Chapter 8
ACOELOMATE BILATERAL ANIMALS
14-1
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
• Active, directed movement most efficient with an
elongated body form with anterior (head), posterior
(head), dorsal, and ventral sides
14-2
General Features
Position and Biological Contributions
•
•
•
•
Simplest animals with primary bilateral symmetry
Mesoderm well-defined
Triploblastic-3 primary germ layers
Highly specialized parasites
14-3
General Features
• Acoelomates-no body cavity
– Typical acoelomates have only one internal space,
the digestive cavity
• Region between the epidermis and digestive cavity is
filled with parenchyma
14-4
15-5
Insert new figure 14.5
14-6
Phylum Platyhelminthes
Characteristics
•
•
•
•
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
14-7
Phylum Platyhelminthes
• Platyhelminthes is divided into four classes:
• Turbellaria-planaria
• Trematoda- digenetic ( 2 hosts) flukes
• Monogenea- monogentic ( 1host) flukes
– Cestoda-tapeworms (1-3 hosts)
– All members of Monogenea and Trematoda (flukes) and
Cestoda (tapeworms) are parasitic
14-8
Phylum Platyhelminthes
– 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
14-9
Phylum Platyhelminthes
Form and Function
• Epidermis and Muscles
– Most tubellarians 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
14-10
14-11
14-12
Phylum Platyhelminthes
• Some turbellarians, and all other members of this
phylum
– Turbellians have a syncytial epidermis
• Nuclei are not separated by cell membranes
• Non-turbellarians- parasitic classes
– Lack cilia and have a tegument
• Under the basement membrane
– Muscle fibers run circularly, longitudinally and diagonally
• Parenchyma cells fill spaces in the body
14-13
14-14
Phylum Platyhelminthes
Nutrition and Digestion
• Cestodes-tapeworms have no digestive system
• Others have a mouth, pharynx, and intestine
• In planarians
– Pharynx may extend through the ventral mouth
• Intestine has three branches
– One anterior and two posterior
• Mouth of trematodes and monogeneans
– Opens near the anterior end
– Pharynx is not extensible
– Intestine ends blindly, varies in degree of branching
14-15
Phylum Platyhelminthes
• Planaria
– Carnivorous and detect food by chemoreceptors
– Food trapped in mucous secretions from glands and
rhabdites
– Wrap themselves around prey
– Extend the proboscis to suck up bits of food
14-16
Figure 8_07
Phylum Platyhelminthes
• Monogeneans and Trematodes
– Feed on host cells, cellular debris, and body fluids
– Proteolytic enzymes from the intestine are secreted for
extracellular digestion
– Phagocytic cells in gastrodermis complete digestion at
intracellular level
– Undigested food egested out the pharynx
• Cestodes-tapeworms
– Rely on the host’s digestive tract
– Lack a digestive system
– Absorb digested nutrients
14-18
Phylum Platyhelminthes
Excretion and Osmoregulation
• Flatworms have protonephridia-flame cells
– Used for osmoregulation
– Beating flagella drive fluids through the weirs to tubules,
into collecting ducts, excreted through pores
– 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
• Marine turbellarians
– Lack these units
– No need to expel excess water
14-19
Phylum Platyhelminthes
Nervous System
• Subepidermal nerve plexus resembles nerve net of
cnidarians
• One to five pairs of longitudinal nerve cords lie under
the muscle layer
• Freshwater planarians
– One ventral pair of nerve cords forming a ladder-type
pattern
– Brain is a bilobed ganglion anterior to the ventral nerve
cords
14-20
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
14-21
Phylum Platyhelminthes
Reproduction and Regeneration
• Fission
– Many turbellarians constrict behind the pharynx and separate
into two animals
– Each half regenerates the missing parts
– Some do not separate immediately, creating chains of zooids
• Regeneration
– If the head and tail are cut off
• Each end grows the missing part
– Extract of heads added to a culture of headless worms
prevents regeneration
14-22
Phylum Platyhelminthes
Class Trematoda
• All trematodes are parasitic flukes
• Most adults are endoparasites of vertebrates
• They resemble turbellaria but the tegument lacks cilia
in adults
• Adaptations for parasitism include:
– Penetration glands-lytic enzymes to enter skin
– Glands to produce cyst material
– Hooks and suckers for adhesion
– Increased reproductive capacity
14-23
Phylum Platyhelminthes
• Some trematodes share several characteristics of
turbellarians
• Similar reproductive, excretory and nervous
systems
• Sense organs are poorly developed
14-24
Phylum Platyhelminthes
Subclass Digenea
• Indirect life cycle in most
– First intermediate host a mollusc
• Definitive or final host
– Vertebrate
– Organisms reproduce sexually in this host
• A 2nd or 3rd intermediate host may be required in the
life cycle
• Parasitize a wide range of hosts
14-25
Phylum Platyhelminthes
• General Digenean Life Cycle
– Egg passes from definitive host in excreta and
must reach water
– Hatches into a free-swimming ciliated larva, the
miracidium
– Miracidium penetrates tissues of a snail
• Transforms into a sporocyst
– Sporocyst reproduces asexually to form sporocysts
or rediae
– Rediae reproduce asexually and form rediae or
cercariae
14-26
Phylum Platyhelminthes
– Cercariae emerge from the snail
• Penetrate a 2nd intermediate host or encyst on objects
• Develop into metacercariae (juvenile flukes)
– Metacercaria develop into adults when eaten by
definitive host when
• Some serious parasites of humans and
domestic animals are digeneans
14-27
Phylum Platyhelminthes
Clonorchis sinensis: Human Liver Fluke
•
•
•
•
Most important human liver fluke
Common in China, Japan, and Southeast Asia
Also infects cats, dogs, and pigs
Heavy infection may cause cirrhosis of the liver and
death of human
• Adult fluke is 10–20 mm long with an oral and ventral
sucker
14-28
Phylum Platyhelminthes
Clonorchis 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
14-29
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
14-30
14-31
Phylum Platyhelminthes
Schistosoma: Blood Flukes
• Over 200 million people infested with schistosomiasis
– Common in Africa, South America, West Indies, and the
Middle and Far East
• Sexes are separate
• 3 species account for most human schistosomiasis:
– Schistosoma mansoni in venules of large intestine
– S. japonicum in venules of small intestine
– S. haemotobium in venules of urinary bladder
14-32
Figure 8_10a
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
14-34
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
14-35
Figure 8_10b
Phylum Platyhelminthes
• Eggs of S. mansoni and S. japonicum
– Damage the intestinal wall
• Eggs of S. haematobium
– Damages the bladder wall
• Control: proper disposal of human wastes
• Schistosome dermatitis (swimmer’s itch)
– Occurs when cercariae penetrate an unsuitable host such
as a human
14-37
Figure 8_11
Phylum Platyhelminthes
Class Monogenea
• Monogenetic flukes were originally placed in
Trematoda
• Some now argue they are sister taxa, both having a
posterior attachment with hooks
• External parasites of fish, especially gills, but a few
are found in bladders of frogs and turtles
• Have direct life cycle in a single host
14-39
Phylum Platyhelminthes
• 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
14-40
14-41
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
• Muscles, excretory and nervous systems similar to
other flatworms
• Lack sensory organs except for modified cilia
14-42
14-43
Phylum Platyhelminthes
• 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
14-44
14-45
Phylum Platyhelminthes
• Chain of proglottids is called a strobila
• Proglottids originate in the germinative zone just
behind the scolex
• Some practice self-fertilization, although the norm is
cross-fertilization from one proglottid to another
• Shelled embryos form in the uterus
– Either expelled or the whole proglottid is shed
14-46
Figure 8_15
Phylum Platyhelminthes
• Nearly all cestodes require two hosts
– Adult is parasitic in the digestive tract of the vertebrate
• Over 1000 species of tapeworms known, infecting
almost all vertebrates
• Most tapeworms do little harm to host
14-48
Phylum Platyhelminthes
Taenia saginata: Beef Tapeworm
• Lives as an adult in the alimentary canal-digestive
tract) of humans
• Juvenile form found in intermuscular tissue of cattle
• Mature adults can reach over 10 meters (32 feet) in
length with over 2000 proglottids
• Scolex has four suckers but no hooks
• Gravid proglottids (with shelled, infective larvae)
pass in feces
14-49
Phylum Platyhelminthes
• Each mature proglottid has muscles and parenchyma
plus male and female organs
• Gravid proglottids usually crawl out of feces
• Proglottids rupture as they dry
– Embryos are viable for five months and are picked up by
grazing
14-50
Phylum Platyhelminthes
Life Cycle
• Cattle swallow shelled larvae that hatch
• They 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 evaginates and waits for
humans to eat undercooked beef.
14-51
Phylum Platyhelminthes
• Once eaten by humans new proglottids develop in
the intestines in 2–3 weeks
• Infected individuals expel numerous proglottids daily
• Infection can be avoided by eating only thoroughly
cooked beef
14-52
14-53
Phylum Platyhelminthes
Taenia solium: Pork Tapeworm
• Adults live in small intestine of humans
• Juveniles live in muscles of pigs
• Scolex has both suckers and hooks
• If eggs or proglottids are ingested by a human instead of pig
– Embryos migrate to organs and form cysticerci
• Cysticercosis commonly occurs in eyes or the brain; blindness,
serious neurological symptoms or death
• Infection can be avoided by eating thoroughly cooked pork
14-54
Brain of person who died from
cerebral cysticercosis
14-55
Phylum Rotifera
Characteristics
• Possess a ciliated crown, the corona, that beats like a
rotating wheel
• Approximately 2000 species
• Aquatic species are mostly benthic
15-56
15-57
Phylum Rotifera
• Rotifer body has a head, trunk and a foot
• Corona is often a pair of cilated discs
– Beating of the cilia help in feeding and locomotion
– Mastax is a muscular pharynx equipped with hard jaws for
grasping and chewing
– Narrow foot has one to four toes and may be
retractile
Attaches with pedal glands that secrete an adhesive
Pair of protonephridial tubules with flame cells empty into a
cloacal bladder that collects excretory and digestive waste
Bilobed brain and sensory organs include eyespots, sensory pits
and papillae
15-58
Phylum Rotifera
Reproduction
• Dioecious
• Males are unknown in many species
• Females are parthenogenetic, producing diploid eggs
that hatch into diploid females
• Females produce
– amictic eggs that form diploid females
by parthenogenesis
or
– Haploid mictic eggs that, if not fertilized, become haploid
males
15-59