Invertebrates: Platyhelminthes, Annelida, and Mollusca

Platyhelminthes Overview

  • Species Diversity: Approximately 20,000 described species.
  • Habitat: Inhabit marine, freshwater, and moist terrestrial environments.
  • Lifestyle: Includes both free-living and parasitic species.
  • Body Structure: Acoelomates – lack a true body cavity.
  • Digestive System: Incomplete digestive system characterized by a gastrovascular cavity.
  • Osmoregulation: Managed via protonephridia, also known as flame cells.

Planarians

  • Habitat: Commonly found in freshwater and damp terrestrial environments.
  • Morphological Features:
    • Eye spots (ocelli) for light detection.
    • Auricles serve as olfactory sensors.
    • Cilia on the body facilitate movement.
  • Cephalization: Exhibits moderate cephalization, a concentration of sensory organs at the head.

Flukes (Trematodes)

  • Lifestyle: Parasitic and digenetic, meaning they require two or more hosts for their lifecycle.
  • Reproductive Strategies:
    • Sexual reproduction occurs in the vertebrate host.
    • Asexual reproduction occurs in an intermediate host.
  • Example Species:
    • Clonorchis sinensis: Human liver fluke.
    • Fasciola hepatica: Sheep liver fluke.

Tapeworms (Cestodes)

  • Characteristics: Exclusive parasites found in vertebrates.
  • Morphological Features:
    • Scolex: Attachment organ to the host.
    • Proglottids: Segmented body parts that contain reproductive organs.
  • Nutrition: Lacks a digestive system, absorbing nutrients directly from the host.
  • Example:
    • Taenia pisiformis: Carnivore tapeworm.

Annelida Overview

  • Common Name: "Little rings" (referring to their segmented bodies).
  • Species Diversity: Approximately 16,500 described species.
  • Habitat: Occupy marine, freshwater, and terrestrial environments.
  • Body Structure: Eucoelomates with a true body cavity, characterized by a complete digestive and closed circulatory system.

Two Clades of Annelids

  • Errantia:
    • Typically mobile and often predatory.
    • Features well-developed jaws and parapodia for efficient movement and respiration.
  • Sedentaria:
    • Generally less mobile; includes burrowers and filter feeders.

Sedentaria Examples

  • Hirudo verbana (Leeches):
    • Used in medicinal practices to improve circulation.
    • Can be ectoparasitic blood suckers or predatory.
  • Lumbricus terrestris (Earthworms):
    • Possesses five pairs of “hearts” (pumping vessels).
    • Clitellum plays a role in reproductive functions.

Mollusca Overview

  • Basic Characteristics:
    • Body Plan: Consists of a head, muscular foot, and a visceral mass containing internal organs.
    • Soft body often covered by a mantle which secretes a calcium carbonate shell.
    • Adolescents possess a radula for various feeding methods.
  • Nervous System: Well-developed with a centralized brain.
  • Circulatory System: Open circulatory system, alongside a complete digestive system.
  • Species Diversity: Nearly 100,000 known species, with a significant number still undiscovered.

Gastropods (Class Gastropoda)

  • Dominance: Largest group of molluscs, including snails, limpets, abalones, and sea slugs.
  • Shell Characteristics: Most species have coiled shells, except for nudibranchs (sea slugs) which lack shells.
  • Feeding Mechanisms: Utilizes radula primarily for grazing on seaweeds; some adopt deposit feeding or suspension feeding strategies, while others are carnivores utilizing the radula to seize prey.

Bivalves (Class Bivalvia)

  • Feeding Behavior: Specialize as filter feeders, including clams, oysters, scallops, and mussels.
  • Body Structure:
    • Compressed body resides within a mantle cavity situated between two shells (valves).
    • Gills expanded and folded fulfill both respiration and filter feeding roles.
  • Body Dynamics: Incorporates siphons for water intake and expulsion during feeding.
  • Movement: Clams burrow using their muscular foot, while oysters and mussels attach to hard surfaces. Scallops can swim freely.

Cephalopods (Class Cephalopoda)

  • Examples: Squids, octopuses, cuttlefishes, and chambered nautilus.
  • Motility: Fast swimming predators facilitating locomotion through water jet propulsion, managing water movement via a siphon.
  • Nervous Organization: Highly complex with advanced visual systems (large eyes & brain).
  • Anatomical Features:
    • Beak-like jaws for crushing and ripping prey.
    • Internal shells, such as the pen in squids.

Chitons (Class Polyplacophora)

  • Unique Structure: Characterized by a dorsal shell comprising 8 plates.
  • Foot Structure: Muscular foot situated ventrally, aiding movement.
  • Feeding: Many species graze on seaweeds and small invertebrates found on rocky substrates.

Dissection Activities

Platyhelminthes

  • Planarians:
    • Observe live specimens of Girardia tigrina through a depression slide while feeding.
    • Identify and label anatomical features like eyespot, auricle, gastrovascular cavity, and pharynx using prepared slides and sketches.
  • Digenetic Flukes:
    • Examine prepared slides of Clonorchis sinensis and Fasciola hepatica, labeling significant anatomical parts.
  • Cestoda (Tapeworms):
    • Study prepared slides of Taenia pisiformis, noting immature and mature proglottids as well as the scolex with suckers.

Annelida

  • Errantia:
    • Visual observation and sketches of Nereis, labeling parts such as parapodia and setae.
  • Sedentaria:
    • Study and sketch Hirudo verbana, noting suckers and their functions.
    • Earthworm dissection for internal structures including the brain, hearts, and reproductive organs.

Mollusca

  • Polyplacophora (Chitons):
    • Visual observation of chiton specimens and anatomical labeling.
  • Gastropoda:
    • Shell sketches of various Conus shells.
  • Bivalvia:
    • Dissection of mussels, identifying internal structures, including gonads and heart.
  • Cephalopoda:
    • Structure examination of nautilus and squid, labeling significant features.