Biology 320 Invertebrate Zoology Fall 2005

1. Biology 320Invertebrate ZoologyFall 2005 Chapter 10 – Phylum Platyhelminthes

2. Phylum Platyhelminthes • 20,000 species of soft, dorsoventrally compressed worms • Mainly marine or freshwater • Three major groups • Class Turbellaria – free-living flatworms • Class Trematoda – parasitic flukes • Class Cestoda – parasitic tapeworms • Most are acoelomate and also lack a hemal system • Small size is an adaptation to living in tight spaces, such as under rocks, in body cavities, etc.

3. Class Turbellaria • Two types, grouped according to size • Microturbellaria • Macroturbellaria • Microturbellaria • The majority of turbellarians at 4500 spp. • 0.5mm – a few mm • Macroturbellaria • Normally several cm • Largest lives in a lake in Russia and can reach 60cm in length

4. Body Wall • Typically have one layer of ciliated epidermis • Cilia may only be present on ventral surface • Name means “whirlpool,” due to ciliary action • Microvilli also present on epidermis • Soft; lack a cuticle

5. Often have a fibrous endoskeleton containing actin filaments • Epidermis is glandular; secretes: • Mucus • Adhesives • Rhabdoids • Rhabdoids • Membrane-bound • Rod-shaped • Released onto epidermal surface • Expand to form mucus • Rhabdite is the most common type

6. Adhesion facilitated by • Glands • Cilia • Muscular suckers • Many have structures known as duo-gland organs • Consists of two different kinds of gland cells in one complex • Viscid gland secretes adhesive • Releasing gland secretes de-adhesive

7. Musculature and Locomotion • Fairly complex musculature • Typical circular and longitudinal • Diagonal • Dorsoventral • Capable of many types of movement • Ciliary gliding • Creeping • Swimming using dorsoventral undulations of lateral body margins • Twisting • Somersaulting

8. Nervous System • Variable • Primitive flatworms • Ring-like brain • One or more nerve cords extending posteriorly • Nerve net • Dugesia, and similar worms • Bilateral brain • Two ventrolateral longitudinal nerve cords • Transverse commissures that give the nervous system an ladder-like appearance

9. Sensory structures • Usually two pigment cup ocelli • Photoreceptors • Many flatworms are negatively phototactic • Some possess statocysts • Balance and orientation • Mechanoreceptors • Senses pressure, touch, etc.

10. Nutrition • Most posses a blind gut used for ingestion and egestion • Extremely long flatworms have a through-gut • A single layer of gastrodermis lines gut • Phagocytes • Glandular • Ciliated in more primitive turbellarians • Gut design varies depending on body size • Microturbellaria have simple, unbranched guts • Macroturbellaria often have digestive ceca to increase SA for digestion, absorption, etc.

12. Location of mouth varies • Usually located mid-ventrally, but anywhere along the ventral midline • Presence / absence / type of pharynx varies • Zero pharynx – no pharynx • Simple pharynx – ciliated tube • Plicate pharynx – long, muscular, protrusible tube. Housed in a sheath when retracted • Bulbous pharynx – muscular sucking bulb; can be everted in some

13. Type of prey is related to pharynx structure and body size • Small - bacteria, unicellular algae, and protozoans • Large - small inverts such as rotifers, insect larvae, crustaceans, and annelids • Most are carnivorous, but some are scavengers • Many methods of prey capture • Wrap around prey • Entangle in mucus (sometimes toxic) • Capture with everted raptorial proboscis (has adhesive cups or hooks) • Pin prey to substrate with adhesive organs • Some even stab with penis stylet

14. May ingest whole prey items or pieces of prey • Eversible pharynx penetrates prey or carrion, and pumps contents into gut • Digestion • Extracellular initially. Pharynx is muscular and often has enzyme releasing pharyngeal glands • Intracellular later • Can withstand long periods of starvation by reabsorbing and metabolizing gut tissue, reproductive tissue, and various connective tissues

15. Symbiosis • Some harbor endosymbiotic photosynthesizers • Some are commensals of molluscs and arthropods • Bdelloura lives on gills of horseshoe crabs • Shares food collected by host • Several are endoparasites of molluscs, crustaceans, and echinoderms

16. Internal Transport • Most lack a coelom and hemal system • Vermiform design and flatness provides large SA:vol • Gas exchange occurs across body wall and general body surfaces • Nutrient circulation and diffusion • Simple gut is not diffusion limited • Digestive ceca provide enough surface area to meet diffusion requirements in larger worms • Gastrodermis has cilia for circulation

17. Excretion • Nitrogen from protein metabolism is in the form of ammonia • Released along with water and other metabolites • Protonephridia are the excretory “organs” • Posses lots of protonephridia, as there is no circulatory system to pump blood to a centralized kidney • Essentially a cup with ciliated terminal cells • Cilia draw fluid into cup • Cup filters fluid through a mesh-like structure • Waste travels through ducts which open to the surface via pores

18. Asexual Reproduction • Regeneration • Amazing regenerators (1/300th of body can grow into an entire worm) • Active area of research • Interesting patterns of regeneration, depending on where the animal is cut • Clonal • Transverse fission – posterior end attaches, anterior end pulls and separates • Paratomy – resembles strobilation in cnidarians • Fragmentation – cyst forms around fragment. Regeneration occurs and cyst hatches • Parthenogenesis – “virgin birth”

19. Sexual Reproduction • Hermaphroditic • Male organs • Two testes that each lead to a sperm duct… • Then to a seminal vesicle (storage) • Penis – copulatory organ that may be armed with a sharp, hardened stylet • Female organs • Gonopore for copulation • Copulatory bursa for short term storage of partners sperm • Seminal receptacle for long term sperm storage

21. Copulation • Reciprocal • Rarely self fertilize • Internal fertilization • Hypodermic impregnation • Eggs • Oviposited from gonopore (singly, clutches, strings) • Relatively few eggs • Carefully spawned and have protective capsules • Some FW spp. produce both summer eggs and resting (winter) eggs • Usually direct development • Some produce planktotrophic larvae

22. Diversity of Class Turbellaria • Order Catenulida • Small, long, and slender • Few epidermal cilia • Head has one statocyst and two ciliated pits • Mouth opens into simple pharynx • Gonads unpaired • Aflagellate sperm • May reproduce via paratomy • Catenula

23. Order Acoela • Less than 2mm long • Many cilia on epidermis • No pharynx • Lack cellular gut • Individual germ cells rather than gonads • Biflagellate sperm • No oviduct, eggs rupture wall • No protonephridia • Amphiscolops

24. Order Macrostomida • Lots of epidermal cilia • Rhabdites • Duo-glands • Small paired ocelli • Simple pharynx • One pair of ventrolateral nerve cords w/ commissures • Aflagellate sperm • Macrostomum

25. Order Polycladida • Many epidermal cilia • Rhabdites and duo-gland organs • Biflagellate sperm • Plicate pharynx • Large, up to 30cm • Oval, flattened • Large gut w/ many ceca • Some produce larvae • May be brightly colored (aposematic in some) Stylochus

26. Order Tricladida • Many epidermal cilia • Rhabdites and duo-gland organs • Biflagellate sperm • Plicate pharynx • Yolky eggs • Large gut with three branches and many ceca • Bdelloua, Dugesia

27. Order Rhabdocoela • Several suborders • Many epidermal cilia • Rhabdites and duo-gland organs • Biflagellate sperm • Protrusible bulbous pharynx • Some have proboscis which can be everted • Some have ventral adhesive disc • Mesostoma

28. Neodermata • Flukes, tapeworms and relatives belong to the taxon Neodermata • Cellular epidermis is later replaced with syncytium called neodermis • Non-ciliated • No intracellular spaces due to syncytium • One multinucleate cell • Nothing unwanted passes

29. Class Trematoda • Flukes • Belong to Subclass Digenea, which means “two generations” • Endoparasites of vertebrates • 11,000 spp.

30. Body Form and Function • Flat • 0.2 mm – 6.0 cm long • Various adhesion devices • Oral sucker surrounding mouth • Ventral sucker • Facultative anaerobes that mainly rely on glycolysis • Neodermis helps protect against host’s digestive enzymes

31. Circular, longitudinal, and diagonal musculature • Nervous system • Similar to that of the turbellarians • Pair of anterior cerebral ganglia (brain) • Longitudinal nerve cords (variable) • Ocelli in infective larval stages (miracidia and cercaria) • Excretory system • Protonephridia • Two longitudinal collecting ducts • Posterior bladder • One nephridiopore

32. Reproduction • Highly organized reproductive system • Constant supply of nutrients from host allow for large egg production • 10,000 to 100,000 that of turbellarians • Male system • Two testes • Two ducts • External seminal vesicle • Cirrus sac (internal seminal vesicle, prostate, copulatory cirrus)

33. Genital atrium shared between male and female reproductive systems; contains one gonopore • Female system • One ovary (germarium) • Oviduct - where seminal vesicles and vitellaria (yolk producing structures) are added • Ootype (sac which encapsulates egg and yolk cells in protein) • Uterus • To genital atrium; out gonopore

34. Some dioecious, but most are monoecious • Most cross-fertilize, but some can self-fertilize • No asexual reproduction in adults • No need, as adults usually have a constant supply of nutrients • Asexual reproduction in some larval stages

35. Trematode Life Cycles • Two or more hosts / infective larval stages • 1st intermediate host is usually a gastropod mollusc (snail) • 2nd intermediate host is usually an arthropod or fish • Definitive host is a vertebrate • Aquatic cycle (may also occur on land) • Eggs are passed in feces • If land in water, hatch as miracidia (infect snails) • Miracidium sloughs off epidermis and metamorphoses into sporocyst, which contains many embryos • Sporocyst embryos form redia (feeding larvae with digestive system; also produce embryos) • Redia embryos develop into cercaria (possess digestive tract, suckers and tail) • Cercaria leave snail, swim, and infect 2nd intermediate host (usually arthropod or fish), and encyst as metacercaria • Definitive host eats infected muscle tissue, and metacercaria develops into adult • Free metacercaria can also be found attached to rocks or aquatic plants

37. Chinese Liver Fluke • Opisthorchis (or Chlonorchis) sinensis • Infects bile ducts of 20 million people in Asia • Adults are 2.5 cm long • Live up to 8 years • Produce 4000 eggs / day • Causes jaundice, gallstones, and possibly liver cancer

39. Blood Fluke • Schistosoma mansoni • Found in neotropics, and Africa • Causes schistosomiasis • 300 million people worldwide • One of three worst parasitic diseases • Dioecious • Male and female permanently paired • Smaller female fits in groove on male • Live in intestinal veins • Eggs laid in venules • Work way into lumen using spikes / enzymes

40. Life cycle is typical of trematodes • Cercaria penetrate human skin w/ enzymes and muscular boring • Enter circulation and transform into adults on way to intestine • First pass through lungs and liver • Damage to organs and vessels from various life stages (eggs are the worst) • Causes inflammation, necrosis, and fibrosis

41. Class Monogenea • 1100 spp. • Typically 1 – 5 mm long, but up to 20 mm • Mainly ectoparasites of aquatic vertebrates; therefore rely on aerobic metabolism • Name means “one generation,” because they lack clonal reproduction • Also lack intermediate hosts • Have a large attachment organ • Known as a haptor • Located posteriorly • Has suckers and hooks

42. Egg develops into a hooked miracidium • Called oncomiracidium • Has two ocelli • Adult • Head (sometimes has muscular oral sucker, or adhesive glands) • Trunk • Haptor • Pharynx secretes protease which degrades host’s epidermis • Feed on: • Cellular debris • Blood • mucus • Remaining body systems are similar to turbellarians and trematodes

43. Life Cycles • Dactylogyrus • Ectoparasite of fish gills • Serious problem in hatcheries • Leads to death by blood loss or secondary infection • Polystoma • Inhabits bladders of old world frogs • Synchronization of life cycles (host and parasite eggs released at same time) • Oncomiracidia attach to tadpole’s gills • When tadpole becomes frog, parasite migrates from gill chamber to bladder

44. Class Cestoda • The tapeworms • 3400 spp. • All endoparasites of vertebrate guts • Lack gut entirely • Long; can reach 25 m

45. Body Form • Ribbon-like • Three regions • Scolex – head with hooks and suckers for attachment • Neck – narrow growth zone containing stem cells • Strobila – segmented trunk consisting of proglottids (segments) • Neodermis allows worm to avoid host’s immune system • Rely almost entirely on anaerobic respiration • Muscular, nervous, and protonephridial system is similar to that of turbellarians and trematodes

46. Reproduction • Strobilation occurs at neck region • Each proglottid has a miniature reproductive system • During copulation, cirrus is everted and inserted into gonopore • When one worm is present • Self fertilize using one proglottid • Serf fertilize between two proglottids • When two or more worms are present • Cross fertilize • Worms have proglottids that are in different stages of sexual maturity • Eggs constantly being produced • Mature (gravid) proglottids break off from strobila and are passed in feces

47. Life Cycles • Usually two or more hosts • Definitive host always a vertebrate • Aquatic and terrestrial life cycles • Diphyllobothrium is a fish tapeworm that can infect humans • Eggs in feces are deposited in water • Hatch into a ciliated, swimming oncosphere larva (has hooks) • Oncosphere is ingested by a copepod • Oncosphere moves to hemocoel of copepod, sheds ciliated epidermis, replaces with neodermis, and develops into a procercoid larva • Fish eats copepod • Procercoid migrates from gut of fish to muscle, and transforms into a juvenile called a plerocercoid (metacestode) • Development is completed in the gut lumen of a fish-eating vertebrate after ingestion of infected muscle tissue

49. Taenia is a terrestrial tapeworm that infects domestic animals and humans • Can reach 20 m but is typically 3 to 5 m • Lifecycle • Eggs are removed in feces • Animal ingests eggs and oncosphere bores through intestinal wall and migrates to skeletal muscle • Oncosphere develops into a cysticercus (another metacestode w/ inverted scolex) • Development is completed in the gut lumen of a vertebrate after ingestion of infected muscle tissue