FLAT WORMS

Phylum Platyhelminthes Simple animals with soft leaflike or ribbonlike bodies, and includes flatworms, flukes, and tapeworms Around 20,000 species Body is unsegmented Bilaterally symmetrical Called “flatworms” because bodies are flattened dorso-ventrally

Phylum Platyhelminthes Acoelomates-solid bodies without a lined body cavity; internal organs lie in a loose tissue called parenchyma 3 body layers: ectoderm, mesoderm, endoderm Show cephalization (concentration of sensory organs at anterior or head end) No circulatory system; body cells exchange oxygen & carbon dioxide directly with environment by diffusion

Phylum Platyhelminthes Single opening into gastrovascular cavity: two-way digestive tract In some a tubular pharynx can be projected Mouth located at the front, ventral end Water balance is maintained by excretory tubules (protonephridia) equipped with flame cells

Phylum Platyhelminthes Nervous system: brain and nerve cords extending along the body Circular, longitudinal, and other muscle fibers enable turning, twisting, and folding of the body Free-living flatworms move by means of a ciliated epidermis and undulations of the body Parasitic flatworms have thick cell layer called tegument covered by a nonliving cuticle covering their bodies as protection inside hosts

Phylum Platyhelminthes Mostly hermaphrodites Sperm and egg are exchanged during mutual copulation; internal fertilization Asexual reproduction by fragmentation and regeneration Parasitic species have complex life cycles - with several larval stages often with two or more different hosts

Classification Class Turbellaria Class Trematoda (flukes) Dugesia (planarian) Class Trematoda (flukes) Schistosoma Class Cestoda (tapeworms) 7

Class Turbellaria (Planaria) Most are marine but include freshwater planarian (Dugesia) Spade-shaped anterior end with light-sensitive eyespots 2 clusters of nerve cells (ganglia) to form a simple brain and a nerve net Capable of simple learning

Class Turbellaria (Planaria) Move by cilia Feed by scavenging or protozoans Mouth located at the end of muscular tube called the pharynx which is extended when feeding Flame cells help remove wastes to excretory pores

Class Turbellaria (Planaria) Hermaphrodites that cross-fertilize eggs; deposited into a capsule until hatching in 2-3 weeks Reproduce asexually by fragmentation

Class Turbellaria (Planaria) Brain Eyespots Pharynx Nerve Cords Gastrovascular Cavity Mouth

Class Trematoda (Flukes) Includes parasitic flukes Most around 1 cm long and oval shaped Require a host to live Have oral and ventral suckers to cling to host & suck blood & body fluids Endoparasites: live inside a host Ectoparasites: live on outside of host

Class Trematoda (Flukes) Covered in tough, unciliated tegument Nervous & excretory systems like turbellarians Hermaphrodites Long, coiled uterus stores & releases 10,000+ eggs Eggs released through genital pore & develop into larva

Class Trematoda (Flukes) Life cycle of sheep liver fluke (Fasciola hepatica): Adult liver flukes live in sheep liver & gall bladder where they mate & form eggs Eggs enter intestines, pass out with feces, & hatch in water Larva enter snails, asexually multiply, then leave snail & form cysts Cysts (dormant larva with hard, protective covering) clings to grass Sheep ingest cysts when they eat grass Cysts hatch in digestive tract & bore through intestines into bloodstream Mature and reproduce in the liver

Class Trematoda (Flukes) Schistosomiasis: disease caused by parasitic blood flukes (Schistosoma); infects people in Asia, Africa, and South America causing intestinal bleeding and tissue decay that can result in death

Class Cestoda (Tapeworms) Adapted for parasitic life Tough outer tegument prevents being digested by host Scolex: anterior end, contains hooks & suckers for attachment to intestine of host

Class Cestoda (Tapeworms) Long, ribbon-like bodies up to 12 m in length Nervous system extends length of body but lacks sense organs Lacks mouth and digestive tract; absorbs digested nutrients from host Grows by making body segments called proglottids

Class Cestoda (Tapeworms) Each proglottid produces eggs & sperm that cross-fertilize with other segments & also self-fertilize (hermaphrodites) Oldest, mature proglottids containing eggs at posterior end; break off & pass out with feces

Class Cestoda (Tapeworms) Life cycle of beef tapeworm: Cattle eat grass with proglottids containing fertilized eggs Eggs hatch into larva & bore through cow’s intestine into bloodstream Larva burrow into cow’s muscle & form cysts Humans eat beef (muscle) & cysts travels to intestines Cyst breaks open & adult beef tapeworm forms