Lophotrochozoa and Coelom Development

SUPERPHYLUM LOPHOTROCHOZOA

• Contains three major phyla:
  • Platyhelminthes (Flatworms)
  • Syndermata (Rotifers & Acanthocephalans)
  • Nemertea (Ribbon worms)

TRUE COELOM DEVELOPMENT

• Protostomes:

  • During gastrulation, mesoderm cells migrate into the blastocoel to form a solid mass.
  • Cavities develop within this mass creating the true coelom, known as schizocoelom (splitting of mesoderm).
  • Example groups: Molluscs, Annelids, Arthropods.

• Deuterostomes:

  • Develop coelom through enterocoely (mesoderm pouches pinch off from the archenteron).
  • Example groups: Echinoderms, Chordates.

CATEGORIES OF TRUE COELOMS

• Enterocoelom:

  • Formed by outpocketing of embryonic gut (enteron).
  • Organisms: Echinodermata, Chordata.

• Schizocoelom:

  • Formed by splitting of mesodermal mass.
  • Organisms: Annelida, Mollusca, Arthropoda.

SHARED FEATURES OF LOPHOTROCHOZOA

• Lophophore:

  • Feeding apparatus with two rows of ciliated tentacles around the mouth.

• Trochophore larvae:

  • Diamond-shaped with two ciliated bands, facilitating movement and food capture.

• Some members lack these features yet remain classified in this superphylum due to molecular phylogenetics.

FOSSIL RECORD

• Evidence dates back to Cambrian period, molecular studies suggest divergence may be earlier.

PHYLUM PLATYHELMINTHES (Flatworms)

• Major Clades:
  • Catenulida: Simple worms.
  • Rhabditophora: Diverse flatworms.
  • Characteristic features:
  • Dorsoventral flattening.
  • Triploblastic structure (ectoderm, mesoderm, endoderm).
  • Bilateral symmetry (right/left sides).
  • Acoelomate (no body cavity).
  • Gastrovascular cavity for digestion (incomplete).

PHYSIOLOGY OF PLATYHELMINTHES

• Diffusion: Primary method for nutrient uptake and gas exchange due to flat shape.
• Excretion: Flame cells maintain osmotic balance.

RHABDITOPHORA

• Planarians:
  • High regenerative abilities, found in various aquatic and moist environments.

TREMATODA (Flukes)

• Obligate parasites with complex life cycles often involving multiple hosts.
• Cause diseases such as Schistosomiasis.

CESTODA (Tapeworms)

• Characteristics:
  • Endoparasitic with no digestive system.
  • Absorb nutrients directly from hosts.
  • Have a scolex for attachment and body segments (proglottids).

SYNDERMATA (Rotifers)

• Feeding structure: Ciliated corona creating water vortex for feeding.
• Size: Typically 50 µm to 2 mm; multicellular with specialized organs despite their small size.
• Reproduces via parthenogenesis in some species, allowing females to produce offspring from unfertilized eggs.