Flatworms and Classification

• Flatworms are acoelomate organisms, meaning they lack a body cavity.
• Flatworms are dorsoventrally compressed.
• Flatworm bodies are solid between the outer surface and the digestive system cavity.
• They include free-living and parasitic forms.
• Free-living forms have primitive sense organs.
• Parasitic forms include important parasites of humans.
• Flatworms have three embryonic tissue layers (triploblastic): ectoderm, mesoderm, and endoderm.
• Ectoderm gives rise to surfaces that cover tissues.
• Mesoderm gives rise to internal tissues.
• Endoderm lines the digestive system.

Class Cestoda

• Includes tapeworms
• Tapeworms are internal parasites mainly of vertebrates.

Class Monogenea

• Ectoparasites of mostly fish.

Class Trematoda

• Includes flukes
• Flukes are internal parasites of humans, mollusks, and other groups.

Class Turbellaria

• Mainly free-living, marine species
• Some species live in freshwater or terrestrial environments.
• Turbellaria are hermaphroditic.

Dugesia (Planaria)

• Common freshwater Turbellaria.
• The head has eyespots that are light sensitive.
• Ganglia surround the eyespots.
• Nerve cords extend from the ganglia along the body.
• Auricles are tactile and olfactory sensory lobes on the head.
• Planaria suck food into their mouth and pharynx.
• Food is then passed into the gastrovascular cavity.
• Tubules connected to excretory pores on both sides of the body make up the excretory system.

Phylum Nematoda

• Roundworms are bilaterally symmetrical, worm-like organisms.
• A strong, flexible noncellular layer called a cuticle surrounds roundworms.
• The cuticle is resistant to digestive enzymes.
• It's only permeable to dissolved gasses, water, and some ions.
• Roundworms are pseudocoelomates.
• A ring of nervous tissue around the pharynx forms a simple nervous system.
• Dorsal and ventral nerve cords run the length of the body.
• Nematodes colonize nearly every conceivable habitat on Earth.
• Many are free-living, serving as decomposers and predators.
• Some nematodes are parasitic, affecting humans and domestic animals.

Examples of Roundworms

• Trichinella causes trichinosis and is ingested from raw or undercooked meat infected with the larvae.
• Hookworms live in the small intestine.
• Ascaris is a common roundworm.
• Whipworms reside in the cecum.
• Pinworms live in the colon and rectum.
• Filarial worms are tropical parasites that cause diseases like elephantiasis and river blindness.

Phylum Annelida

• Includes earthworms, polychaetas, and leeches.
• Annelids are found in marine, freshwater, and terrestrial habitats.
• Water or humidity is critical for survival, especially in terrestrial habitats.
• Annelids are protostomes "segmented worms” due to metamerism (true segmentation).
• Metamerism increases efficiency of movement and allows for greater body complexity.
• All annelids except leeches have chitinous hair-like structures called setae, projecting from their cuticle.

Class Polychaeta

• Includes sand worms, tube worms, and clam worms.
• Most have well-developed, paired, paddle-like appendages called parapodia.
• Polychaetas are abundant and are predators on small invertebrates.
• They also serve as food for fish and large invertebrates.

Class Oligochaeta (Earthworm)

• Earthworms are the most abundant members of the class.
• Possess a clitellum and have few, reduced setae.

Class Hirudinea (Leeches)

• A primary difference between leeches and other annelids is the development of suckers.
• Suckers are located at the anterior and posterior ends.
• Leeches lack setae.
• The segmentation of the body wall may not correspond to the internal segmentation of the coelomic cavity.
• This adaptation helps leeches elongate when ingesting blood.

This lesson covers the phylum Platyhelminthes (flatworms) and their classification. Flatworms are acoelomate, dorsoventrally compressed organisms with three tissue layers. The major classes include Cestoda (tapeworms), Monogenea, Trematoda (flukes), and Turbellaria.