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 What is the function of the numerous branches of the digestive tract of Dugesia? What is meant by the term cephalization? The scolex is common to the class _____________? This flatworm class lacks a digestive tract. What does the term hermaphrodidic mean?

Answers to Review Questions (Flatworms) They add surface area for increased absorption of nutrients. The concentration of sense organs in the anterior end. Cestoda. 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.