Study Notes on Flatworms (Phylum Platyhelminthes)

SC.912.L.15.2: Explain how organisms interact with the living and nonliving components of their environment, including how organisms obtain energy and nutrients.
Attendance and make-ups notification.
Course Content: Chapter 9C (Part 1) – Describing morphology, ecology, and diversity of flatworms (Phylum Platyhelminthes).
Students will complete Chapter 9C (Part 1) In-Class Worksheet.
No homework assigned for Marine Science for the respective dates (Fri - 1/9/26 and Mon - 1/12/26).

Some marine worms show the development of true organs. In contrast, sponges do not have tissues, instead having various cell types, and cnidarians possess tissues but lack organized organs.
Symmetry Types:
- Radial Symmetry: Beneficial for organisms that are sessile or drift with currents.
- Bilateral Symmetry: Found in organisms that crawl or swim in a directed manner.
- Anterior: Front end of the organism, often housing the brain or command center for the nervous system and sensory organs (e.g., eyes).
- Posterior: Rear end of the organism (tail).
- Dorsal: Upper side of the organism, opposite the belly.
- Ventral: Lower side of the organism (belly).

Bilateral Symmetry: Body can be divided into equal right and left halves.
Anterior: The front (head) of an animal.
Posterior: The rear (tail) of an animal.
Dorsal: Back or upper side of an organism, opposite to the belly.
Ventral: Refers to the belly or lower side.
Ectoderm: The outer layer of cells in an embryo that forms the skin and nervous system.
Endoderm: The innermost layer of cells in an embryo that develops into the digestive tract.
Mesoderm: The middle layer of cells (between ectoderm and endoderm) that develops into muscles, bones, and other organs.
Proglottid: Each reproductive unit found in some flatworms.

Hermaphrodite: An organism possessing both male and female reproductive organs.
Regeneration: The ability of some animals to regrow lost or damaged body parts.
Coelom: A body cavity present in some animals.
Coelomate: An animal with a coelom between the digestive tract and outer body wall.
Acoelomate: An animal that lacks a coelom.
Nerve Cord: A bundle of nerve fibers running the length of some animals to transmit signals.
Eyespots: Clusters of light-sensitive cells that help the organism react to environmental changes.

Features:
- Dorsoventrally flattened body structure.
- Display bilateral symmetry.
- Acoelomate body plan: No true body cavity; all organs are contained within a solid tissue called parenchyma.

Turbellaria: Free-living and non-parasitic.
- Features:
  - Possess eyespots to detect light.
  - Exhibit regeneration capabilities upon injury.
- Example: Planaria.
Trematoda: Parasitic flukes.
- Features:
  - Have complex life cycles with multiple hosts.
- Examples:
  - Liver flukes (Clonorchis, Fasciola).
  - Blood flukes (Schistosoma).
Cestoda: Parasitic tapeworms.
- Features:
  - Reside in the digestive tracts of vertebrates.
  - Composed of segmented bodies that can grow to significant lengths.
- Examples:
  - Human, pork, and Asian tapeworms (various species of Taenia).

The space between the inner and outer body surfaces is filled with solid tissue (not gel-like, as seen in sponges).
The gastrovascular cavity, resembling that of cnidarians, includes a single opening for food entry.
Many species exhibit a triangle-shaped head.

Feeding Habits: Free-living predators and scavengers, feeding on:
- Small organisms (bacteria, algae, protozoans).
- Some larger animals (snails, oysters, barnacles).
Habitat:
- Can be found in marine, freshwater, and terrestrial environments, often under rocks or in moist soils.
Respiration: Absorbs dissolved oxygen through their skin.
Reproduction:
- Capable of sexual reproduction (being hermaphrodites).
- Also reproduce through asexual regeneration.
Sensory Structures: Possess eyespots and sometimes pigment cups known as ocelli (“little eyes”).

Hermaphroditic Reproduction: They can simultaneously impregnate one another.
Mating Ritual: Known as penis fencing; involves two individuals attempting to impregnate each other using their penises, with the successful penetrator acting as the male, thus avoiding the energy costs related to healing and egg development.

Many flatworms exhibit the capability to regenerate missing or damaged body parts, primarily seen in Planaria. This process is a form of asexual reproduction known as regeneration.

Nervous System:
- Central Nervous System: Comprises a collection of nerve cells referred to as the brain, responsible for processing information.
- Peripheral Nervous System: Includes nerve cords and transverse nerves running along the entire body, tasked with carrying information to and from the brain; coordinates muscular movements.
Digestive System:
- Gastrovascular cavity with a singular opening (mouth) for food intake.
- Pharynx: A tube-like structure assisting in food intake.
Excretory System:
- Comprises flame cells and an interconnected network of excretory canals that lead to openings known as excretory pores.
- Functions to collect dissolved wastes and fluids and maintain water balance.

Flame Cells: Specialized cells that collect dissolved wastes and excess fluids.
Excretory Canals: A network that facilitates the transport of these fluids.
Excretory Pores: Openings in the skin that release waste fluids to the environment.

Classification: Invasive species of flatworm located in several Caribbean islands.
Origin: Believed to have been introduced through soil in imported potted plants.
Ecological Impact:
- Prey on and significantly reduce populations of snails, slugs, earthworms, insects, and spiders that act as decomposers.
- Disruption to nutrient cycling and effects on soil structure and nutrient content due to the depletion of decomposers.
- Impact on bird populations, as birds rely on snails, earthworms, and insects for food.

Parasitic Relationship: The Platydemus manokwari is a host of a parasitic nematode that can infect humans, leading to meningitis (infection and inflammation of the brain and spinal cord membranes).

Marine flukes negatively impacting fish, seabirds, and mollusks.
Example: Dactylogyrus, a marine fluke, lives on the gills of marine fish, causing respiratory issues.

Anatomy: Includes structures like hooks, testis, ovary, and various larval stages.

Adult Fluke: Lays eggs on the skin, gills, and fins of marine fish, leading to irritation.
Infections: Can also extend to crustaceans like lobsters, first identified in the mouthparts of lobsters in Norway during the 1960s.

Provides anatomical details such as mouth, pharynx, intestine, and various stages of embryonic development.