Phylum Platyhelminthes Overview

Phylum Platyhelminthes

• Flatworms (~34,000 described + ~36,500 undescribed)
• One class free-living, three classes parasitic
• 80% of described species are parasitic

General Characteristics

• Acoelomate, triploblastic, and bilaterally symmetrical
• Free-living flatworms are the most primitive bilateral animals
• Free-living flatworms are the first group to evolve a true mesoderm
• Some species have anterior brains connected to ventral nerve cords
• Mesoderm develops into parenchyma tissues that lie between the outer body wall and gut endoderm
• No specialized circulatory system

Body Architecture

• Locomotion: Movement by beating ventral cilia; muscle contractions called pedal waves, which allow looping
• Sensory Structures: Eyespots, auricles, cerebral ganglia
• Excretory System: Protonephridia for osmoregulation (filtering waste)

Digestive System

• Food enters and unmetabolized wastes leave through the same opening
• Metabolized wastes are diffused through their flat body
• Protonephridia (first kidney) also eliminates other metabolic wastes, like ammonia, urea, and amino acids.

Reproduction

• All flatworms in the 4 classes are simultaneous hermaphrodites
• Eggs and sperm fertilize anytime when one flatworm encounters another.
• In general, most individuals cannot fertilize themselves

Class Turbellaria

• 16% of all flatworms
• Mostly aquatic (avoid dehydration), some freshwater and terrestrial
• Size ~1 cm long and ciliated
• Possess a primitive nervous system (including a cerebral ganglion)
• Unique structure called rhabdites that secrete mucus to avoid desiccation and predation
• Diverse digestive system types (no well-defined, to unbranched, three or multibranched gut)

Class Cestoda ("Tapeworms")

• Internal parasites (endoparasitic) primarily of vertebrates
• ~135 million people have tapeworm infections worldwide
• Small anterior, hooked attachment organ (scolex)
• Body divided into segments (proglottids) arising behind the scolex
• Absence of a digestive tract
• Proglottids function primarily for reproduction.

Class Monogenea

• Mostly ectoparasites of fishes
• Posterior attachment organ (haptor/opisthaptor) including suckers, complex attachment hooks, and sclerites.
• Larva (oncomiracidium) bearing 3 bands of cilia and usually 1 or 2 pairs of eyes.
• Very high species and sites specificity
• ~1100 species described; ~20,000 more believed to exist
• Life cycle does not involve intermediate hosts

Class Trematoda ("Flukes")

• Outer body layer is unciliated syncytial tegument
• Trematode bodies resemble turbellarians, but with mouth opening, blind-ended digestive tract, and no segments.
• Parasites ingest host tissues and blood through mouth
• Complex life cycle including 5 stages (miracidium, sporocyst, redia, cercariae, metacercariae)
• Often requiring 2+ intermediate hosts, typically mollusks
• Some species have sexual reproduction in the definitive host