Platyhelminthes
Chris Racz
Examples
Turbellaria: Free living flatworms, Planarians, Symsagittifera roscoffensis,
Microstomum caudatum.
Cestoda: Cyclophyllidea, Pseudophyllidea, Taenia saginata,
Diphyllobothrium, Taenia.
Trematoda: Digenea, Aspidogastrea, tissue flukes, blood flukes
Monogenea: Polyopisthocotylea, Monopisthocotylea, Gyrodactylus,
Dactylogyrus, Neobenedenia, earthworms
Characteristics














Bilaterally symmetrical
Body is made of 3 layers of tissues with organs/organelles (tripoblastic)
No internal cavities
No anus, classified as a blind ending gut
Regurgitates through mouth using protonephredia
Nervous system containing longitudinal fibers instead of a net
Flat because they lack circulatory and respiratory organs
Hermaphrodite sexual reproduction
Cestoda, Trematoda, and Monogenea are parasites; Turbellaria are non-parasitic.
Soft bodied
Hermaphroditic
Have neoblasts---stem cells that give rise to the other cell types in the body
Made of epidermal cells that are typically multiciliated, each cell bearing multiple
cilia rather than only one
Acoelomates
Morphology: Basic Structure
The structure of platyhelminthes are
bilaterally symmetric, containing three
cell layers. They have definitive
nervous tissues and sensory organs
at one end of their body, distinctly
showing the head and tail. They lack
circulatory and respiratory organs
and do not have an internal body
cavity. It has two eye-slots on its
head, and body is covered in ciliated
epidermis.
Morphology: Systems
Digestive system consists of the mouth, pharynx, and a branching intestine where the
nutrients are absorbed. The excretory system has unique flame cells, which help
excrete waste through their body. When cilia beat, they move water into the tubules
and out pores in the body called nephridiospores. The muscle system, made of muscle
fibers, is below the epidermis and is used for locomotion. The nervous system is made
of longitudinal nerve cords and smaller lateral nerves.
Embryology
Like most animals, platyhelminthes have 3 germ layers to protect the embryo: the
ectoderm, endoderm and mesoderm. The mesoderm is the embryonic layer that
forms in between the ectoderm and endoderm. It has no true body cavity.
Turbellaria are hermaphrodites, fertilizing their eggs internally by copulation. Larger
ones mate by penis fencing, a duel trying to impregnate the other. the fluke has a
long coiled tube, or uterus, that stores eggs until release through a genital pore. A
fluke may produce tens of thousands of eggs at a time. A few survive predation
and environmental destruction and hatch into larvae, or immature organisms.
Cestoda contains both male and female reproductive organs. Cross-fertilization
between two adjacent worms is typical, but self-fertilization between proglottids can
also occur. After fertilization the egg-packed proglottid breaks off from the adult
and is eliminated with the host’s feces. Monogenea form a mucus coat around part
of their intertwined bodies. Each injects sperm into the mucus. Sperm from one
worm moves to the pouchlike seminal receptacle of the other. Simultaneously eggs
in the body cavity move through oviducts to the female genital pore. After several
days a mucus and chitin sheath is secreted by the clitellum, a swelling around the
sex organs. As the worm wriggles to slip the sheath off its body, eggs and sperm
are joined and fertilization occurs.
Biochemical Evidence
James V. McConnell discovered that
flatworms had memory RNA when he cut
them in half. After cutting them in half,
they regenerated into two full flatworms
Life History of a blood fluke
A female fluke fits into a groove inside of the males body. Once inserted, the blood
flukes reproduce sexually in the human host. Once fertilization occurs, the eggs
exit the hosts feces. The egg develops in water to form ciliated larvae, which infect
snails as the intermediate host. Asexual reproduction within the snail results in
another motile larva, which escapes the host. The larvae penetrate the skin and
blood vessels of human inside water contaminated with the infected human feces.
Summary
Platyhelminthes are flatworms with bilateral symmetric body. They are a very
interesting specimen due to memory RNA, capable of reforming after being sliced
in half. Platyhelminthes consists of the unsegmented flatworms, which includes
both free-living and parasitic species. They have bilateral symmetry, and can move
by using layers of muscles, or in some species, by gliding along a slime trail using
cilia. Flatworms are slightly less developed than segmented worms due to their
lack of a circulatory system and complete digestive system. Instead, flatworms
absorb nutrients through their skin and excrete wastes using specialized "flame
cells." Some flatworms have primitive light-sensing "eyes" that allow them to move
either towards or away from light, while other species have different types of
sensors on their bodies, including chemical, balance, and water movement
receptors. Most species of flatworms reproduce either sexually or asexually.