17. phylum platyhelminthes full explanation for zoology students

1. PHYLUM PLATYHELMINTHES
By
SYED SHAKEEL SHAH
Department of Zoology
University of Narowal

2. Introduction
• The phylum Platyhelminthes contains over 20,000
animal species.
• Size from 1 mm or less to 25 m, in length.
• Their mesodermally derived tissues include a loose
tissue called parenchyma that fills spaces between
other more specialized tissues, organs, and the
body wall.

3. • Depending on the species, parenchyma may
provide skeletal support, nutrient storage, motility,
reserves of regenerative cells, transport of
materials, structural interactions with other tissues,
modifiable tissue for morphogenesis, oxygen
storage, and perhaps other functions yet to be
determined.
• This phylum has an organ-system level of
organization.

4. Classes
(1) the Turbellaria consists of mostly free-living
flatworms
(2) Monogenea
(3) Trematoda
(4) Cestoidea contain solely parasitic species.

5. Characteristics of the phylum
Platyhelminthes
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

6. CLASS TURBELLARIA: THE FREE-
LIVING FLATWORMS
• These are mostly free-living bottom dwellers in
freshwater and marine environments, where they
crawl on stones, sand, or vegetation.
• Turbellarians are named for the turbulence that
their beating cilia create in the water.
• Turbellarians are predators and scavengers.

7. • The few terrestrial turbellarians known live in the
humid tropics and subtropics.
• Although most turbellarians are less than 1 cm 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.

8. Body Wall
• As in the Cnidaria, the ectodermal derivatives include
an epidermis that is in direct contact with the
environment.
• Some epidermal cells are ciliated, and others contain
microvilli.
• A basement membrane of connective tissue separates
the epidermis from mesodermally derived tissues.
• An outer layer of circular muscle and an inner layer of
longitudinal muscle lie beneath the basement
membrane.
• Other muscles are located dorsoventrally and obliquely
between the dorsal and ventral surfaces.

9. • 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.
• The gastrodermis secretes enzymes that aid in
digestion, and it absorbs the end products of digestion.

10. • On the ventral surface of the body wall are several
types of glandular cells of epidermal origin.
• Rhabdites are rodlike 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
turbellarian to a substrate.
• Releaser glands secrete a chemical that dissolves the
attachment as needed.

11. Locomotion
• Body is bilaterally symmetrical.
• Turbellarians are primarily bottom dwellers that
glide over the substrate.
• They move using cilia and muscular undulations.
• The densely ciliated ventral surface and the
flattened body of turbellarians enhance the
effectiveness of this locomotion.

12. 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.
• Other turbellarians have digestive tracts that are lobed. 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.

13. • The turbellarian pharynx functions as an ingestive
organ.
• It varies in structure from a simple, ciliated tube to
a complex organ developed from the folding of
muscle layers.
• In the latter, the free end of the tube lies in a
pharyngeal sheath and can project out of the
mouth when feeding.
• 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.

14. • Sensory cells (chemoreceptors) on their heads help
them detect food from a considerable distance.
• Food digestion is partially extracellular.
• Pharyngeal glands secrete enzymes that help break
down food into smaller units that can be taken into
the pharynx.
• In the digestive cavity, phagocytic cells engulf small
units of food, and digestion is completed in
intracellular vesicles.

15. FIG: Digestive Systems in Some Orders of Turbellarians. (a) No pharynx and digestive cavity. (b)
A simple pharynx and straight digestive cavity. (c) A simple pharynx and unbranched digestive
cavity. (d) A branched digestive cavity. (e) An extensively branched digestive cavity in which the
branches reach almost all parts of the body.

16. 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.
• 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.

17. • Protonephridia are networks of fine tubules that
run the length of the turbellarian, along each of its
sides.
• Numerous, fine side branches of the tubules
originate in the parenchyma as tiny enlargements
called flame cells.
• Flame cells (so named because, in the living
organism, they resemble a candle flame) have
numerous cilia that project into the lumen of the
tubule.
• Slitlike fenestrations (openings) perforate the
tubule wall surrounding the flame cell.

18. • The beating of the cilia drives fluid down the tubule,
creating a negative pressure in the tubule.
• As a result, fluid from the surrounding tissue is
sucked through the fenestrations into the tubule.
• The tubules eventually merge and open to the
outside of the body wall through a minute opening
called a nephridiopore.

19. FIG: Protonephridial System in a Turbellarian. (a) The protonephridial system lies in the
parenchyma and consists of a network of fine tubules that run the length of the animal on each
side and open to the surface by minute nephridiopores. (b) Numerous, fine side branches from
the tubules originate in the parenchyma in enlargements called flame cells. Small black arrows
indicate the direction of fluid movement.

20. 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 turbellarian’s body position in reference to the
direction of gravity.
• Some turbellarians have a more centralized nerve net with
cerebral ganglia.

21. • 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.
• Lateral branches called commissures (points of union)
connect the nerve cords.
• Nerve cords and their commissures give a ladderlike
appearance to the turbellarian nervous system.
• 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.

22. FIG: Nervous Systems in Three Orders of Turbellaria. (a) Convoluta has a nerve net with a
statocyst. (b) The nerve net in a turbellarian in the order Polycladida has cerebral ganglia and
two lateral nerve cords. (c) The cerebral ganglia and nerve cords in the planarian, Dugesia.

23. • Turbellarians respond to a variety of stimuli in their
external environment.
• Many tactile and sensory cells distributed over the
body detect touch, water currents, and chemicals.
• Auricles (sensory lobes) may project from the side
of the head.
• Chemoreceptors that aid in food location are
especially dense in these auricles.

24. • Most turbellarians have two simple eyespots called
ocelli (sing., ocellus).
• These ocelli orient the animal to the direction of light.
(Most turbellarians are negatively phototactic and
move away from light.)
• Each ocellus consists of a cuplike depression lined with
black pigment.
• Photoreceptor nerve endings in the cup are part of the
neurons that leave the eye and connect with a cerebral
ganglion.

25. 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, 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.
• Turbellarians are monoecious, and reproductive
systems arise from the mesodermal tissues in the
parenchyma.

26. • Numerous paired testes lie along each side of the
worm and are the sites of sperm production.
• Sperm ducts (vas deferens) lead to a seminal vesicle
(a sperm stroage organ) and a protrusible penis.
• The penis projects into a genital chamber.
• The female system has one to many pairs of ovaries.
• Oviducts lead from the ovaries to the genital
chamber, which opens to the outside through the
genital pore.

27. • Even though turbellarians are monoecious,
reciprocal sperm exchange between two animals is
usually the rule.
• This cross-fertilization ensures greater genetic
diversity than does self-fertilization.
• During cross-fertilization, the penis of each
individual is inserted into the copulatory sac of the
partner.
• After copulation, sperm move from the copulatory
sac to the genital chamber and then through the
oviducts to the ovaries, where fertilization occurs.

28. • Yolk may either be directly incorporated into the
egg during egg formation, or yolk cells may be laid
around the zygote as it passes down the female
reproductive tract past the vitellaria (yolk glands).
• Eggs are laid with or without a gel-like mass.
• A hard capsule called a cocoon (L., coccum,
eggshell) encloses many turbellarian 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.

29. • Autumn capsules have thick walls that can resist
freezing and drying, and they hatch after
overwintering.
• Development of most turbellarians is direct—a
gradual series of changes transforms embryos into
adults.
• A few turbellarians have a free-swimming stage
called a Müller’s larva.
• It has ciliated extensions for feeding and
locomotion.
• The larva eventually settles to the substrate and
develops into a young turbellarian.

30. FIG: Asexual Reproduction in a
Turbellarian. (a) Just before division and
(b) just after. The posterior zooid soon
develops a head, pharynx, and other
structures. (c,d) Later development.
FIG: Triclad Turbellarian Reproductive
System. Note that this single
individual has both male and female
reproductive organs.

31. Thank You…!