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

Explore the fascinating world of Phylum Platyhelminthes, commonly known as flatworms. This quiz covers their general characteristics, body architecture, locomotion, sensory structures, and digestive systems. Delve into the diverse classes of these organisms, with a focus on their adaptive strategies.