Flatworms, Mollusks, and Annelids Study Notes

Complete Digestive Tract: A digestive system with two openings where digestion occurs in a continuous tube.
Radula: A file-like feeding organ found in mollusks used for scraping food.
Hemocoel: A body cavity in some invertebrates that contains blood and circulates it directly in the body.
Segmentation: The division of an organism’s body into repeated sections or segments.
Coelom: A fluid-filled body cavity surrounded by muscle in which the internal organs develop and are contained.

Flatworms are classified with mollusks and annelids in the phylum Lophotrochozoa due to their unique larval forms and feeding structures.
Flatworms have a solid body and an incomplete or absent gut. The lack of a circulatory system limits their size and allows nutrient and oxygen transport solely by diffusion.
Three classes of flatworms include:
- Planarians: Free-living, non-parasitic flatworms with sensory organs and a simple brain.
- Flukes: Parasitic flatworms that primarily infect humans and other animals (e.g., Schistosoma causing schistosomiasis).
- Tapeworms: Parasites that absorb nutrients from the host's digested food and reproduce by breaking off segments filled with fertilized eggs.

Mollusks have a complete digestive tract, allowing more efficient digestion and nutrient absorption.
Three main features shared by mollusks:
- Radula: A scraping organ for feeding found in most mollusks.
- Mantle: Tissue that covers internal organs, often secreting a calcium shell.
- Ctenidia: Gills used in respiration, modified in land species to absorb oxygen from air.
The hemocoel circulates blood through spaces between cells and extends into a muscular foot, which aids in movement and feeding.
Major Classes of Mollusks:
- Gastropoda: Snails and slugs, including herbivores and carnivores.
- Pelecypoda (Bivalves): Clams, oysters, and mussels. Mostly filter feeders.
- Cephalopoda: Squid and octopus, known for their intelligence and complex behavior.
- Additional classes include Scaphopoda, Polyplacophora, Aplacophora, and Tryblidiidae.

Annelids possess a segmented body plan, featuring a repeated structure of organs and body systems within each segment.
They have a closed circulatory system where blood flows within vessels, allowing for a more efficient distribution of resources.
The coelom serves as a hydrostatic skeleton, aiding movement through contraction of muscles that change the shape of each segment.
Annelids can reproduce asexually through fragmentation or sexually, with many species being hermaphrodites.
Three groups of annelids include:
- Earthworms: Important for soil fertility as they consume organic waste.
- Marine Worms: Varied diets including predation on small invertebrates.
- Leeches: Some are blood-feeders while others prey on small animals.

Planarians vs. Flukes and Tapeworms: Planarians are free-living and non-parasitic, while flukes and tapeworms are primarily parasitic with complex life cycles.
Mollusks vs. Annelids: Mollusks generally exhibit more diversity in form and lifestyle, while annelids are characterized by their segmented body plan and closed circulatory system.