lecture 9

Preparation for the Exam

• Ensure you have all necessary materials:

  • Clothes and shoes suitable for the exam environment.

  • Pens, pencils, erasers, calculators, and any other tools required for test-taking.

• Doors will be locked until we let you in. Arrive early to avoid any last-minute issues.

• Relax and stay calm before the exam; practice deep breathing if you feel anxious. Remember, you will do fine if you have prepared well.

• Don't forget to complete your weekly online quizzes to reinforce your learning.

• If you've fallen behind, take advantage of lecture resets next week to catch up on missed content.

• Remember at the end of this week, you will be halfway through the trimester, so it's a good time to assess your progress and adjust your study habits if necessary.

Introduction to Animals Without a Backbone

• Focus on invertebrates, specifically:

  • Flatworms: Members of the phylum Platyhelminthes include free-living and parasitic forms.

  • Roundworms: Represented by the phylum Nematoda, known for their slender body shape.

• Highlight the diversity of invertebrates, including their beauty and complexity in ecosystems, showcasing their roles in food webs and biodiversity maintenance.

Evolutionary Relationships of Invertebrates

• Animals are classified based on body structures and evolutionary developments:

  • Parazoans (e.g., sponges): Lack differentiated tissues, showcasing a simple body organization.

  • Eumetazoans: Possess differentiated tissues, allowing for more advanced physiological functions.

• Body symmetry variations include:

  • Asymmetrical (e.g., sponges): No symmetry in body structure.

  • Radially symmetrical (e.g., cnidarians): Body parts arranged around a central axis.

  • Bilaterally symmetrical (e.g., most invertebrates): Body divided into left and right halves that mirror each other.

• Distinction between Protostomes and Deuterostomes:

  • Protostomes: Develop mouth first from the blastopore in embryonic development, which includes many invertebrate groups.

  • Deuterostomes: Develop the anus first from the blastopore, with notable taxa including echinoderms and chordates.

• Importance of the triploblastic structure (three germ layers):

  • Ectoderm: Develops into skin and nervous system.

  • Mesoderm: Forms muscles, circulatory system, and other organs.

  • Endoderm: Develops into the digestive tract and other internal organs.

General Characteristics of Flatworms (Phylum Platyhelminthes)

• Approximately 55,000 species; some are microscopic and found in diverse habitats like freshwater, marine, and terrestrial environments, showcasing a wide range of adaptations.

• Physical characteristics:

  • Dorsoventrally flattened body: This allows for a larger surface area to volume ratio, enhancing respiration and nutrient absorption processes. This body shape also helps them move efficiently in aquatic and moist environments.

  • Acoelomate body plan: Lacks a true body cavity, leading to efficient nutrient transport due to direct diffusion through cells. This is especially important because they often rely on external environments for nutrients.

  • Gastrovascular cavity: Serves as both digestive and circulatory systems, where food enters for digestion and waste is expelled through the same opening, highlighting a simple yet effective anatomical design.

• Respiration and circulatory systems: Water is critical for survival; respiration occurs across the body surface through diffusion, making them vulnerable to environmental changes but efficient in resource-poor habitats.

• Locomotion: Can swim and move using cilia on their ventral surface. Additionally, muscular contractions enable them to navigate various environments, with some species employing adaptive movements depending on their habitat.

• Nervous system: Flatworms possess a simple nervous system with a pair of cerebral ganglia, serving as a primitive brain, and two ventral nerve cords. They also have sensory structures like eyespots to detect light and dark variations in their environment.

• Reproductive strategies:

  • Sexual reproduction: Many flatworms are hermaphroditic, possessing both male and female reproductive structures which allows for flexibility in mating.

  • Asexual reproduction: Reproduction can occur through fission or regeneration, where lost body parts can grow back, demonstrating remarkable adaptability and resilience.

Specific Types of Flatworms

Free-Living Flatworms (Class Turbellaria)

• Includes organisms like Planaria, predominantly free-living predators or scavengers that inhabit various environments, from freshwater streams to damp soil.

• Locomotion techniques: Movement is assisted by cilia on their epidermis, creating a thin layer of mucus that facilitates gliding, and muscular contractions enable more powerful movements when needed.

• Digestive system: They exhibit a simple but efficient digestive system, with a mouth located on the ventral side and a pharynx for ingesting food, pushing it into the intestine where enzymatic digestion occurs.

• Behavioral characteristics: Exhibit complex behaviors including learning capabilities and can regenerate lost body parts, showing advanced adaptations that assist them in survival.

Parasitic Flatworms (Classes Trematoda and Cestoda)

• Trematodes (Flukes):

  • They can be internal or external parasites with complex life cycles involving multiple intermediate hosts; often including snails and vertebrates.

  • Identify hosts using specialized structures such as suckers and hooks, causing significant health issues in humans, like fatigue and diarrhea. Their complex life cycles can also contribute to issues such as zoonotic diseases.

  • Example: Clonorchis sinensis (liver fluke) can lead to liver disease and other serious complications.

• Cestodes (Tapeworms):

  • Adaptations for parasitism include hooks and suckers at the scolex for attachment to the intestinal walls of hosts.

  • They lack a digestive system; instead, they absorb nutrients directly across their skin through a specialized cuticle, oftentimes leading to significant nutrient deficiencies in their hosts.

  • Proglottid segments: Serve as reproductive units, capable of shedding in feces, assisting in the propagation of their life cycles. Lifecycles often include several intermediate hosts, such as pigs and cows, underscoring their dependence on varied ecosystems for survival.

General Characteristics of Roundworms (Phylum Nematoda)

• Approximately 61,000 species; characterized by their elongated, cylindrical bodies that allow for effective movement through soil and other substrates by wriggling or swimming through moist environments.

• Possess a pseudocoelom: A body cavity that provides space for internal organs but is not a true coelom. This structure plays a crucial role in nutrient transport and organ function, enabling a higher degree of complexity compared to flatworms.

• Digestive system: Complete digestion occurs with a mouth, pharynx, intestine, and anus offering efficient processing of food, allowing for optimized nutrient absorption and energy acquisition.

• May be free-living or parasitic, playing significant roles in soil health by breaking down organic matter and improving soil structure, or causing diseases in humans and animals, impacting agricultural and ecological systems.

• Utilize a cuticle that is periodically molted (ecdysis) which allows for growth, adaptation to their environments, and protects them from external pathogens.

• Reproductive strategies: Typically include internal fertilization, with sexually dimorphic characteristics; males being smaller than females which is a notable adaptation for their reproductive success.

Notable Nematode Parasites

• Wuchereria bancrofti: Causes lymphatic filariasis, leading to elephantiasis due to obstruction of the lymphatic system, posing serious public health challenges in endemic regions.

• Dirofilaria immitis: Known as canine heartworm, transmitted by mosquitoes and can be life-threatening if left untreated, affecting both canine and feline populations.

• Ascaris lumbricoides: A common intestinal parasite in humans, known for excessive egg production, leading to serious digestive complications and malnutrition, particularly in underdeveloped regions.

Summary

Invertebrates, particularly flatworms and roundworms, exhibit significant diversity in forms, life strategies, and evolutionary advancements. Understanding their anatomy, reproductive strategies, ecological roles, and health impacts is crucial for the exam, providing insights into the vast complexity of life without backbones.