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 and 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

Planarians are capable of asexual reproduction via fission 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