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ZOO 2040
Biology of Animals
Topic 6
Acoelomates (“Flatworms”)
Acoelomate Characteristics
Phylum Platyhelminthes (‘flatworms’)
First animals with bilateral symmetry and cephalization: concentration of
sense organs at the head end and ‘left-right’ orientation improves
recognition of positive/negative stimuli.
■  Protostomes with spiral embryonic!
cleavage (determinate = early fixing of!
embryo cell ‘fate’): mouth derives!
from blastopore.
■  Triploblastic–have all three germ layers:
■ 
■ 
■ 
• In some, parenchyma is made of non-contractile muscle cell bodies.
■ 
Excretion and osmoregulation!
controlled by flame cells located!
in protonephridia (absent in!
some groups).
■  No respiratory or circulatory!
system (diffusional exchanges!
by cells through the body wall).
■ 
Beginning of organs and organ!
system level of organization:
• Rudimentary nervous system (‘ladder-like’).
• Beginning of excretory/osmoregulatory system.
■ 
Phylogeny uncertain: some may have!
attained acoelomate condition secondarily.
Class Turbellaria
Phylum Platyhelminthes (continued)
■ 
■ 
■ 
Mostly free-living and aquatic.!
Range from 0.5-50 cm.
■  Most have a simple, incomplete!
digestive tract, in which the!
mouth leads to a pharynx, then to!
temporary spaces containing!
cells that take in food particles by!
phagocytosis (intracellular digestion):
Nonparasitic forms with a simple, incomplete gut (gut is missing in
parasitic species).
Movement in some controlled by longitudinal, circular, and oblique
layers of muscle. Others move along slime trails through the beating of
epidermal cilia.
Most can reproduce sexually or asexually. Most are monoecious, but
have developed ways of avoiding self-fertilization.
■ 
• Development may be direct (eggs hatch into tiny worms that resemble the adults) or
indirect (with a ciliated trochophore larval form).
• Flukes and tapeworms may have complex life cycles.
■ 
■ 
Cephalized nervous system, consisting of a head ganglion (‘brain’)
usually attached to longitudinal nerve cords that are interconnected
across the body by transverse branches:
• In some flatworms, cephalization has included the development in the head region of
light-sensitive organs (ocelli).
• Other sense organs can include chemoreceptors, statocysts, and receptors that sense
water movement (rheoreceptors).
• Solid body with no coelom (no body cavity!
other than gut).
• Ectoderm (outer), mesoderm (middle), and!
endoderm (inner).
• Mesoderm well defined and not!
derived from ectoderm or endoderm.
■ 
About 1 mm to >40 m long (but very flat < 1 mm thick).
Cellular mesodermal parenchyma replaces gelatinous mesoglea of
cnidarians and ctenophores:
• Mostly carnivorous, feed on tiny invertebrates (located by chemoreceptors).
• Polyclads have a folded pharynx and a many-branching gut. Triclads have threebranched gut.
Parasitic or free-living (turbellarians, some of which are symbiotic, live
in freshwater and marine environments).
Almost no fossil record:
■ 
Most move by laying down slime from!
marginal adhesive glands in the epidermis:
• Glide along these ‘slime trails’ by beating!
epidermal cilia and/or rhythmic muscular!
contractions originating at the head end.
• Small ones swim with cilia.
• A few trace fossils have been reported that were probably made by platyhelminths.
• Fossil trematode eggs have been found in Egyptian mummies and in dried dung of
Pleistocene ground sloths.
• Trematode larvae that parasitize mollusks may leave pits or thin spots on the inside
of the shell, and these pits may be recognized on fossil shells. Trematode larvae also
irritate mollusks and cause pearl formation.
■ 
Over 4500 known species (may be another!
10,000). 12 Orders (Acoela most primitive),!
about 120 families.
Class Monogenea
Class Trematoda
■ 
■ 
Parasitic flukes: most are ectoparasites!
of vertebrates, primarily gills and!
skin of fishes (important parasites in!
crowded aquaculture conditions),!
some live in urinary bladder of frogs!
or turtles, one is an eye parasite of!
hippopotamuses).
Also all parasitic flukes: most!
are endoparasites of vertebrates!
(several have enormous effect on!
humans, e.g., liver, lung, and!
blood flukes (schistosomiasis).
■  Body is covered with a tegument
• Allows avoidance of host immune!
system and digestive juices.
• Peculiar epidermal arrangement, in which!
the main cell bodies are deep, separated from the!
cytoplasm next to the exterior by a layer of muscle (but connected to the exterior
layer by cellular projections).
• Exterior layer is syncytial (i.e., continuous, not broken by cell membranes).
Lacks cilia in adults.
• Very high specificity of host and attachment!
site (some found only at base of gill filaments,!
others at tips).
Adults have an attachment organ (opisthaptor) on!
posterior end, with suckers, hooks and sclerites. Also!
often have an anterior adhesive organ (prohaptor) with!
suckers and adhesive glands.
■  Direct life cycles with single host: sexual maturity reached while
attached, eggs hatch into ciliated larvae (oncomiracidia) that disperse or
attach to the same host.
■  About 1,100 described species (about 19,000 more are believed to be
undescribed). Formerly thought to be trematodes (adults of both are
similar), but seem closer to cestodes based on molecular evidence.
■ 
No opisthaptor; instead have one or two suckers. Have penetration
and/or cyst-producing glands. Resemble turbellarians in having a
relatively well developed gut. Muscular, excretory, and reproductive
systems also relatively complete.
■  Over 10,000 spp. (probably another 5,000 spp.) in over 200 families
■ 
• Most have complex life cycles w/ numerous larval stages (including: miracidium,
redia, cercaria, and metacercaria) that parasitize one or more species different
from final (definitive) host of adults (where sexual reproduction occurs).
Page 1
ZOO 2040
Biology of Animals
Topic 6
Acoelomates (“Flatworms”)
Life cycle of human lung fluke, Paragonimus westermani
Life cycle of human liver fluke Clonorchis sinensis
■ 
Human infected by!
eating undercooked!
fish.
■  Metacercariae ‘ecyst’!
in small intestine and!
migrate into the bile!
ducts of the liver.
■  Worms mature and lay!
eggs, which pass out of!
the common bile duct!
to the intestines and!
are shed with feces!
into water.
■  Egg containing miracidium!
hatches after being eaten by a snail!
and develop into cercariae.
■  Cercariae exit from snail and penetrate fish skin, and encyst as
metacercariae in fish muscle.
■ 
Adult P. westermani are found most often in the lungs of the human
host: adult worms measure about 10 mm in length, 5 mm in width, and
5 mm in thickness:
• Produce eggs that are passed into the air passages of the lungs, and the eggs are
coughed up and spit out, or swallowed and passed in the host’s feces.
• The eggs develop and hatch, and the first intermediate host is a snail, they
develop into cercariae, leave the snail, and infect the second intermediate host (a
crab or crayfish).
■ 
The definitive host (human) is infected when it eats a crab or crayfish
infected with metacercariae (crabs and crayfish are generally not eaten
raw even in areas of endemic paragonimiasis, however, so transmission
to humans probably occurs most often via pickled or marinated crabs or
crayfish (in which metacercariae remain viable), or via contaminated
hands or cooking utensils).
• The metacercariae excyst in the host human’s small intestine and the immature
worms penetrate the host’s small intestine, migrate through the abdominal
cavity, penetrate the diaphragm, and finally burrow into the lungs.
• The resulting disease, pulmonary paragonimiasis can be disabling, but is rarely
fatal.
• However, the worms are often found in ectopic locations such as the brain, heart,
or spinal cord, and when present in these locations, the infections can be fatal.
Life cycle of human blood flukes, Schistosoma spp.
Trematodes and Amphibian Malformations
Schistosomes are unusual trematodes, in that the sexes are separate, they reside
in the blood vessels of the definitive host, and there are no second intermediate
hosts in their life cycles.
■  A number of species of schistosomes can cause schistosomiasis in humans, but
most human infections are caused by S. mansoni, S. haematobium, or S.
japonicum, which collectively range throughout most of Africa, parts of SE Asia
and NW South America, and some islands in the Caribbean:
■ 
■ 
• Humans are infected by the worm while bathing.
• Worm enters blood stream and eventually resides in veins of abdominal cavity, where they
mature and lay eggs, which enter the intestinal tract or bladder and are shed into water
along with urine or feces.
• As the eggs of the schistosomes penetrate the walls of the veins and the small intestine or
urinary bladder, they cause a significant amount of damage. The tissues hemorrhage, so
blood often appears in the urine or feces. As the infection progresses the tissues become
inflamed and fibrotic and unable to function normally.
• Many of the eggs produced by the female worms do not escape from the veins, but are
swept up in the circulatory system and deposited in the liver. The liver responds to the
presence of the eggs by encapsulating them in a fibrous granuloma.
• The damage to the small intestine (or urinary bladder) and liver accumulates over time and
results in a chronic, disabling disease that can be fatal.
■ 
■ 
Eggs containing miracidia hatch and infect a snail, developing into cercariae,
which exit from the snail and penetrate human skin.
Class Cestoda (‘tapeworms)
Life cycle of beef tapeworm Taenia saginata
All endoparasites, primarily of vertebrates. Ribbon-like, often very
long (>10 m). Have penetration and/or cyst-producing glands.
■  Syncytial tegument, with lots of cytoplasmic projections to increase
surface area (no digestive organs, nutrient uptake is by absorption).
■  Anterior end (scolex) generally has several suckers with!
numerous hooks for attachment.
■  Just behind the scolex is a narrower region (neck), from!
which many sections (proglottids) serially bud (several/day)
■ 
• Primarily involved in sexual reproduction: each proglottid contains!
both male (up to 1,000 testes)and female (multiple ovaries)!
reproductive organs (each segment, as well as the whole animal!
is a simultaneous hermaphrodite).
• Each proglottid contains around 50,000 eggs, generally fertilized by!
sperm from a neighboring cestode, but may be fertilized by same!
cestode or proglottid.
• Mature proglottids at posterior end of tapeworm are shed (or burst open),
releasing fertilized eggs into host’s gut, which are shed in feces.
• Usually eggs are consumed by one or more intermediate hosts (vertebrate and
invertebrate), hatching into an oncosphere larva that penetrates the host’s
intestinal wall and encysts in its body.
• Encysted parasite must be eaten by another intermediate or definitive host.
■ 
Amphibians with extra limbs
have been known for
centuries, but this has
generally been in the form of
missing digits or limb parts,
and has typically affected
<5% of the population.
Recently, there has been an
increase in the percent of
population members affected
(to 15-90%, often in multiple
species at a single site) and in
the severity of the
abnormalities (including
missing, incompletely
developed, and partially
missing limbs, extra digits
and limbs, and misshapen
eyes, tails, skin lesions, and
other deformities.
Over 3,500 known spp. in >60 families
Page 2
Biology of Animals
ZOO 2040
Topic 6
Acoelomates (“Flatworms”)
■ 
Phylum Mesozoa (‘middle animals’)
Mesozoan Classes
Discovered by van Beneden (1876), presumed ‘missing link’
between protozoa and metazoa. 50 species.
Class Rhombozoa, 75 spp.
Live as commensals!
in cephalopod kidneys.
Small (0.75-7mm): adult nematogens!
(A) consist of a layer of ciliated!
epithelium surrounding an!
elongated axial cell that is!
attached caudally to the host.
■  There may be a poorly-defined!
‘head’ (calotte ) which has!
shorter and thicker cilia than!
the elongate body (‘trunk’).
■  Reproduction poorly understood, apparently a true alternation of generations:
■ 
• Most researchers believe these organisms to be degenerate flatworms
that belong to the phylum Platyhelminthes.
• Others consider them to be colonial protozoa, perhaps ciliates.
■ 
■ 
Small (0.5mm - 7 mm), composed of 20-30 cells in two (nongerm) layers: ectoderm and endoderm. Ciliated epidermis:
•  Some believe that the inner layer is derived from mesoderm, and that
the endoderm (which is associated with digestive organs in other
animals) has been lost as an adaptation to a parasitic existence
No symmetry, body cavity, or organs. Extracellular digestion.
Excretion by individual cells. Diffusional respiration.
■  Parasites or commensals of marine invertebrates.
■  Complex (but poorly understood) life histories: asexual
(plasmodium) and sexual life cycle stages.
■  Two very different classes (= different phyla?):
■ 
• New individuals (vermiform larvae) produced asexually from w/in axial cell of
adults. These larvae exactly resemble their parents; break out through the body wall
(initially free-swimming, but become reattached to the host kidney).
• When the cephalopod host attains sexual maturity, nematogens transform into
sexually active rhombogens (B), which differ in the nature of their axial cell and the
larvae they produce (ciliate infusoriform larvae by fusion of male and female
gametes). Larvae expelled from the host in urine; their subsequent fate is unknown.
Phylum Nemertea
Mesozoan Classes
‘Ribbon worms’,!
‘nemerteans’,!
‘nemertines’.
■  About 900 spp.
■  Elongated, dorso-!
ventrally flattened,!
unsegmented ‘worms’.
■  Several cm to 60 m(!)!
long.
■  Like flatworms, they have mesodermal parenchyma, ciliated
exteriors, and secrete mucus for locomotion by ciliary beating,
but otherwise very different.
■  Mostly shallow water marine, some freshwater or in tropical soils
(unlike flatworms, they can burrow). Many are commensals with
invertebrates (especially arthropods and mollusks). A few are
parasites.
■ 
Class Orthonectida (Orthonecta), 10 spp.
Parasitize numerous invertebrates, including!
brittle stars, bivalves, polychaetes, and!
nemerteans (‘horsehair worms’).
■  Move by ciliary gliding (body is capable of!
limited undulatory ‘swimming’).
■  Body an ‘epithelial sac’, housing dioecious!
reproductive cells:
■ 
• When crowded, reproductive cells develop gonad-like!
structures that make male/female gametes.
• Egg and sperm release synchronized to improve chances of fertilization.
• Eggs fertilized outside body, develop into ciliated larvae that infect another
(or the same) host.
• First instar larvae metamorphose into plasmodia that can cause damage to
the host through suppression of sexual organs (parasitic sterilization).
■ 
Plasmodium different from rhombozoans, multiplies repeatedly by
mitosis, forming a multinucleate mass that divides to produce males
and females (adult forms thus also develop asexually from agametes).
Phylum Nemertea (cont.)
Phylum Nemertea (cont.)
Have a true circulatory system, but no heart or valvular!
blood vessels, some have hemoglobin.
■  Cephalized nervous system:
■ 
■ 
• Ring-shaped cephalic ganglion with 4 ganglia (2 dorsal, 2 ventral).
• 5 longitudinal nerves (2 lateral trunks, 2 dorsolateral, 1 mid-dorsal).
• Up to 80 photoreceptor. Chemo- and mechanoreceptors, statocysts.
■ 
Food capture by a hollow, muscular!
proboscis (not the same as the pharynx of!
turbellarians, which is an extension of the!
gut) floating in a hollow, tubular, fluid-!
filled cavity (rhyncocoel)
• When muscles around the rhyncocoel contract,!
compression of fluid in rhyncocoel everts proboscis!
and shoots it out through the proboscis pore.
• Proboscis may glue to prey, or prey may be speared!
with a sharp stylet (those with a stylet also have!
paralyzing venom).
• Proboscis is retracted with a retractor muscle,!
brings food to the mouth.
• Food is swallowed whole, or foregut is extruded!
to permit digestion.
• One-way digestive tract (separate mouth!
and anus).
Dioecious: may start out as male and later become female,!
some can reproduce asexually by fission,!
and they can regenerate:
• Most have external fertilization, but some!
are viviparous (nourish the young inside the!
female.
• Larvae may or may not metamorphose.
• Marine forms have a ciliate!
pillidium larva, with a mouth,!
but no anus
• Only a portion of these!
larvae develop into the!
juvenile (from a series of!
imaginal discs that gradually!
fuse together, somewhat!
similar to insects).
■ 
Page 3
Excretion through a pair of lateral tubes with!
many branches, opening to the outside via one!
or more pores. Wastes picked up by flame cells and carried by ducts to
the outside of the body.
ZOO 2040
Biology of Animals
Topic 6
Acoelomates (“Flatworms”)
Phylum? Superphylum? Acoelomorpha
Phylum? Class? Acoela (“no coelom”)
A disputed phylum of marine, soft-bodied animals
with larvae similar to the planula larvae of
cnidarians.
■  They have also been proposed as a superphylum
including the Acoela, Nemertodermatida and
Xenoturbellida. Most species are free-living, though
some live on the surface of other organisms
(ectocommensals).
■  Traditionally, they were considered to belong to the
phylum Platyhelminthes. In 2004, molecular studies
demonstrated that they are a separate phylum.
■  Although their taxomony is still in dispute, most
researchers believe them to be basal among the
Bilateria, slightly more derived than the Cnidaria.
About 20 families. Usually small
(<2 mm, but up to 15 mm)
invertebrates resembling
flatworms, but simpler anatomy.
Considered to be the most
primitive bilateral animals.
■  Formerly treated as an Order!
of Platyhelminthes, recent!
molecular analyses suggest!
that Acoela diverged even before!
the Platyhelminths evolved.
■  Often included as as one of two
classes within the phylum
Acoelomorpha (in which case, it
contains the majority of species).
■ 
■ 
Phylum? Class? Acoela (“no coelom”)
■ 
Phylum? Class? Nemertodermatida
Have a simple nervous system and a saclike gut.
■ 
• Digestion is accomplished by means of a syncytium that
forms a vacuole around ingested food.
• No epithelial cells lining the digestive vacuole, although there
is sometimes a short pharynx leading from the mouth to the
vacuole.
• All other bilateral animals (except tapeworms) have a gut
lined with epithelial cells.
■ 
■  Like
platyhelminths, they have no circulatory or
respiratory systems, but also lack flame cells.
■  No true brain or ganglia, have a network of nerves
beneath the ciliated epidermis, with the nerves are
slightly more concentrated towards the head end.
■  Sensory organs include a statocyst and, in some, very
primitive pigment-spot ocelli capable of detecting light.
■ 
Phylum? Xenoturbellida (“strange flatworms”)
■ 
■ 
■ 
■ 
Six species: Xenoturbella bocki!
was first described as a flatworm!
in the Order Acoela in 1949,!
from sediments off the coast!
of Scotland). X. westbladi!
was described in 1999 from!
sediments off the coast of!
Sweden. Additional species have!
been found in deeper ocean!
environments.
Wormlike, up to 4 cm, covered!
by locomotory cilia.
No digestive tract (or any other!
organs), but do have statocysts.
Based on both 18S rRNA and!
mitochondial genes, as well as!
gamete development (oogenesis), they appeared to be related to primitive
bivalved mollusks (protobranchs). Other molecular evidence (See your text)
suggests that they are deuterostomes, but this is considered unlikely. Analysis
of the four new species has resulted in their now being considered to be a very
early bilateral animal, earlier than either flatworms or molluscs.
Page 4
Very small (1-2 mm) also!
resembling flatworms.
Like Acoela, were!
formerly treated as an!
Order within the!
Platyhelminthes.
Likely related to the!
Acoela and, like them,!
probably diverged!
from platyhelminths!
before the platyhelminths diversified into
parasitic Orders.