Download Platyhelminthes

Platyhelminthes
After completing this tutorial you should be able
to:
Explain the basic body plan of members of the
phylum Platyhelminthes.
Identify representatives of the classes Turbellaria,
Trematoda, and Cestoda.
Compare and contrast the anatomy and
morphology of free-living and parasitic flatworms.
Describe the symmetry of this group.
Platyhelminthes is a phylum of
flatworms.
Class Turbellaria (planarians) is free-living.
Trematoda (flukes)
Cestoda (tapeworms) are parisitic.
Members of the phylum Platyhelminthes are dorsoventrally
flattened with body composed of three different tissue layers:
ectoderm, endoderm, and the mesoderm.
These animals are bilaterally symmetrical.
This phylum demonstrates an organ-system level of organization.
The front or anterior portion of the body bears most of the sense
organs as compared to the rear or posterior of the body.
Most flatworms have a single opening to the digestive tract and
they have no respiratory or circulatory systems; but are simple
and flat enough so diffusion is sufficient for these processes.
Protonephredia and flame cells regulate water balance.
Turbellaria the Planarians
Planaria
They are usually found in slow-moving streams near stones, leaves
or debris. The head of these animals has lateral tactile projections
termed auricles. There are also photoreceptor sensory organs
termed ocelli on the anterior dorsal surface. Their nervous system
is based on two longitudinal ventral nerve cords connected by the
anterior ganglia, or primitive brain. This concentration of sense
organs in the anterior end is termed cephalization. Planarians
secrete a slime track over which they glide. Gliding is
accomplished by beating the epidermal cilia in the slime track.
These flatworms ingest food by means of a tube-like pharynx
located on the midventral line. Digestion begins extracellularly with
enzymes secreted onto the food before it is sucked into the
intestine by the pharynx. The intestine may have lateral branches,
adding surface area for increased absorption of nutrients. The
mouth, pharynx, and intestine make up the entire digestive tract. A
cross section of Dugesia will show the ectoderm which produces
the epidermis. The layer surrounding the digestive tract is derived
from endoderm. The space between the two layers is filled with
tissues from the mesoderm.
Three layers
A cross section of Dugesia
will show the ectoderm
which produces the
epidermis. The layer
surrounding the digestive
tract is derived from
endoderm. The space
between the two layers is
filled with tissues from the
mesoderm.
Trematoda the Flukes
Members of the class Trematoda consist of the parasitic
flukes. The flukes live within one or more host animals
during their life cycle. They have a well-developed
digestive system with the mouth at the anterior end.
They are characterized by a thick cuticle as well as one
or more suckers surrounding the mouth. These suckers
are used for attachment to the host's internal body
surface. These organisms are typically hermaphroditic
(have male and female sex organs). With the aid of a
drawing of Clonorchis sinensis (a human liver fluke)
locate the: oral sucker, pharynx, esophagus, excretory
bladder, ovary, uterus, seminal receptacle, testes,
seminal vesicles and vas deferens
Flukes
Liver fluke
Cestoda the tapeworms
Members of the class Cestoda are endoparisitic
tapeworms which completely lack a digestive
tract. At the anterior end a scolex possessing
hooks and suckers is found which attaches to
the host's digestive system. Posterior to the
scolex is the neck region which leads to sections
termed proglottids. Each proglottid possesses
both male and female reproductive organs.
Farther down, the gravid proglottids contain
thousands of fertilized eggs. In the drawing and
pictures of Taenia pisiformis (tapeworm) observe
the scolex with hooks and suckers, proglottids,
ovary, testes, uterus, and excretory canal.
Tapeworms
Tapeworm proglottid
Review
1. What is the function of the numerous branches
of the digestive tract of Dugesia?
2. What is meant by the term cephalization?
3. The scolex is common to the class
_____________?
4. This flatworm class lacks a digestive tract.
5. What does the term hermaphrodidic mean?
Answers to Review Questions
(Flatworms)
1. They add surface area for increased
absorption of nutrients.
2. The concentration of sense organs in the
anterior end.
3. Cestoda.
4. Cestoda.
5. Having functional male and female sex
organs.
Scientists identify gene required
for flatworms to maintain their
stem cells
Freshwater flatworms,
called planaria, possess
extraordinary regenerative
capabilities by virtue of a
population of stem cells
they maintain throughout
their lives. Researchers
recently identified a key
gene that maintains
planarian stem cells.