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).
Figure 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.
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|
||Sting prey, two-way digestive tube, i.e. food enters and waste exits the mouth||Marine or aquatic|
||Mouth and anus with small body cavity, parasitic forms with suckers instead of mouth||Aquatic or as parasites in animal hosts|
||Diverse; filter feeders or Detrivores||Aquatic or on land, in soil or sediments|
||Diverse; filter feeders or using scraping teeth, called radula||Marine, aquatic, on land|
||Highly developed mandibles and mouth parts||Most diverse phylum of animals
Exoskeleton made of chitin
||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.
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
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