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.