Advantages to having a Body Cavity

(pseudocoelom or coelom)
• Space for development of organs and organ systems
• Body is more flexible, advantageous for Locomotion
• Gut can move independently of the body wall
• Fluid in the cavity allows for Hydrostatic Skeleton
• Fluid in cavity protects internal organs
• Fluid in cavity can act as a Transport/Circulatory System

Advantage of a True Coelom:


Phylum Rotifera

  • Pseudocoelomate
  • Filter-Feeding
  • Microscopic
  • Mostly Freshwater
  • Complete Digestive Tract (with extracellular digestion)

Some Rotifer Species Consist Exclusively Of Females

Phylum Nemotoda 

Nematode Characteristics

  • Most free-living, some notorious parasites
  • Psuedocoelomate
  • Open Circulatory System
  • Complete Digestive Tract
  • Cuticle that is molted (Nemotodes are Ecdysozoans)
  • No specialized organs for respiration or excretion
  • Cephalization
  • Mostly dioecious
  • Exhibit Eutely


  • A noncellular covering of collagen
  • Permeable to water and gases
  • Free living nematodes must live in water or moist soil
  • Shed cuticle during development=Ecdysis (molting)

-Complete Digestive Tract

-Extracellular Digestion

Nematode Reproduction and Development

  • Mostly diocious, copulate with internal fertilization
  • No morphologically distinct larval stage, young emerge from eggs as miniature adults
  • Chromosome diminuation 
  • Eutely – constant number or cells