Platyhelminthes Acoelomate Flatworms

Today’s Objective TSW take notes on the phylum Platyhelminthes including economic importance, an overview of class characteristics, and ecological niches.

General Flatworm Characteristics Acoelomate – no body cavity coelom- solid mesenchyme (mesoderm) First phylum to show a definite cellular mesoderm Bilaterally symmetrical Triploblastic; 3 distinct tissue or germ layers Dorsoventrally flattened

General Characteristics (cont.) Respiration by diffusion Digestive system extensively branched One opening for feeding and waste removal Lack an anus Primitive kidneys – protonephridia with “FLAME CELLS”

General Charcterisitcs (cont.) First evidence of cephalization primitive brain Some have eyespots Directional movement No true respiratory system or circulatory system Usually hermaphroditic

Class Turbellaria Free-living flatworms Some freshwater (Planaria), marine, or terrestrial Most move by circular and longitudinal muscles Some move by using cilia (or cilia + mucus)

Turbellarian Feeding - Carnivorous Wrap body around prey Entangling with mucous Ingest the whole prey or suck its juices through a hardened stylet (modified pharynx) in many species the pharynx is completely eversible and can envelope the entire prey there is no anus so ingestion and egestion are through the mouth

Turbellarian Reproduction Sexual Reproduction Most hermaphroditic Both sex organs discharge gametes into a common chamber even though this is true- cross fertilization is the most common mode of reproduction Some self fertilization can and does occur Asexual Reproduction Fragmentation and Regeneration

Dugesia

Class Monogenea Parasitic Flukes Only one host Most external parasites Gyrodactylus Polystoma Can cause major economic losses to fisheries

Class Trematoda Digenetic Flukes Complex life history, involves 2 or more hosts Live in every type of environment 5,000+ known species Live mostly in digestive system or associated organs (all parasitic)

Some Trematodes of Importance Clonorchis – Chinese human liver fluke Fasciola hepatica – Liver fluke Schistosoma – Blood fluke Know this life cycle! Where they are present it is hazardous to bathe, drink, wade in or dip your arms in infested waters!

Class Cestoda - Tapeworms Long, Flattened White or Yellow in color All parasitic! Adults almost always live in the intestines of their hosts Body has protective cuticle Head (scolex) has multiple disks of hooks for attachment

Cestodes (cont.) Can reach up to 50 ft. in length! No specialized sense organs Reproduce using proglottids (segments) Contain flame cells, nerves, muscles, male and female reproductive organs Shed in feces of host Taenia saginata – beef tapeworm Taenia solium – pork tapeworm

Tapeworm Structure