Transcript Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. CHAPTER 8 Acoelomates 14-1

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CHAPTER 8
Acoelomates
14-1
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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
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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 Platyhelminthes
Characteristics
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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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Phylum Platyhelminthes
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 Platyhelminthes
Nutrition and Digestion
• Cestodes have no digestive system
• Others have a mouth, pharynx, and intestine
Excretion 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
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Phylum Platyhelminthes
Sense 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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Phylum Platyhelminthes
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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Phylum Platyhelminthes
• 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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Phylum Platyhelminthes
Classification of Phylum Platyhelminthes
• Class Turbellaria
• Class Trematoda
• Class Monogenea
• Class Cestoda
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Phylum Platyhelminthes
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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Phylum Platyhelminthes
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 Platyhelminthes
Sheep 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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Phylum Platyhelminthes
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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Phylum Platyhelminthes
• 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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Phylum Platyhelminthes
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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Phylum Platyhelminthes
• 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 Platyhelminthes
Class 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 Platyhelminthes
Class 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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Phylum Platyhelminthes
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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Phylum Platyhelminthes
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 Nemertea
Feeding 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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Gnathiferans and Smaller Lophotrochozoans
CHAPTER 9
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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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