Acoelomorpha, Platyzoa, and Mesozoa Chapter 14
Dec 21, 2015
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 SymmetryRadially 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 AnimalsAnimals 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.
AcoelomatesThese 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 & LophotrochozoaClades 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 growLophotrochozoa 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 PlatyzoaClade Platyzoa is a unique group of
lophotrochozoan protostomes that contain Platyhelminthes, Gastrotricha and Gnathifera.
Phylum PlatyhelminthesMembers of phylum
Platyhelminthes live in marine, freshwater, and damp terrestrial habitats.
Phylum PlatyhelminthesFlatworms are
flattened dorsoventrally and have a gastrovascular cavity.Extracellular
digestion.Undigested food is
egested through the pharynx.
Phylum PlatyhelminthesThe 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 PlatyhelminthesThe nervous system
consists of a ladder-like network of nerves and a bilobed brain.
Many have large ocelli – light sensing organs.
Phylum PlatyhelminthesMany can reproduce
asexually as well as sexually.Asexual
reproduction via fission.
Sometimes the new individuals remain attached – chains of zooids.
Monoecious
TaxonomyFlatworms (phylum Platyhelminthes) are
divided into four classes:Class Turbellaria – ex. Planaria
Not monophyleticClass Trematoda – parasitic flukesClass Monogenea – parasitic monogenetic flukesClass Cestoda - tapeworms
Phylum Platyhelminthes
Class TurbellariaTurbellarians are nearly all free-living and
mostly marine.
Class TurbellariaThe best-known turbellarians, commonly called
planarians, have light-sensitive eyespots and centralized nerve nets.
Class TrematodaTrematodes 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.
PseudocoelomatesPseudocoelomates 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 CavityA 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.
PseudocoelomatesPseudocoelomates 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 CycliophoraEntoproctaEctoproctaBrachiopodaPhoronida
Phylum GastrotrichaGastrotrichs 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 GnathiferaPossess 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 GnathiferaRotifera and Acanthocephala
Presumed sister taxa.Form a clade called Syndermata.Have eutelic syncytial epidermis.
Constant number of nuclei.Grouping is controversial.
Phylum GnathostomulidaPhylum Gnathostomulida includes the jaw
worms.Very small - <2mm.Live in interstitial spaces of fine coastal
sediments.Can endure low O2.
Phylum GnathostomulidaFeed by scraping bacteria and fungi from the
substratum with a pair of jaws on the pharynx.AcoelomateSexual stages include males, females, and
hermaphrodites.Fertilization is internal.
Phylum MicrognathozoaMicrognathozoans 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 MicrognathozoaSimple 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 RotiferaMembers of the
phylum Rotifera are pseudocoelomate and have three embryonic germ layers (triploblastic).
Complete digestive system.
Phylum RotiferaDioecious (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 RotiferaRotifers 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 AcanthocephalaAll 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 AcanthocephalaProboscis 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 AcanthocephalaBody 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 AcanthocephalaNo 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 AcanthocephalaAcanthocephalans 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 MesozoaPhylum Mesozoa is considered a “missing lin
k” between protozoa and metazoa.
Have a simple level of organization.Minute, ciliated, and wormlike animals.
All live as parasites in marine invertebrates.
Phylum MesozoaMost 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 MesozoaRhombozoans 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 MesozoaOrthonectids parasitize a variety of
invertebrates.
Reproduce sexually and asexually.Asexual reproduction consists of a
multinucleated mass called a plasmodium.
Phylogeny of MesozoansSome 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.