclass Digenea Fasciola hepatica/fascioloides magna (slides of miricidia, cercariae, adult) Leucochloridium variae (slides) Schistosoma sp (slide of cercariae, adults in copula) Ornithodiplostomum ptychocheilus (live metacercariae, SEM diagrams, other lab materials) Dicrocoelium dendriticum (slides) Class Cestoda Subclass Cestodaria Gyrocotyle (preserved specimen) Bothriocephalus sp. (slide mount) Dipylidium caninum (slides) Echinococcus granulosis (slides of adult and larvae)

2 4. Functional Morphology and General Biology A. Class Turbellaria The 4, 500 species of Turbellarians include mostly free-living flatworms. Most are marine, but many freshwater types are found in our area. There are 2 striking features of the Turbellarians. One is their extremely complex hermaphroditic reproductive system. This, coupled with their small size and the fragility of the body, means that it is rarely feasible for us to see much anatomical detail, apart from that in preserved material. Thus, we will only study a few of the more recognizable types, and only superficially. The second feature is their colour. Surprisingly, most free-living flatworms are brightly coloured. We will get no appreciation for this at all, because freshwater ones, and commercially-available Planaria sp., are drab. Place one or more live planarians into a dish of fresh water and observe them under low power on a dissecting microscope. Planarians are usually negatively phototactic. Test this idea using whatever means you think appropriate. They also have a definite righting response. Test this as well. Observe also their means of locomotion. We should also be able to observe feeding and the action of the muscular pharynx. Lastly, make a wet mount and transfer to your compound microscope to examine the typical turbellarian digestive system. You may be able to observe the beating of cilia covering the body surface that aid in locomotion. Refer to figures in Chapter 8 of Pechenik to help you understand the Turbellarian body plan. Next, examine the whole-mount slides of Planaria under the compound microscope. The anterior end is marked by the paired eye spots and the ear-like flaps, called auricles. The muscular pharynx is located in the middle of the animal, within the pharyngeal chamber. To feed, the worm protrudes the pharynx out through the mouth. The pharynx connects to the digestive tract, which in this case is three-branched. This is about all you can discern from this slide. Switch to the commensal, Bdellaura, found on the gills of horseshoe crabs. Note that this species lacks auricles, but has eyespots. Carefully examine the area under the eyespots to find the paired cerebral ganglia and then the paired nerve cords that pass posteriorly from them. Posterior to the pharynx is an area where the male and female reproductive systems come together and open to the outside via a gonopore. The penis bulb should be visible as a conical mass in the atrium. Lastly, examine a slide of a cross-section through the body of a planarian. Notice the lack of a body cavity. Use the appended figure to find parenchyma tissue, epidermis, rhabdoids, basement membrane, circular and longitudinal muscles and the 2 longitudinal nerve cords. The cells lining the gut are irregular columnar cells that are well supplied with food vacuoles. The other preserved species (Temnocephala sp.) is ecto-commensal on the gills of marine crustaceans. Although they may seem a bit obscure to those of us on the Prairies, they provide clues to adaptations hat may have lead to the evolution of the parasitic platyhelminths from free-living Turbellarian ancestors.

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4 B. Class Trematoda All trematodes are parasitic on the inside or outside of their hosts. Final hosts are always a vertebrate. They are similar in general shape and morphology to the turbellarians, but the similarities end there. They differ in having an external tegument and one or two suckers for attachment. Their life-cycles are usually fantastically complex, with up to 4 hosts involved in the various stages. First intermediate hosts are almost always gastropod mollusks. Focus on the following 4 features of trematode biology.

5 General life cycle. It is important that you spend some time learning the generalized life cycle of a typical trematode. Refer to figure 8.19 to help you follow the complexities of a standard life cycle. The life cycle of all trematodes involves a free-living ciliated miracidium stage that hatches from the egg. Think of the miracidium as a swimming germ ball. Given that they are approximately the size of Paramecium, but are obviously multicellular, their complexity is astounding. The sole function of all miracidia is to recognize and penetrate its correct species of snail. After active invasion of the snail, the miracidia initiates a remarkable sequence of asexual reproduction. The first developmental stage within the snail involves the formation of sporocysts. Within the sporocysts, redia are formed which in turn, produce thousands of free-swimming cercaria. Cercariae exit the snails into water, secrete a resistant cyst (usually in a further host) and encyst as metacercariae. When these are ingested by a suitable final host, hermaphroditic adults grow, reproduce and the cycle repeats. I will take you through the dissection of a common fathead minnow so that you can observe living examples of several stages of an example trematode. Trematode functional morphology. In this exercise, we will first cover the basic anatomy of an adult fluke and then briefly consider diversity by examining some others. Fig in Pechenik describes basic trematode morphology. You should refer to this figure as you examine the following material. Examine the slide of the small liver-fluke of sheep and cattle, Dicrocoelium dendriticum, under a dissecting scope. Identify the narrow anterior end and locate the anterior sucker surrounding the mouth at the distal end. Just posterior to the sucker is the pharynx that then branches off into the digestive cecae which has many branches. Just posterior to the intestinal bifurcation is the prominant ventral sucker, a unique muscular structure used for attachment. Posterior to this structure lies the female reproductive organs, ending in a common genital pore. You should be able to observe the dark, much-branched ovary and the sinuous uterus, completely packed with eggs. Posterior to the female systems, between the 2 intestinal branches in the middle of the animal, are the branched testes. Between the diverticula of the intestine are the extensive yolk glands. At this stage of information overload, step back and ask yourself some general questions. How does this animal feed? Why is so much of the animal committed to reproduction? Why such extensive yolk glands? What is the advantage of hermaphroditism to this group? How does the worm cause pathology in its host? Pick another species from your slide box (e.g. Echinostoma) for comparison. Try to identify as many of the features noted above. Schistosomes. Forget everything you ve learned up to now on trematode morphology and life-cycles. A quick look at figures 8.16 aand 8.19 will convince you. It is unfortunate that the most important group of trematodes (in terms of human misery) is a group that breaks all the rules. From your slide box you will notice that they are dioecious worms (see S. japonicum or S. mansoni); the males have a special canal in which females permanently reside. Second, they lack a metacercarial stage so that

6 cercariae released from snails directly penetrate the definitive host. After penetration, they mature in the liver and mesenteric veins. Pathology in humans is associated with the eggs, which tend to accumulate in the liver, lymph, spleen and bladder. In this case, pathology is not actually due to the worms themselves, but to the host s immune response to the eggs. The host response functions to encase the eggs, and in so doing, increases their diameter and their pathogenicity. This phenomenon is known as immunopathology. Eggs from a single worm can cause injury to various tissues, and a worm can lay eggs for up to 30 years. Note the morphology of the egg of S. mansoni from your slide box. That small spike on the egg is a characteristic feature. Note also the morphology of the cercariae, and the forked tail. Within the cercariae body you should see an oral sucker and ventral sucker. It is avian schistosome cercariae that is the source of the local problem known as swimmers itch. Be sure you understand the cause of swimmers itch, the hosts involved (generally) and why it is so difficult (probably impossible) to control.

7 C. Class Cestoda All cestodes (or tapeworms) are internal parasites of vertebrates. They differ from trematodes in that they lack any type of digestive tract and they have a segmented body. Each segment contains a set of male and female reproductive organs. All cestodes feed by absorbing nutrients across their tegument, and all possess an anterior holdfast on the head region called a scolex (Fig and 8.13). The entire body is designed for little more than converting absorbed nutrients into eggs. The gravid segments are the most posterior portion of the worm. Functionally, they are sacs filled with eggs. All tapeworms have complex life cycles, usually involving invertebrate intermediate hosts. In this section, we will cover the basic anatomy of a typical cestode and then briefly examine cestode diversity. The primitive Cestodarians Be sure to observe the preserved specimen of Gyrocotyle. These are considered primitive cestodes and their ancestors probably form the link between Turbellarians, Trematodes and Cestodes. These specimens were dissected from the intestine of a ratfish on the West Coast. Very little is known about the biology of this group. It is thought that the life cycle involves a ciliated larvae which directly penetrates the host (otherwise how else could it get into the intestine?). Another interesting feature of this parasite is that ALL ratfish in a population are always infected with TWO individuals; very rarely do you find one or three! Its morphology is also bizarre, with an anterior acetabulum (similar to the trematodes) and a posterior, folded rosette which it uses to fold into the spiral valve of its primitive hosts. The Eucestodes (the true cestodes) The true cestodes typically have suckers and hooks on the scolex and a series of proglottids housing the reproductive structures. For a basic understanding of cestode morphology and life-cycles, we will examine specimens of Diphylobothrium and Dipylidium. The first is a common parasite of fish; the last is a common parasite of dogs. Focus on 1) the differences in life-cycle patterns and 2) basic adult morphology of the head regions (scolex, suckers and hooks) and the posterior sections. Dipylidium caninum is a common cestode of dogs. Focus attention on basic anatomy and life-history (see Fig in text). Each section has both female and male reproductive systems as well as nervous and excretory systems. For our purposes, we will not focus on the details of these systems. Suffice to say that they are very similar to the system noted in trematodes, but simply repeated over and over in the various segments. Note the two vaginal openings and bilobed ovary in each mature section. The uterus contains numerous egg capsules, each with eggs. Note the complex structure of the scolex. The intermediate host for this species is the common flea, where the larvae develop. Dogs become infected when they accidentally ingest fleas when they groom.

8 Life-cycle of Diphyllobothrium spp. Species in this genus can infect humans, particularly in northern cultures where eating raw fish is a cultural norm. They are also important because they can be pathogenic to the second intermediate host; the large white cysts also infect hosts such as lake trout and whitefish, making them unmarketable. As such, it is an important problem for northern economies, many of which rely on commercial fisheries for sustenance. Many researchers have studied this group to try to find a method of control. None have succeeded. Your slide box contains specimens of D. latum, the broad fish tapeworm of humans. Gravid sections of worm pass out of the final host (a carnivore, often humans), releasing thousands of tiny eggs. These hatch, releasing a larval stage very similar in form and function to the trematode miracidia. These swim and penetrate the first intermediate host, usually a copepod. There, they penetrate the host s gut, encyst and then await ingestion by the next host, usually trout, whitefish or ciscoe. After ingestion, the parasite penetrates the fishes gut, travels to the muscles and encysts again as a plerocercoid. 100 s of these large cysts can be found in fish from some lakes. When the fish is ingested by a carnivore (usually gulls or loons in the north), the worms excyst and over the span of the following year, become gravid. Biology of Echinococcus granulosis. Use the specimens in your slide box and diagrams in your text to understand the lifecycle, basic morphology and disease implications of this fascinating cestode. As for the Schistosomes, the species that breaks all the rules, is also the one which is most important as an infection of humans. Given your understanding of the life-cycle, how do humans become infected with this lethal parasite?

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10 5. Platyhelminth diversity Echinostoma this is a parasite of local ducks, geese and muskrats. Typical pond snails act as first and second hosts (cercariae leave the snail, only to find another one for encystment). These are good specimens to observe functional anatomy. Observe the characteristic row of spines surrounding the oral sucker. What is their function? Leucochloridium this is the adult stage of one of the more famous trematodes. It is found in the cloaca of passerine birds. This is the species which causes drastic changes in the appearance of infected snail tentacles. Notice its small size, prominent reproductive organs and prominent suckers. Halipegus this species is found under the tonges of frogs. It is also a good species to observe functional morphology. The two posterior ovaries and single testes are obvious, as are the hundreds of lateral yolk glands. This species is unusual in that it uses four hosts in its life-cycle (snails, copepods, odonate larvae and frogs). Calamus this bizarre species is from the intestine of pilot whales. Notice the lateral spines that cover the tegument (to aid in attachment). The suckers and some of the internal anatomy should also be obvious. Rhopalias this is another weirdo collected from the small intestines of pocket gophers in Florida. Notice the odd gland/spines arrangement on both sides of the oral sucker. Apparently, these are used to stab the intestinal wall to aid in attachment. 6. Questions for discussion 1. Compare the number of eggs in the uterus of Bdellaura with those in parasitic platyhelminths, such as Fasciola. One characteristic of parasitic platyhelminths compared to the free-living ones, is a notoriously high reproductive rate. Why is this so? 2. Consider the uniqueness of the dioecous life-history strategy of Schistosomes. No other parasitic platyhelminths do this. If they are to be dioecous, then problems associated with mate finding and mate choice, become important. How might you expect mate finding to occur? How do they resolve this major problem once a mate is found? 3. What are some of the life-cycle adaptations that have evolved to enable trematodes and cestodes to colonize terrestrial habitats? 4. How has the ancestoral turbellarian body plan been altered to permit a parasitic way of life in the trematodes and cestodes?

11 5. Adult schistosomes live in the periphral blood stream of their hosts. All other bloodinhabiting parasites utilize blood-feeding vectors (recall malaria) to transmit their offspring to other hosts. Schistosomes have a major problem in that their eggs must somehow get from the blood to the faeces (or urine), and then get into water. How do they pull this off? What is the link between this mechanism and the disease known as Schistosomiasis? 6. It is very simple for the vertebrate immune system to attack parasites that live in the blood (unless they are intracellular like Plasmodium spp.). How can it be that the adult Schistosomes can avoid host immunity and live in a host for up to 30 years? 7. Outline the differences and similarities between the digestive and reproductive systems of Cestodes Vs. Trematodes. 8. In comparison to almost all other cestodes, the life-cycle of Echinococcus is bizarre. Can you envision some of the selection pressures that may have lead to the evolution of this strategy?