2. Cnidaria and Phylum Ctenophora

Phylum Cnidaria and Phylum Ctenophora
  • Simplest Eumetazoa: True animals with a tissue grade of organization.
  • Cells have a basal membrane and intracellular junctions, a more advanced feature compared to Porifera.
Phylum Ctenophora
  • Includes sea gooseberries and comb jellies.
  • Exhibit movement through rows of fused cilia known as ctenes.
  • All are marine pelagic predators with about 80 species documented.
  • Characteristic feature: Planktonic lifestyle facilitated by ctenes.
Animal Tree of Life
  • Ctenophores are considered the sister group to all other animals (Moroz & Kohn, 2015; Whelan et al., 2017).
Phylum Cnidaria
  • Primarily marine with a few freshwater species.
  • Abundant and diverse, with approximately 10,000 species.
  • Body Forms:
    • Polyp (sessile) - Example: sea anemone.
    • Medusa (slow swimming, pelagic) - Example: jellyfish.
  • Life cycle may include both forms.
Habitat Characteristics of Water
  • Prevents drying out.
  • Provides support and buoyancy.
  • Facilitates waste removal and oxygen diffusion for respiration.
  • High specific heat maintains thermal stability.
  • Filters harmful UV rays and disperses offspring, reducing competition.
Cnidarian Organization
  • Displays tissue grade of organization with two cell layers:
    • Epidermis.
    • Gastrodermis.
  • Cell layers are one cell thick with limited differentiation, including:
    • Musculo-epithelial.
    • Glandulo-epithelial.
    • Cnidocytes: stinging cells containing nematocysts.
  • Exhibits radial symmetry.
Cnidarian Body Wall Structure
  • Two layers: diplobastic (epidermis and gastrodermis) with mesoglea in between.
  • Body wall lacks certain organs, emphasizing a simple structure.
Nervous System
  • Features a diffuse nervous system:
    • Double nerve net present in the epidermis and gastrodermis.
  • Cnidocytes: activated through nematocyst discharge with a coiled stinging thread.
Class Anthozoa
  • Greek word “anthos” means “flower”; includes corals, sea anemones, and sea pens.
  • Characteristics:
    • Solitary or colonial polyps; medusa stage absent.
    • Presence of siphonoglyphs for feeding.
    • Large gut cavity divided by mesenteries.
    • Radial symmetry exhibited by either hexamerous (6-part) or octamerous (8-part).
Hydrostatic Skeleton
  • Comprises an enclosed fluid that provides structural support.
  • Offers flexibility and allows for muscle movement but limits lifting capacity.
  • Locomotion mechanism in polyp includes contraction of muscles to inflate body shape.
Subclass Hexacorallia (Zoantharia)
  • Characterized by large polyps with two siphonoglyphs and tentacles in multiples of 6.
  • Includes colonial stony corals in the order Scleractinia.
  • All connected by coenosarc, facilitating nutrient sharing between polyps.
Class Scyphozoa
  • Large jellyfish species with predominance of medusa stage in life cycle.
  • Characterized by tetramerous (4-part) radial symmetry and complex gut system.
  • Distinctive features include absence of a velum on bell margin and presence of numerous micromouths.
Class Hydrozoa
  • Moss-like organisms including structures like the Portuguese man-of-war.
  • Life cycle often has both polyp and medusa forms; may be predominantly polyp.
  • Tend to be colonial, displaying polymorphism with specialized zooids.
Summary of Cnidaria
  • Tissue Grade of Organization: cells organized into two layers with limited differentiation.
  • Body Types: Presence of both polyp and medusa forms; in some classes, one form dominates the life cycle.
  • Radial Symmetry: Allows for interactions with the environment in all directions.