The phylum Platyhelminthes includes over 20,000 species of triploblastic and acoelomate flatworms, categorized into four classes: Turbellaria, Monogenea, Trematoda, and Cestoidea. These organisms exhibit diverse lifestyles, from free-living and predaceous to strictly parasitic, with unique adaptations such as lacking a digestive tract in tapeworms. Key characteristics include bilateral symmetry, complex reproductive systems, and various methods of locomotion and digestion.

1. TRIPLOBLASTIC AND
ACOELOMATE BODY
PHYLUM PLATYHELMINTHES

2. Phylum Platyhelminthes
Gr. Platys, flat and helmins, worm. This
phylum contains over 20,000 animal species.
Flatworms range in adult size from 1mm or
less to 25m in length. The phylum is divided
into four classes:
1. Turbellaria
2. Monogenea
3. Trematoda
4. Cestoidea

3.  Class Turbellaria
Mostly free-living and aquatic; external surface usually
ciliated; predaceous; possess rhabdities, protrusible
proboscis, frontal glands, and many mucous glands;
mostly hermaphroditic. Convoluta, Notoplna, Dugesia.
Over 3,000 species.

4.  Class Monogenea
Monogenetic flukes; mostly ectoparasites on vertebrates
(usually on fishes; occasionally on turtles, frogs,
copepods, squids); one life-cycle form in only one host;
bear opisthaptor. Disocotyle, Gyrodactylus, Polystoma.
About 1,100 species.

5.  Class Trematoda
Trematodes; all are parasitic; several holdfast devices
present; have complicated life cycles involving both
sexual and asexual reproduction. Over 10,000 species.

6.  Class Cestoidea
All parasitic with no digestive tract; have great reproductive
potential; tapeworms. About 3,500 species.
Subclass Cestodaria
Body not subdivided into proglottids; larva in crustaceans,
adult in fishes. Amphilina, Gyrocotyle. About 15 species.
Subclass Eucestoda
True tapeworms; body divided into scolex, neck, and strobila;
strobila composed of many proglottids; both male and female
reproductive systems in each proglottid; adults in digestive
tract of vertebrates. Protocephalus, Taenia, Echinococcus,
Taeniarhynchus; Diphyllobothrium. About 1,000 species.

7. Cestoidea

8. Characteristics
1. Usually flattened dorsoventrally, triploblastic,
acoelomate, bilaterally symmetrical
2. Unsegmented worms (members of the class Cestoidea
are strobilated)
3. Incomplete gut usually present; gut absent in
Cestoidea
4. Somewhat cephalized, with an anterior cerebral
ganglion and usually longitudinal nerve cords
5. Protonephridia as excretory/osmoregulatory structures
6. Hermaphroditic; complex reproductive systems

9. The free-living flatworm
Members of the class Turbellaria (turbel-lare-ah)
(L.turbellae, a commotion aria, like) are mostly free-
living bottom dwellers in freshwater and marine
environments, where they crawl on stones, sand, or
vegetation. Most turbellarians are less than 1cm long, the
terrestrial, tropical ones may reach 60 cm in length,
Coloration is mostly in shades of black, brown, and gray,
although some groups display brightly colored patterns.

10. Flatworm

11. Body wall
As in the Cnidaria, the ectodermal derivatives include an
epidermis that is in direct contact with the environment.
Other muscles are located dorsoventrally and obliquely
between the dorsal and ventral surfaces. Between the
longitudinal muscles and the gastrodermis are the loosely
organized parenchymal cells.
The innermost tissue layer is the endodermally derived
gastrodermis. It consists of a single layer of cells that
comprise the digestive cavity.

12. Cnidarians

13. Cont…
Rhabdities are rod like cells that swell and form a
protective mucous sheath around the body, possibly in
response to attempted predation or desiccation. Adhesive
glands open to the epithelial surface and produce a
chemical that attaches part of the turbellarians to a
substrate. Releaser glands secrete a chemical that
dissolves the attachment as needed.

14. Locomotion
Turbellarians are the first group of bilaterally
symmetrical animals. Bilateral symmetry is usually
characteristic of animals with an active lifestyle.
Turbellarians are primarily bottom dwellers that glide
over the substrate. They move using cilia and muscular
undulations. As they move, turbellarians lay down a
sheet of mucus that aids in adhesion and helps the cilia
gain traction.

15. Digestion and nutrition
Some marine turbellarians lack the digestive cavity
characteristic of other turbellarians. This blind cavity
varies from a simple, unbranched chamber to a highly
branched system of digestive tubes. From an
evolutionary perspective, highly branched digestive
systems are an advancement that results in more
gastrodermis closer to the sites of digestion and
absorption, reducing the distance nutrients must diffuse.

16. Cont..
The turbellarians pharynx functions as an ingestive
organ. Most turbellarians, such as the common planarian,
are carnivores and feed on small, live invertebrates or
scavenge on larger, dead animals; some are herbivores
and feed on algae that they scrape from rocks.

17. Digestive Systems in Some Orders of
Turbellarians.
1. No pharynx and digestive cavity.
2. A simple pharynx and straight digestive cavity.
3. A simple pharynx and unbranched digestive cavity.
4. A branched digestive cavity.
5. An extensively branched digestive cavity in which
the branches reach almost all parts of the body.

18. Exchanges with the environment
The turbellarians do not have respiratory organs; thus,
respiratory gases (CO2 and O2) are exchanged by
diffusion through the body wall. Most metabolic wastes
(e.g., ammonia) are also removed by diffusion through
the body wall.

19. Cont..
In marine environments, invertebrates are often in
osmotic equilibrium with their environment. In
freshwater, invertebrates are hypertonic to their aquatic
environment and thus must regulate the osmotic
concentration (water and ions) of their body tissues.
The evolution of osmoregulatory structures in the form
of protonephridia enabled turbellarians to invade
freshwater.

20. Flame cells
Protonephridia (Gr. protos, first nephros, kidney) (sing.,
protonephridium) are networks of fine tubules that run
the length of the turbellarians, along each of its sides
Numerous, fine side branches of the tubules originate in
the parenchyma as tiny enlargements called flame cells.

21. Nervous System and Sense Organs
The most primitive type of flatworm nervous system,
found in worms in the order Acoela e.g., Convoluta spp.
is a subepidermal nerve plexus. This plexus resembles
the nerve net of cnidarians. A statocyst in the anterior end
functions as a mechanoreceptor (a receptor excited by
pressure) that detects the turbellarians body position in
reference to the direction of gravity.

22. Cont..
Some turbellarians have a more centralized nerve net
with cerebral ganglia. The nervous system of most other
turbellarians, such as the planarian Dugesia, consists of a
subepidermal nerve net and several pairs of long nerve
cords.

23. Primitive brain
Neurons are organized into sensory (going to the
primitive brain), motor (going away from the primitive
brain), and association (connecting) types—an important
evolutionary advance with respect to the nervous system.
Anteriorly, the nervous tissue concentrates into a pair of
cerebral ganglia (sing., ganglion) called a primitive
brain.

24. Reproduction and Development
Many turbellarians reproduce asexually by transverse
fission. Fission usually begins as a constriction behind
the pharynx. The two (or more) animals that result from
fission are called zooids (Gr., zoon, living being or
animal), and they regenerate missing parts after
separating from each other. Sometimes, the zooids
remain attached until they have attained a fairly complete
degree of development, at which time they detach as
independent individuals.

25. Regeneration and fission

26. Cont..
Eggs are laid with or without a gel-like mass. A hard
capsule called a cocoon (L., coccum, eggshell) encloses
many turbellarians eggs. These cocoons attach to the
substrate by a stalk and contain several embryos per
capsule. Two kinds of capsules are laid. Summer
capsules hatch in two to three weeks, and immature
animals emerge. Autumn capsules have thick walls that
can resist freezing and drying, and they hatch after
overwintering.

27. Some Important Trematode Parasites
of Humans
The Chinese liver fluke, Clonorchis sinensis, is a
common parasite of humans in Asia, where over 30
million people are infected. The adult lives in the bile
ducts of the liver, where it feeds on epithelial tissue and
blood. The adults release embryonated eggs into the
common bile duct. The eggs make their way to the
intestine and are eliminated with feces.

28. The Tapeworms
The most highly specialized class of flatworms are
members of the class Cestoidea (ses-toide-ah) (Gr.
kestos, girdle eidos, form), commonly called either
tapeworms or cestodes. All of the approximately 3,500
species are endoparasites that usually reside in the
vertebrate digestive system. Their color is often white
with shades of yellow or gray. Adult tapeworms range
from 1 mm to 25 m in length.

29. Adaptations
Two unique adaptations to a parasitic lifestyle
characterize tapeworms:
(1) Tapeworms lack a mouth and digestive tract in all
of their life-cycle stages; they absorb nutrients directly
across their body wall.
(2) Most adult tapeworms consist of a long series of
repeating units called proglottids. Each proglottid
contains a complete set of reproductive structures.

30. Some Important Tapeworm Parasites
of Humans
One medically important tapeworm of humans is the beef
tapeworm. Adults live in the small intestine and may
reach lengths of 25 m. About 80,000 eggs per proglottid
are released as proglottids break free of the adult worm.
As an egg develops, it forms a six-hooked (hexacanth)
larva called the onchosphere.

31. Bladder worm
As cattle (the intermediate host) graze in pastures
contaminated with human feces, they ingest oncospheres
(or proglottids). Digestive enzymes of the cattle free the
oncospheres, and the larvae use their hooks to bore
through the intestinal wall into the bloodstream. The
bloodstream carries the larvae to skeletal muscles, where
they encyst and form a fluid-filled bladder called a
cysticercus (pl., cysticerci) or bladder worm.

32. References
Zoology book by Millar and Harley (5th addition)