Phylum Platyhelminythes

Platyhelminthes

~ 20,000 extant species Parasitic + free-living Unsegmented flatworms

Platyhelminthes Triploblastic, acoelomate, bilaterally

symmetrical Incomplete gut; absent in some parasitic forms Cephalization of nervous system Protonephridia: excretion and osmoregulation Hermaphroditic

Support

Hydrostatic skeleton

Elastic body wall

Body musculature

Taxonomy

Class Turbellaria

Class Monogenea

Class Trematoda

Class Cestoda

Class Turbellaria

– Free-living flatworms

– Most are aquatic

– Epidermis cellular and ciliated

Feeding and digestion in Turbellaria Consume invertebrates (few herbivores and

omnivores) Locate food via chemoreception A few are symbiotic

Turbellaria digestive system

Mouth, pharynx, intestine = incomplete gut Pharyngeal glands produce mucus and

proteolytic enzymes Digestion extracellular, then phagocytization

in intestine

Turbellaria nervous system Sense organs

– Tactile receptors cover body - concentrated anteriorly

– Chemoreception = location of food– Statocysts = gravity detection and

orientation– Photoreceptors

Inverted pigment cup ocelli Negative phototaxis

Turbellaria nervous system

Variable: simple net-like to cephalized & bilateral

Ladder-like NS = more recently evolved

Class Monogenea

Monogenetic flukes (life cycle = one host)– Body covered by tegument– Oral sucker reduced or absent– Ectoparasitic (usually fish)

Class Monogenea

Monogenetic flukes (life cycle = one host)– Eggs hatch into ciliated larvae = oncomiracidia– Mature and find host

Class Trematoda

Digenetic flukes (multiple hosts)– Body with tegument– One or more suckers present– Internal parasite

Fluke Digestive System Feed on host tissues and fluids (muscular

pharynx)– Or, material in host gut

One-way digestive tract: mouth, muscular pharynx, short esophagus, intestinal cecae

Fluke Nervous System Ladder-like Cerebral ganglion Suckers with tactile receptors (bristles

and spines)

Sexual repro flukes

Hermaphroditic Mutual cross fertilization Male structures

– Variable testes Monogenetic = many Digenetic = two

– Sperm-to sperm duct, copulatory apparatus, eversible cirrus

Sexual repro flukes Female Structures

– Ovary to oviduct to ootype– Oviduct joined by vitelline duct– Seminal receptacle = blind pouch off of

oviduct– Single uterus sometimes modified as vagina

near female gonopore

Fluke reproduction Mutual cross-fertilization Sperm stored in seminal receptacle Eggs - oviduct to ootype then fertilized r-selected strategy (high fecundity)

a – acetabulum d - vitelline ducts f - vitelline follicles o - oral sucker oe – oesophagus oo – ootype ov – ovary ph – pharynx sr - seminal receptable t – testis u - uterus

Fluke life-cycles

Monogenetic– One host– Mostly external parasites of fish

Digenetic– Two or more hosts– Mostly internal parasites

Fluke life-cycles: Chinese liver fluke

Fluke life-cycles

Digenetic Fasciola = sheep liver fluke– Multiple hosts– Internal parasite of vertebrates– Intermediate host usually gastropod

Fluke life-cyclesSchistosoma mansoni

Schistosomiasis = disease with problems from egg production, fevers, eggs lodged in various tissues

Schistosomiasis

Schistosoma spp. cause swimmer’s itch

Class Cestoda

Tapeworms– Internal parasite– Body with tegument– Body with anterior scolex, short neck and

proglottids– No digestive system

Tapeworms = cestodes Locomotion

– Sedentary: adult on host intestinal wall– Capable of muscular undulations

Attachment– Scolex– Anterior with hooks or adhesive pad

Tapeworm digestion

No mouth, no digestive tract

Nutrients absorbed across tegument

Tapeworm Nervous System

Cerebral ganglion; nerve ring in scolex Each proglottid has additional ganglia;

connect to longitudinal nerve cords Sensory organs reduced, tactile receptors in

scolex

Sexual Repro: tapeworms

Hermaphroditic Mutual cross-fertilization Self-fertilization in some

Proglottids

– Numerous testes along margins– Collecting tubules to coiled sperm duct – Vas deferens to genital pore

Proglottids

– Two ovaries– Uterus = blind sac

Tapeworm sex and fertilization Cirrus of each mate inserted into genital pores Sperm stored, eggs fertilized in oviduct Capsule material and yolk cells stored in uterus When mature, proglottids break free

Beef Tapeworm Life Cycle

Pork Tapeworm Life Cycle