Platyhelminthes is a phylum composed of three classes of flatworms.
The class Turbellaria (planarians) is free-living, while the remaining classes
including the Trematoda (flukes) and Cestoda (tapeworms) are parisitic.
Members of the phylum Platyhelminthes are dorsoventrally flattened
with a triploblastic body composed of three different tissue layers:
ectoderm, endoderm, and the mesoderm. These animals are bilaterally
symmetrical, meaning that a slice directly through the middle can produce
two mirror images. 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.
The best known member of the class Turbellaria is the free-living
planarian, Dugesia. 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.
Water balance or osmoregulation is maintained by protonephridia
which terminate in specialized flame cells. 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.
Members of the class Trematoda consist of the parisitic 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 hermaphrodidic (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.
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.
Review Questions
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)
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