Phylum Platyhelminthes General Characteristics They exhibit bilateral symmetry

Phylum Platyhelminthes

General Characteristics • They exhibit bilateral symmetry: anterior and posterior ends are different; so are the dorsal (top) and ventral (bottom) surfaces • The platyhelminths also exhibit some degree of cephalization Commonly referred to as the 'flatworms' because their bodies are dorsoventrally flattened. • They are acoelomates • This phylum (and all remaining phyla) possess 3 germ layers (=triploblastic) • The mesoderm (third germ layer) gives rise to muscles, various organ systems, and the parenchyma, a form of solid tissue containing cells and fibers

Outer Body Covering • The body of some platyhelminthes (e. g. , turbellarians) is covered by a ciliated epidermis • Epidermal cells contain rod-shaped structures called rhabdites that when released into the surrounding water, expand form a protective mucous coat around the animal • The outer body covering of other platyhelminthes (e. g. , parasitic forms) is a non-ciliated tegument • The tegument is referred to as a syncytial epithelium

Organ Systems of the Platyhelminthes Digestive System • Some of the flatworms possess a digestive system, with a mouth, pharynx, and a branching intestine from which the nutrients are absorbed • The intestine, with only one opening, is a blind system

Organ Systems of the Platyhelminthes cont. Excretory System (osmoregulation) • A network of water collecting tubules adjacent to flame cells or a protonephridia • When cilia beat they move water into the tubules and out the body through pores called nephridiopores

Organ Systems of the Platyhelminthes con’t Muscular System • Below the epidermis are layers of circular and longitudinal muscle fibers; used in locomotion Nervous System • Includes: anterior cerebral ganglia, longitudinal nerve cords, and some lateral nerves • Most free living planarians and parasitic larval forms possess a variety of sensory organs (e. g. , eye spots, statocysts, rheoreceptors)

Organ Systems of the Platyhelminthes cont. Reproductive System • Most are capable of some form of asexual reproduction (e. g. , many turbellarians reproduce by fission) • Most flatworms are hermaphroditic; however, they often pair with other individuals to exchange gametes

Platyhelminthes Taxonomy

Class Turbellaria • Free-living flatworms; mostly marine organisms • Range in size from microscopic (interstitial species between sand grains) to extremely large (two feet) Locomotion • Most move by means of cilia and mucous • Muscle contractions also permit turning, twisting and folding of the body

Class Turbellaria con’t Nutrition • Turbellarians are carnivores and prey on other animals or eat dead animal remains. • Planarians have a muscular pharynx that they can insert into their prey and then pump to bring in food fragments • These animals have a highly divided gut to greatly increase the surface area for digestion and absorption Senses • They have well developed sensory structures, including eyespots, mechanoreceptors, and chemoreceptors

Class Turbellaria con’t Reproduction • Planarians are capable of asexual reproduction via fission • Also capable of regeneration; exhibit both anterior- posterior and lateral polarity • They are hermaphrodites but usually exhibit cross-fertilization • The penis of some turbellarians is modified as a hollow stylet; sperm tranfer is by hypodermic impregnation, in which the copulating partners stab each other and inject sperm

Class Trematoda • Flukes that live as parasites either on or in other organisms. • Outer body lacks cilia; tegument has a layer of glycoproteins that are important in protection and absorption • Possess 2 suckers: 1. Oral sucker which attaches to organs of the host 2. Ventral sucker or acetabulum; used to attach to host tissues

Types of Hosts • Often have complex life cycles that alternate between sexual and asexual stages. • Most require at least 2 different kinds of hosts to complete their life cycle: 1. Definitive host (primary host) • The host in which the parasite matures and reproduces (sexually) • The host in which eggs are released 2. Intermediate host • Hosts in which larval stages develop and undergo asexual reproduction • Results in an increase in the number of the individuals

General Life Cycle - Chinese liver fluke, Clonorchis sinensis • Adults live in the bile ducts of humans, dogs, and cats • There are 2 intermediate hosts: a snail and a fish • Eggs are passed out of the definitive host and hatch as ciliated larvae called miracidia • The miracidia penetrates a snail molluscan host and becomes a sporocyst • They undergo asexual reproduction producing larvae called rediae • Rediae often asexually produce more rediae, but will eventually give rise to larvae called cercariae • They leave the molluscan host and penetrate fish • They encyst in the fish tissues as the metacercaria • Consumption of infected fish results in the metacercaria excysting in the gut and migrating to the bile duct

Schistosoma • Schistosoma spp. is a common blood fluke of Southeast Asia that causes shistosomiasis • Humans are the definitive host; snails are the intermediate host • In humans its eggs ultimately penetrates and damages intestinal tissue and tissue of the bladder • A source of constant inflammation and eventually leads to deterioration of liver, spleen and other organs

Class Cestoda General Morphology • Nonciliated tegument composed of glycoprotein • The anterior region is called a scolex; often armed with suckers and hooks • Extending from the neck is a series of proglottids; contain the sex organs and eggs; no digestive system • Mature eggs released through an opening in the proglottid or leave the host when the proglottids are separated from the main body of the worm.

Beef Tapeworm, Taeniarhynchus saginatus • Definitive host humans; intermediate host cattle • Eggs are shed with human feces; infected persons defecate in a pasture and the eggs are ingested by cattle • Eggs hatch giving rise to oncosphere larvae that bore into the intestinal wall and get into the circulatory system to be transported to muscle • Here the larvae develop into the cysticercus stage (=the bladder worm) with the inverted scolex • If uncooked beef is consumed the cysticercus is freed and the scolex everts, forming the adult • Symptoms include loss of weight, chronic indigestion, diarrhea