5.2 Applications and skills

5.2.4 Recognising animal phyla

Despite all the problems of grouping species by their external features, it is still useful to do so when it comes to complex, multicellular organisms like animals.

Classifying animal phyla

We tend to look at some specific features when classifying animals:

  • Body symmetry – how are the body parts balanced? If there is a single plane of symmetry that splits the body into left and right halves, this is called bilateral symmetry. If there are a number of planes that split the body up like spokes on a wheel, this is called radial symmetry. Radially symmetrical animals do not have ‘left’ or ‘right’ sides.
  • Organisation – in the embryo stage, some animals develop from only one layer of cells, while others develop from two or three layers. Animals that develop from three germ cell layers have greater complexity and specialisation in their tissues and organs. 
  • Feeding mechanism – since all animals are heterotrophic, it makes sense to focus on how they feed themselves. More complex animals have a single digestive tube running from mouth to anus, and associated organs (e.g. liver, kidneys).

radial and bilateral symmetryFigure 5.2.4a – Radial vs bilateral symmetry
Unlike the lobster, the coral polyp does not have an anterior (forward) or posterior (backward) orientation. The dorsal (top) and ventral (bottom) sides of both animals are distinguishable.

Activity: External recognition features of selected animal phyla

Study Figures 5.2.4b-k and match them to the phyla described below.

Kingdom Animalia
All animals are heterotrophic organisms that are able to locomote.
Phylum Body symmetry Organisation/body plan Feeding mechanism Habitat
Porifera None Different types of cells – no real tissues Filter feeders Marine
Cnidaria Radial
  • Two tissue layers
  • Reproduce sexually and asexually, producing polyps
Sting prey, two-way digestive tube, i.e. food enters and waste exits the mouth Marine or aquatic
Platyhelmintha Bilateral
  • Three tissue layers
  • Soft body, flattened dorso-ventrally
  • Rudimentary sensory organs located in head
Mouth and anus with small body cavity, parasitic forms with suckers instead of mouth Aquatic or as parasites in animal hosts
Annelida Bilateral
  • Three tissue layers
  • Segmented body
  • Developed organ systems
  • Highly locomotive because of segments
Diverse; filter feeders or Detrivores Aquatic or on land, in soil or sediments
Mollusca Bilateral
  • Three tissue layers
  • External shell of calcium carbonate (some exceptions)
  • Many have highly developed sensory organs
Diverse; filter feeders or using scraping teeth, called radula Marine, aquatic, on land
Arthropoda Bilateral
  • Three tissue layers
  • Articulated body (joints)
Highly developed mandibles and mouth parts Most diverse phylum of animals
Exoskeleton made of chitin
Chordata Bilateral
  • Three tissue layers
  • Hollow dorsal neural tube and tail (most have a backbone)
Complex digestive system with associated organs Marine, aquatic, on land

Did you know?

Viruses are not living things, since they do not possess organelles capable of performing any of the functions of life (i.e. metabolism, respiration, sensitivity, growth, reproduction, excretion and nutrition, or ‘MRS GREN’). That’s why viruses are not classified in any of the three Domains. 

Vase sponge

Figure 5.2.4b – Branching vase sponge Callispongya vaginalis


Figure 5.2.4c – Radial symmetry in jelly fish of the class Scyphozoa


Figure 5.2.4d – Tapeworms can grow to more than 10m long in a human host

Polychaete worms

Figure 5.2.4e – Polychaete worms are highly motile, but those protusions are not legs


Figure 5.2.4f – Snails


Figure 5.2.4g – Squid and snails are members of the same phylum


Figure 5.2.4h – Crab


Figure 5.2.4i – Spider


Figure 5.2.4j – All of these animals have common features (crab, spider, fruitfly)


Figure 5.2.4k – The numerous members of this phylum are fish