Platyhelminthes, Mollusca, and Annelida

INVERTEBRATES: PHYLUM PLATYHELMINTHES (FLATWORMS)

  • Characteristics of Flatworms
    • Flattened bodies, hence the name 'flatworms'.
    • Incomplete digestive system with a single opening (mouth/anus).
    • Acoelomate: lacks a coelom, with organs embedded in mesoderm tissue.
    • Types of flatworms:
    • Free-living (example: planarians)
    • Internal parasites (example: flukes, tapeworms)

PLANARIANS

  • Description

    • Non-parasitic, free-living flatworms that exhibit bilateral symmetry and basic cephalization.
    • Arrow-shaped head with auricles for sensory detection, housing chemosensory and tactile nerve cells.
    • Possess a pair of light-sensitive eyespots.
    • Muscular body structure allows for varied movements; movements facilitated by gland-secreted mucus and cilia on the ventral surface.

  • Feeding Mechanism

    • Prey captured by wrapping around it and pinning it down using slime.
    • Muscular pharynx extends from the mouth to the gastrovascular cavity for digestion; it acts like a trunk to ingest food.
    • Lacks a circulatory and respiratory system. Nutrients and oxygen are directly delivered, and waste exits via the pharynx.

  • Reproductive System

    • Mostly reproduce sexually; they are hermaphroditic with both testis and ovaries.
    • Capable of cross-fertilization.
    • Known for regenerative capabilities; can regenerate from sections lost due to injury.

  • Nervous System

    • Ladder-type nervous system, characterized by two lateral nerve cords and transverse nerves.
    • Contains paired ganglia at the anterior end which functions as a primitive brain.

PARASITIC FLATWORMS

  • Characteristics

    • Obligate parasites that cannot survive outside a host.
    • Loss of cephalization; sensory structures replaced by hooks/suckers for attachment.
    • Simplified nervous and complex reproductive systems with most of the body dedicated to reproduction.

  • Types

    • Flukes: Named for the organs they inhabit (e.g., liver fluke, lung fluke).
    • Primary host supports the parasite in its reproductive stage; e.g., humans for Schistosoma (causes Schistosomiasis).
    • Complex life cycles involving multiple hosts.
    • Tapeworms: Featuring a scolex for attachment and proglottids for reproduction.
    • Fertilized proglottids fill with eggs before elimination in feces to infect secondary hosts.
    • Length varies significantly (up to 20m!).

PHYLUM MOLLUSCA

  • General Information
    • Second most diverse animal group with over 100,000 named species.
    • Inhabit terrestrial, freshwater, and marine environments; exhibit varied feeding habits (herbivores, carnivores, detritivores).
    • Triploblastic, protostome, coelomate phylum with a three-part body plan: visceral mass, mantle, and foot.

MOLLUSCA ANATOMY

  • Nervous System

    • Comprises ganglia connected by nerve cords.
    • Cephalization varies; some have less advanced systems (clams) while others have highly developed systems (squids, octopuses).

  • Visceral Mass

    • Contains internal organs, including those for digestion, circulation, and reproduction.

  • Mantle

    • Covers visceral mass and is responsible for secreting shells in some mollusks (snails, bivalves).

  • Foot

    • Muscular organ used for movement, attachment, or food capture.
    • Many mollusks possess a radula, a unique feeding structure.

  • Circulatory System

    • Generally open; hemolymph (invertebrate blood) is pumped through vessels and direct diffusion occurs for nutrient and oxygen exchange.

MOLLUSCA DIVERSITY

  • Major Groups
    • Bivalves: Clams and oysters, with two-part hinged shells, lack of cephalization, primarily filter feeders.
    • Gastropods: Largest class including slugs and snails, can be terrestrial or aquatic, exhibit diverse feeding mechanisms and modes of reproduction.
    • Cephalopods: Highly advanced, includes squids and octopuses, with complex nervous systems and locomotion through jet propulsion.

PHYLUM ANNELIDA – SEGMENTED WORMS

  • Characteristics

    • Segmented body structure along the length; well-developed coelom that functions as a hydrostatic skeleton.
    • Anneliida includes Oligochaetes (earthworms), Polychaetes (marine worms), and Hirudinea (leeches) with distinct adaptations for each class.

  • Anatomy

    • Complete digestive system and a nervous system consisting of a brain and ganglia for each segment.
    • Closed circulatory system with ventral and dorsal blood vessels and paired nephridia in each segment for waste removal.

  • Reproductive System

    • Clitellum present for reproduction; responsible for mucus cocoon generation housing fertilized eggs.
    • Anneliida are generally hermaphroditic and cross-fertilize.

  • Leeches

    • Typically freshwater, can act as predators or parasites.
    • Suckers facilitate attachment to hosts; anticoagulants present in saliva used in medicine for blood flow regulation.