Flatworms: Platyhelminthes Taxonomy

• The presentation is about invertebrate taxonomy and focuses on flatworms (Platyhelminthes) and roundworms (Aschelminthes).

Flatworms (Phylum Platyhelminthes)

• Flatworms are part of the kingdom Animalia
• Examples of flatworms include Planaria, flukes, and tapeworms.

Flatworm General Characteristics

• Flatworms are characterized as acoelomate and triploblastic.
• These organisms exhibit bilateral symmetry
• Flatworms can live freely in marine and fresh water environments, and can be parasitic.
• Flatworms are dorso-ventrally flattened
• Epidermis has either cilia or tegument
• Possess hook/sucker organs of attachment and adhesion
• Flatworms have an incomplete digestive system.
• Cestodes do not have a digestive system

General Characteristics

• Flatworms have protonephridia excretory systems.
• Flatworms' nervous system is ladder-like
• They lack respiratory, circulatory, and skeletal systems.
• Flatworms lack exo- or endoskeletons
• Hard parts may contain spines, hooks, or teeth.
• Most flatworms are hermaphroditic (monoecious), though Schistosomatidae is an exception.
• Some flatworms reproduce asexually via fragmentation (fission) and regeneration.
• Trematodes cross-fertilize, while cestodes self-fertilize
• Fertilization is internal in flatworms

Flatworm Classes

• Class Turbellaria consists of free-living flatworms like Planaria.
• Class Trematoda has flukes (leaf worms) such as Fasciola and Schistosoma.
• Class Cestoda includes tapeworms, such as Taenia sp.

Class Turbellaria (Free Living Flatworms)

• Turbellarians are primarily free-living and mostly marine.
• Some can be endo-commensals or parasites.
• These organisms have a ciliated epidermis.
• Their digestive system is incomplete
• The mouth is located ventrally.
• The body is unsegmented
• Possess tangoreceptors and chemoreceptors
• Their life cycle is simple
• Regeneration is present

Planaria Anatomy

• Planaria elongates
• Animals with a flattened triangular anterior end or "head"
• Have a tapered posterior end
• Possess two black eyespots middorsally
• The mouth is located on the ventral surface near the middle of the body.

Body Wall Histology

• Under the basement membrane is layered muscle fibers (circular, longitudinal and dorsoventral).
• The space between internal organs and muscles contain parenchyma, a loose meshwork with no definite walls.
• They lack a body cavity.

Locomotion

• Worms glide across surfaces using cilia and mucus from the ectoderm.
• Muscular contraction allows for movement.

Digestive System

• The digestive system includes a pharynx and three branches of intestinal caeca.
• No anus exists, the mouth is used for both ingestion and egestion.

Excretion and Osmoregulation

• They are first known organism to have a specialized excretory organ
• Flame cells and numerous branched excretory tubules work to collect waste.
• Two excretory canals within in the parenchyma collect in 2 longitudinal excretory canals.
• Excretory canals open on each side and opens dorsally through minute pores.

Flame Cell Structure and Function

• Smallest terminal tubules end with flame cells
• Nucleated cells enclose a central cavity
• A collection of cilia hangs in cavity, and moves characteristically.
• Flame cells capture nitrogenous waste from the parenchyma.
• Waste and water pass into excretory tubules to be excreted.

Nervous System

• Two cerebral ganglia
• Two nerve cords, connected by transverse strands.
• Form a ladder shape in the flatworm

Reproduction

• Asexual reproduction through regeneration or binary fission
• Sexual reproduction through self or cross-fertilization
• Fertilized eggs are deposited on submerged objects and hatch into young which then develop to adults.
• Metamorphosis: the transformation from immature form to adult in two or more distinct stages

Trematoda (Leaf Worms - The Flukes)

• Flukes are ecto- and endoparasites
• They have a tegument covering their body
• Flukes are leaf-like in shape
• Have well-developed suckers
• An oral sucker surrounds the mouth.
• Attached with a ventral sucker
• The digestive system is incomplete
• They have a single ovary and two or more testes

Fluke Anatomy

• The oral sucker and ovary are located near the anterior
• Ventral sucker is on the underside near the anterior
• There is also a visible uterus

Body Wall

• The body wall consists of tegument, basement membrane, circular muscles, Golgi apparatus, nucleus, nuclear membrane, and tegument secreting mesenchmal cells

Fluke Life Cycle

• The life cycle includes stages involving snails, water plants, humans, sheep, cattle, and passage through feces.
• Stages include: Sporocysts, Rediae, Cercariae, Metacercariae, Excyst, and Adults

Schistosoma

• A genus of trematodes commonly known as blood flukes
• Responsible for schistosomiasis (Bilharziasis) in human

Schistosoma Morphology

• Females range from 12 to 26 mm in size.
• Males range from 6 to 22 mm in size.
• Three main species that infect humans are Schistosoma haematobium, S. japonicum, and S. mansoni.

Schistosoma Life Cycle

• The cycle includes release into water, penetration of skin, cercariae losing tails, and migration to blood in the liver.
• Paired adult worms migrate to the mesenteric venules of the bowel/rectum or to the venous plexus of the bladder.

Cestoda (Tapeworms)

• Tapeworms are endoparasites in the intestine of vertebrates.
• Their body is covered with tegument
• The body divides into a scolex, neck, and segments/proglottids
• The scolex is equipped with adhesive structures (hooks and suckers).
• Mouth and digestive systems are absent.
• Segments (proglottids) contain one or two sets of complete hermaphroditic reproductive systems.
• Life cycle is complicated

Taenia Anatomy

• Scolex: Contains hooks and discs to attach to the host
• Proglottids: Segments that contain reproductive organs
• Mature, Immature and Gravid sections
• Genital pore
• Strobila

Mature Proglottids

• Composed of excretory canals, sperm ducts, genital pores
• Also contains tested, uterus, vagina, ovary, Mehlis' gland, and vitelline gland

Taenia Life cycle

• Oncospheres hatch, penetrate the intestinal wall, and circulate to musculature.
• Humans are infected by ingesting raw or undercooked infected meat.

Phylum Aschelminthes

• One of two body plan clades

General characters (Aschelminthes)

• Aschelminthes are triploblastic and pseudocoelomate.
• They have a body covering (from ectoderm), a muscle layer(from mesoderm), and a digestive tract (from endoderm).
• They are cylindrical, non-segmented worms with bilateral symmetry.
• Their body wall is composed of a cuticle,epidermis, and a single longitudinal muscle layer.
• The cuticle is a delicate, transparent, and elastic membrane covering the outer surface of the body

Ascaris Anatomy

• A synctial epidermis
• Longitudinal muscles
• Pseudocoel, intestine, uterus, oviduct, and muscle cells

Digestive System (Aschelminthes)

• Have a complete digestive system
• The alimentary canal a long tube that starts with a mouth and ends with an anus.

Excretory System (Aschelminthes)

• The excretory system is composed of renette cells connected to two lateral canals.
• These open anteriorly by a single pore.

Respiratory & Circulatory System (Aschelminthes)

• These are absent

Nervous System (Aschelminthes)

• Includes a circum-oesophageal nerve ring (brain) with dorsal and ventral nerve cords.

Reproductive System (Aschelminthes)

• Sexes are separate
• Females are generally larger than males.
• Have direct internal fertilization and development

Ascaris lumbricoides Life Cycle

• Infection occurs by ingestion of the embryonated egg
• Once eggs hatch they release larvae L3
• Larvae enter circulation and migrate to the lungs
• Larvae are coughed up and reingested
• Maturation occurs in small intensities