Acoelomates Phylum Platyhelminthes Phylum Nemertea Phylum Gnathostomulida Taxonomy

Acoelomates Phylum Platyhelminthes Phylum Nemertea Phylum Gnathostomulida

Taxonomy

Acoelomate Phyla • Platyhelminthes – Flat worms • Nemertea – Ribbon worms • Gnathostomulida – Jaw worms

Why bilateral symmetry? ü Movement toward prey or host – Cephalization – Directional sense organs • Chemoreceptors • Ocelli (light sensing eyespots) • Rheoreceptors (sense water currents)

Acoelomates Key Features • Three germ layers – Ectoderm, endoderm, mesoderm – Mesoderm forms muscle and mesenchyme • Organ-system level of organization – Cephalization – Excretory system – Some have circulatory and one-way alimentary canal

Acoelomates Body Plan

Phylum Platyhelminthes • Specialized Cells – Rhabdites - secrete mucous sheath – Tegument - outer covering of syncytial cells – Flame cells – Osmoregulation – Some have endolecithal egg cells – yolk contained within egg cell

Platyhelminthes Cross Section

Tegument • Syncytium – Apical cell membrane missing

Flame Cell • Osmoregulation – Beating flagella creates negative pressure to draw fluid into chamber – Water is directed through channels then through an external pore

Major Classes of Platyhelminthes • Class Turbellaria – Planaria – free living • Class Trematoda – Liver flukes – endoparasites • Class Monogenea – Fish ectoparasites • Class Cestoda – Tape worms – endoparasites

Class Turbellaria • Key Features – Ladder like nervous system – Eye spots (Ocelli) – Auricles - chemoreceptors – Free living flatworms – Muscular and ciliary movement – Carnivorous

Planarian Body Plan

Class Trematoda • Similar in structure to Turbellaria • Special adaptation for endoparasitic life style – Cyst glands – Suckers and hooks for griping host • Digenetic – complex life cycle with multiple hosts

Liver Fluke

Liver Damage Caused By Flukes • Scar tissue • Blocked bile ducts

Flukes in Liver Section

Fasciolopsis - Anterior End

Schistosoma mansoni • Blood fluke • Male and female are dioecious

Schistosoma cercaria

Class Cestoda • Tapeworms - endoparasites • Key Features – Lack a digestive system – Composed of reproductive units called proglottids – Adult tegument covered in microvilli-like projections – increases surface area, non-ciliated

Tapeworm • New proglottids are added just behind the scolex

Tapeworm Tegument • Microvilli help with food adsorption • Sensory cells – no other sense organs

Mature Proglottid • Reproductive unit • Mature proglottids are either shed or produce shelled embryos • Each proglottid has both male and female organs

Tape Worm Life Cycle

Taenia solium Pork tapeworm

Taenia Cyst in Muscle

Phylum Nemertea • Ribbon worms – Ciliated epidermis – Locomotion – gliding over slime track and muscular contraction – Complete digestive tract – mouth and anus – Extracellular digestion – Proboscis to capture prey

Amphiporus