Animal Complexity

Animal phyla can be differentiated according to a number of features of increasing complexity

  • These features include body symmetry, number of body openings, body segmentation and the presence of a notochord

Body Symmetry

  • Body symmetry describes the alignment of body parts around a central axis (radial = circular plane ; bilateral = linear plane)
  • Porifera are the most primitive of animals and lack any body symmetry (asymmetrical) as they have no true tissues or organs
  • Cnidaria are sedentary or slow moving and have radial symmetry, so as to experience the environment equally from all directions 
  • All other animals have bilateral symmetry, which promotes development of a head and allows for streamlined, directional movement

Types of Body Symmetry

body symmetry

Body Openings

  • Porifera are the most basic invertebrate and have no body openings – instead they exchange materials through pores
  • Cnidaria and platyhelminths have a singular body opening, leading to a saccular digestive system
  • All other specified phyla have two body openings (i.e. a separate mouth and anus), resulting in a tubular digestive system
  • Having two body openings allows for specialisation of digestive processes as food is travelling in a single direction

Platyhelminth – Saccular Digestive System


Body Segmentation

  • Segmentation of body parts allows for the specialisation of function in these different areas
  • Organisms with more basic body plans (including porifera, cnidaria and platyhelminths) lack distinctive body segmentation
  • Organisms with more developed body plans have segmented bodies (e.g. annelids, mollusca, arthropoda, chordata)
  • Sometimes body segmentation may not be clearly visible – all molluscs have a mantle, visceral mass and muscular foot but these segments may not be readily apparent through visual inspection

Body Segments in Mollusca

body segments


  • All chordates share four key features: a notochord, a hollow dorsal nerve tube, pharyngeal slits and a postanal tail
  • These features are always present in an embryonic state, but might not always persist into adulthood
  • In vertebrates (a sub-phyla), the notochord is largely replaced and surrounded by vertebra, forming a backbone

Chordate Features

chordate features