chapter 8

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CHAPTER 8 CHAPTER 8

AcoelomatesAcoelomates

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General Features

• Animals that actively seek food, shelter, home sites,

and mates require a different set of strategies and body organization than radially symmetrical sessile organisms

• Two major evolutionary advances – Cephalization

• Concentrating sense organs in the head region– Primary bilateral symmetry

• Body can be divided along only 1 plane of symmetry to yield 2 mirror images of each other

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General Features

• Triploblastic• Acoelomates

– Typical acoelomates have only one internal space, the digestive cavity

• Region between the epidermis and digestive cavity is filled with parenchyma

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Phylum Acoelomorpha

Characteristics• Small flat worms less than 5 mm in length• Typically live in marine sediments; few are pelagic• Have a cellular ciliated epidermis• Parenchyma layer contains small amount of ECM and circular,

longitudinal, and diagonal muscles

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Phylum PlatyhelminthesCharacteristics• Commonly called flatworms• Vary from a millimeter to many meters in length• Some free-living; others parasitic• The parasitic clades

– Share an external body covering called a syncytial tegument or neodermis

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Phylum Platyhelminthes

• Platyhelminthes is divided into four classes: Turbellaria, Trematoda, Monogenea, and Cestoda– Class Turbellaria

• Mostly free-living forms • Most are bottom dwellers in marine or freshwater• Freshwater planarians

– Found in streams, pools, and hot springs• Terrestrial flatworms limited to moist places

– All members of Monogenea and Trematoda (flukes) and Cestoda (tapeworms) are parasitic

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Form and Function• Epidermis and Muscles

– Most have cellular, ciliated epidermis on a basement membrane

– Rod-shaped rhabdites • Swell and form a protective mucous sheath

– Most turbellarians have dual-gland adhesive organs

• Viscid gland cells fasten microvilli of anchor cells to substrate

• Secretions of releasing gland cells provide a quick chemical detachment

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Phylum PlatyhelminthesNutrition and Digestion • Cestodes have no digestive system• Others have a mouth, pharynx, and intestineExcretion and Osmoregulation • Flatworms have protonephridia

– Used for osmoregulation– Wall of the duct beyond the flame cell bears folds or microvilli

to resorb ions and molecules– Majority of metabolic wastes

• Removed by diffusion across the cell wall– Collecting ducts join and empty at nephridiopores

• Marine turbellarians– Lack these units– No need to expel excess water 14-13


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Phylum PlatyhelminthesSense Organs• Active locomotion favored cephalization and

evolution of sense organs• Ocelli (light-sensitive eyespots)

– Present in turbellarians, monogeneans, and larval trematodes

• Tactile and chemoreceptive cells – Abundant, especially in the ear-shaped auricles

• Statocysts (equilibrium) and rheoreceptors (sense direction of water currents) in some

• Sensory nerve endings found in– Oral suckers and genital pores of parasitic groups

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Reproduction and Regeneration

• Fission– Many turbellarians constrict behind the pharynx and separate

into two animals– Each half regenerates the missing parts

• Provides for rapid population growth– Some do not separate immediately, creating chains of zooids

• Regeneration– If the head and tail are cut off

• Each end grows the missing part; it retains polarity– Extract of heads added to a culture of headless worms

prevents regeneration

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Phylum Platyhelminthes• Some asexual reproduction occurs in intermediate

hosts• Nearly all are monoecious but cross-fertilize• Endolecithal eggs• Some turbellarians and all other groups have female

gametes with little yolk– Yolk is contributed by separate organs, vitellaria– Vitelline ducts bring yolk cells to the zygote (ectolecithal

development)– Zygote and yolk cells surrounded by eggshell move into

the uterus

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• Larval trematodes emerge as ciliated larvae– Penetrate a snail or eaten by a host

• Cestodes hatch only after being consumed by a host– Many different animals can serve as intermediate hosts

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Classification of Phylum Platyhelminthes• Class Turbellaria• Class Trematoda• Class Monogenea• Class Cestoda

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Class Turbellaria – Mostly free-living– Range from 5 mm to 50 cm long– Others move by cilia

• Glide over a slime track secreted by adhesive glands

• Rhythmical muscular waves pass backward from the head

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Class Trematoda• All trematodes are parasitic flukes• Most adults are endoparasites of vertebrates• They resemble ectolecithal turbellaria but the

tegument lacks cilia in adults• Adaptations for parasitism include:

– Penetration glands– Glands to produce cyst material– Hooks and suckers for adhesion– Increased reproductive capacity

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Phylum PlatyhelminthesSheep Liver Fluke• Fasciola hepatica

– First digenean whose life cycle was described• Adult fluke lives in bile passageways in the liver of

sheep and other ruminants• Eggs are pass out in feces • Miracidia hatch and penetrate snails to become

sporocysts • After two generations of rediae

– Cercaria encyst on vegetation and await being eaten by sheep

• When eaten, metacercariae develop into young flukes

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Clonorchis sinensis Life Cycle • Adults live in bile passageways of humans and other

fish-eating mammals• Eggs containing a complete miracidium are shed into

water with feces• The eggs hatch only when ingested by snails of

specific genera• Miracidium enters snail tissue and transforms into a

sporocyst• Sporocyst produces one generation of rediae, which

begin differentiation

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• Rediae pass into the snail liver– Continue embryonation into tadpole-like cercariae

• Cercariae escape into water– Make contact a fish in the family Cyprinidae– Bore into fish muscles or under scales– Shed tail and encyst as metacercariae

• A mammal eats raw fish – Cyst dissolves and flukes migrate up bile duct

• Heavy infection can destroy the liver and result in death

• Control of parasites– Destroy snails and thoroughly cook fish

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Schistosoma Life Cycle• Eggs discharged in human feces or urine• In water, eggs hatch as ciliated miracidia• Must contact a particular species of snail to survive • In the snail, they transform to sporocysts • Sporocysts produce cercaria directly• Cercariae escape the snail and swim until they

contact bare human skin• Cercariae pierce the skin and shed their tails

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• Enter blood vessels and migrate to the hepatic portal blood vessels

• Develop in the liver and they migrate target sites• Eggs released by females are extruded through gut

or bladder lining and exit with feces or urine• Eggs that remain behind become centers of

inflammation

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Phylum PlatyhelminthesClass Monogenea

• External parasites of fish, especially gills, but a few are found in bladders of frogs and turtles

• Have a single host• Posterior hooks may become the posterior

attachment organ of the adult, the opisthaptor• Opisthaptors vary widely (hooks, suckers, clamps)

– Withstand the force of water flow• Some serious economic problems in fish farming

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Phylum PlatyhelminthesClass Cestoda

• Tapeworms have long flat bodies with scolex – Holdfast structure with suckers and hooks

• Scolex is followed by a linear series of reproductive units or proglottids

• Lack a digestive system• Tegument is syncytial and has no cilia• Entire surface of cestodes is covered with projections

(microtriches) similar to microvilli seen in the vertebrate small intestine– Microtriches increase the surface area for food absorption

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Taenia saginata: Beef Tapeworm• Lives as an adult in the digestive system of humans• Juvenile form found in intermuscular tissue of cattle• Mature adults can reach over 10 meters in length

with over 2000 proglottids• Gravid proglottids (with shelled, infective larvae)

pass in feces

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Life Cycle• Cattle swallow shelled larvae that hatch as

oncospheres• Oncospheres use hooks to burrow through the

intestinal wall into blood or lymph vessels• When they reach voluntary muscle, they encyst to

become bladder worms (cysticerci)• When the infected meat is eaten, the cyst wall

dissolves and the scolex attaches to intestinal mucosa

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Phylum Mesozoa

• Considered a “missing link” between protozoa and metazoa

• Have a simple level of organization– Minute, ciliated, and wormlike animals

• All live as parasites in marine invertebrates• Most composed of only 20 to 30 cells arranged in two

layers– Layers are not homologous to germ layers of other

metazoans

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Phylum Nemertea

Characteristics• Often called ribbon worms • Have a long muscular tube, the proboscis• Most are less than 20 cm long

• Proboscis is an eversible organ – Protruded from a rhynchocoel for defense and catching prey

• Proboscis is everted by fluid pressure and retracted by muscles

• Has a sharp-pointed stylet at the tip

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Phylum NemerteaFeeding and Digestion • Carnivorous: Feed on dead or living prey• Slime-covered proboscis wraps around prey• Stylet pierces and holds prey until it is thrust into

mouth• Pours a neurotoxin, tetrodotoxin (the toxin in puffer

fishes) on its prey

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Phylum Nemertea

Excretion and Respiration• Near the edge of body is a lateral tube with branches

and flame cells• Wastes picked up from parenchymal spaces by flame

cells are carried out excretory ducts• Protonephridia are so closely associated with

circulatory system that they are truly excretory rather than simply osmoregulatory in function as in flatworms

• Respiration occurs through the body surface

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CHAPTER 9CHAPTER 9

Gnathiferans and Smaller LophotrochozoansGnathiferans and Smaller Lophotrochozoans

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Protostomia

Phylogeny• Large clade sometimes called a superphylum• Protostome phyla are divided between two large

clades– Lophotrochozoa and Ecdysozoa

• Lophotrochozoa (10 Phyla)– Ancestors possessed complex cuticular jaws

• Gnathostomulida• Micrognathozoa• Rotifera• Acanthoephala

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Protostomia– 6 other lophotrochozoan phyla

• Gastrotricha – Tiny aquatic animals that may be closely related to

gnathiferans• Molecular characteristics place the following with

Lophotrochozoa – Cycliophora– Entoprocta– Ectoprocta– Brachiopoda– Phoronida

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chapter 8

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