Acoelomorpha,
Platyzoa, and Mesozoa
Chapter 14
Bilateria
 Most animals have
bilateral symmetry.
 The vast majority of
animal species
belong to the clade
Bilateria, which
consists of animals
with bilateral
symmetry and
triploblastic
development.
Bilateral Symmetry
 Radially symmetrical animals have the world
coming at them from all directions.
 They can catch prey coming from any direction.
 Animals that begin to move about actively
seeking food need a different body
organization.
 Distinct head end with sensory structures.
 Cephalization
Bilateral Symmetry
 Animals with bilateral symmetry have a
distinct head end and can be divided into right
and left halves.
Acoelomate Bilateral Animals
 Animals that have no space between their gut
and body wall are said to be acoelomate.
 These animals are also triploblastic – they
have three embryonic germ layers.
 Organ-system level of organization – more
division of labor among their organs.
Acoelomates
 Although flatworms undergo triploblastic
development, they are acoelomates.
Acoelomates
 These acoelomate phyla are protostomes and have
spiral cleavage.
 Most have determinate cleavage.
 These are the simplest animals with an excretory
system.
 Acoelomate phyla belong to the superphylum
Lophotrochozoa
Phylum Acoelomorpha
 Group contains ~350 species.
 Members were formerly in
Class Turbellaria within phylum
Platyhelminthes Small flat
worms less than 5 mm in
length.
 Typically live in marine
sediments; few are pelagic.
 Some species live in brackish
water.
 Most symbiotic but some
parasitic.
Phylum Acoelomorpha - Digestion
and Nutrition
 Incomplete digestive
system - no anus.
 In many acoels, the
gut and pharynx are
absent.
 Phagocytotic cells
digest food
intracellularly when
food is passed into
temporary spaces.
Phylum Acoelomorpha Reproduction
 Monoecious
 Following fertilization some or all cleavage
events produce a duet-spiral pattern of new
cells.
 May be a defining character for acoelomorphs.
Ecdysozoa & Lophotrochozoa
 Clades within Protostomia
 Modern molecular phylogenies have grouped
acoelomate and coelomate taxa together within the
protostomes.
 Protostomes now divided into two large clades:
Ecdysozoa and Lophotrochozoa
 Ecdysozoa possess a cuticle that is molted as
their bodies grow
 Lophotrochozoa share either an odd horse-shoe
shaped feeding structure, the lophophore or have
unique larval form called the trochophore
Ecdysozoa & Lophotrochozoa
• Trochophore larvae
• Minute, translucent, and roughly top-shaped
• Have a prominent circlet of cilia and sometimes
one or two accessory circlets
• Occur in the early development of other marine
members of Annelida and Mollusca and are
assumed to be the ancestors of such groups
• Trochophore-like larvae also occur in some
Platyhelminthes, Nemertean, Echiura and
Sipunculida groups.
Clade Platyzoa
 Clade Platyzoa is a unique group of
lophotrochozoan protostomes that contain
Platyhelminthes, Gastrotricha and Gnathifera.
Phylum Platyhelminthes
 Members of phylum
Platyhelminthes
live in marine,
freshwater, and
damp terrestrial
habitats.
Phylum Platyhelminthes
 Flatworms are
flattened
dorsoventrally and
have a
gastrovascular
cavity.
 Extracellular
digestion.
 Undigested food is
egested through the
pharynx.
Phylum Platyhelminthes
 The osmoregulatory
system consists of
protonephridia
(excretory or
osmoregulatory
organs closed at the
inner end) with flame
cells.
 Most metabolic
wastes removed by
diffusion across the
body wall.
Phylum Platyhelminthes
 The nervous system
consists of a ladderlike network of
nerves and a bilobed
brain.
 Many have large
ocelli – light sensing
organs.
Phylum Platyhelminthes
 Many can reproduce
asexually as well as
sexually.
 Asexual
reproduction via
fission.
 Sometimes the new
individuals remain
attached – chains of
zooids.
 Monoecious
Taxonomy
 Flatworms (phylum Platyhelminthes) are
divided into four classes:
 Class Turbellaria – ex. Planaria
 Not monophyletic
 Class Trematoda – parasitic flukes
 Class Monogenea – parasitic monogenetic flukes
 Class Cestoda - tapeworms
Phylum Platyhelminthes
Class Turbellaria
 Turbellarians are nearly all free-living and
mostly marine.
Class Turbellaria
 The best-known turbellarians, commonly called
planarians, have light-sensitive eyespots and
centralized nerve nets.
Class Trematoda
 Trematodes live as parasites in or on other
animals.
 They parasitize a wide range of hosts.
Class Trematoda
 Subclass Digenea,
digenetic flukes,
have a complex
life cycle with a
mollusc (snail) as
the first host and a
vertebrate as the
final, or definitive,
host.
Class Monogenea
 All monogeneans
are parasites.
 Often found in the
gills or external
surfaces of fishes.
Class Cestoda
 Tapeworms (Class
Cestoda) are also
parasitic and lack a
digestive system.
 The scolex is
equipped with suckers
and hooks for
attachment to the host.
 Each proglottid
contains a set of
reproductive organs.
Class Cestoda
 Cestodes usually
require at least two
hosts.
 Adult cestodes are
parasites in the
digestive tracts of
vertebrates.
Pseudocoelomates
 Pseudocoelomates have a body cavity (the
pseudocoel) between the gut (derived from
endoderm) and body wall (derived from
mesoderm).
 Derived from the blastocoel.
Advantages of a Body Cavity
 A body cavity, pseudocoel or coelom, has
several advantages. It provides:
 Greater freedom of movement.
 Space for development of organ systems.
 A simple means for circulation of materials around
the body.
 Storage place for waste products.
 A hydrostatic organ.
Pseudocoelomates
 Pseudocoelomates do not form a clade.
 Some are part of superphylum Lophotrochozoa,
others are in superphylum Ecdysozoa.
 All share the pseudocoelomate body plan.
Lophotrochozoa (10 Phyla)
 Ancestors possessed complex cuticular jaws – Clade
Gnathifera




Gnathostomulida
Micrognathozoa
Rotifera
Acanthocephala
 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
Phylum Gastrotricha
 Gastrotrichs appear similar
to rotifers, but without the
ciliated corona and have a
bristly looking body.
 Members of the phylum
Gastrotricha are
pseudocoelomate and have
three embryonic germ layers
(triploblastic).
 Complete digestive system.
 Hermaphroditic or
parthenogenetic.
Clade Gnathifera
 Possess small cuticular jaws with a
homologous microstructure.
 Numbers of pairs of jaws vary.
 Gnathostomulida, Micrognathozoa, and
Rotifera are tiny, free-living, aquatic animals.
 Acanthocephalans are worm-like endoparasites
living as adults in fish or other vertebrates.
Clade Gnathifera
 Rotifera and Acanthocephala
 Presumed sister taxa.
 Form a clade called Syndermata.
 Have eutelic syncytial epidermis.
 Constant number of nuclei.
 Grouping is controversial.
Phylum Gnathostomulida
 Phylum Gnathostomulida includes the jaw
worms.
 Very small - <2mm.
 Live in interstitial spaces of fine coastal
sediments.
 Can endure low O2.
Phylum Gnathostomulida
 Feed by scraping bacteria and fungi from the
substratum with a pair of jaws on the pharynx.
 Acoelomate
 Sexual stages include males, females, and
hermaphrodites.
 Fertilization is internal.
Phylum Micrognathozoa
 Micrognathozoans are tiny animals that live
interstitially (between sand grains).
 Body consists of a two-part head, a thorax, and
abdomen with short tail.
 Move using cilia and have a unique ventral
ciliary adhesive pad that produces glue.
 Three pairs of complex jaws.
Phylum Micrognathozoa
 Simple gut
 Anus opens to outside only
periodically.
 Reproductive system is not
well understood:
 Only female
reproductive organs
have been identified.
 May reproduce
parthenogenetically.
 Cleavage and
subsequent
development have not
been studied.
Phylum Rotifera
 Members of the
phylum Rotifera
are
pseudocoelomate
and have three
embryonic germ
layers
(triploblastic).
 Complete digestive
system.
Phylum Rotifera
 Dioecious (separate sexes)
but some species
parthenogenetic (females
produce diploid eggs).
 Some are parthenogenetic
during part of the year,
depending on environmental
conditions.
 Thick shelled eggs that can
withstand harsh conditions
are sometimes produced.
Phylum Rotifera
 Rotifers have a ciliated crown, the corona,
that is characteristic of the phylum.
Phylum Rotifera
 Rotifers come in a wide
range of colors and
shapes.
 Shapes often
correspond to lifestyle
(floaters, swimmers,
sessile).
 They may be individual
or colonial.
 Mostly freshwater.
 Benthic and pelagic
forms.
Phylum Acanthocephala
 All spiny-headed worms (Phylum
Acanthocephala) are parasites in the
intestines of vertebrates.
 Over 1100 species known.
 Occur worldwide and parasitize fish, birds, and
mammals.
 Larvae develop in crustaceans or insects.
Phylum Acanthocephala
 Proboscis has rows of recurved spines that
penetrate and may rupture host intestines.
 Proboscis with hooks can be inverted into a
proboscis receptacle by retractor muscles.
Phylum Acanthocephala
 Body somewhat flattened.
 About 80% of tegument is a lacunar system of
fluid-filled canals that may distribute nutrients
and remove wastes from muscles.
 No heart - function provided by lacunar fluid.
 Both longitudinal and circular body wall
muscles are present.
Phylum Acanthocephala
 No respiratory system.
 Protonephridia with flame cells, if present,
perform excretory functions.
 Nutrients are absorbed across the tegument,
which bears some enzymes - no digestive tract.
 Dioecious
 No species normally parasitizes humans.
Phylum Acanthocephala
 Acanthocephalans penetrate the intestinal wall
with spiny proboscis.
 Remarkably little inflammation on host wall, but pain
of infection is intense.
 Larval acanthors burrow through beetle
intestine.
 Develop into juvenile cystacanths in the insect
hemocoel.
 Pigs become infected by eating grubs.
Phylum Mesozoa
 Phylum Mesozoa is 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.
Phylum Mesozoa
 Most composed of only 20 to 30 cells arranged
in two layers.
 Layers are not homologous to germ layers of
other metazoans.
 Two classes, Rhombozoa and Orthonectida,
are so different that some authorities place
them in separate phyla.
Phylum Mesozoa
 Rhombozoans live in




kidneys of benthic
cephalopods.
Adults called vermiforms
and are long and slender.
Inner, reproductive cells give
rise to vermiform larvae.
When overpopulated,
reproductive cells develop
into gonad-like structures
producing male and female
gametes.
Larvae are shed with host
urine into the seawater.
Phylum Mesozoa
 Orthonectids parasitize a variety of
invertebrates.
 Reproduce sexually and asexually.
 Asexual reproduction consists of a
multinucleated mass called a plasmodium.
Phylogeny of Mesozoans
 Some consider these organisms primitive
flatworms and place them in phylum
Platyhelminthes.
 Mesozoans are identified as lophotrochozoan
protostomes based on molecular data, but are
not placed in Platyzoa.