28.3 Superphylum Lophotrochozoa: Flatworms, Rotifers, and Nemerteans - Biology 2e

Overview

• Lophotrochozoa Characteristics
  • Triploblastic: have three germ layers
  • Bilaterally symmetrical
  • Protostomes (the blastopore becomes the mouth)
  • Some possess lophophore feeding apparatus or trochophore larvae
  • Include acoelomate, pseudocoelomate, and eucoelomate phyla

Phylum Platyhelminthes (Flatworms)

• General Features

  • Acoelomate organisms
  • Include free-living and parasitic forms
  • Lack lophophore and trochophore larvae
  • Monophyly supported by molecular analyses

• Lineages

  • Catenulida (chain worms): reproduce asexually by budding
  • Rhabditophora: include parasitic trematodes and cestodes

• Physiological Processes

  • Predators or scavengers
  • Possess a branching gastrovascular cavity
  • Excretory system with flame cells for osmoregulation
  • Lack circulatory and respiratory systems; rely on diffusion
  • Mostly monoecious with internal fertilization

• Diversity

  • Traditionally divided into Turbellaria, Monogenea, Trematoda, Cestoda
  • Free-living flatworms, like planaria, capable of regeneration
  • Monogeneans: ectoparasites of fish with simple life cycles
  • Trematodes (flukes): internal parasites with complex life cycles
  • Cestodes (tapeworms): live in hosts' intestines, absorbing nutrients

Phylum Rotifera

• General Features

  • Microscopic, mostly aquatic animals
  • Named for their ciliated corona

• Anatomy

  • Divided into head, trunk, and foot
  • Possess cuticle, skeletal and visceral muscles

• Diversity and Reproduction

  • Classes: Bdelloidea, Monogononta, Seisonidea
  • Some species exhibit sexual dimorphism and haplodiploidy
  • Bdelloid rotifers reproduce exclusively by parthenogenesis

Phylum Nemertea (Ribbon Worms)

• General Features

  • Marine and freshwater species
  • Known for contractile properties and bilateral symmetry

• Unique Characteristics

  • Eversible proboscis within a rhynchocoel used for food capture

• Morphology

  • Soft, unsegmented bodies
  • Possess a closed circulatory system derived from the embryo’s coelom

• Reproduction

  • Mostly dioecious; some freshwater species hermaphroditic
  • External fertilization
  • Possess powers of regeneration

Key Takeaways

• Lophotrochozoans are a diverse group featuring various forms, from free-living to parasitic.
• Flatworms are significant for their simplicity and parasitic species, impacting many hosts.
• Rotifers play a crucial role in aquatic ecosystems due to their feeding habits.
• Nemerteans are unique for their proboscis mechanism and closed circulatory systems.