Biodiversity of Animals

The concept of a phylum

A phylum is a major grouping in animal classification, based on shared body plan features. Of the roughly 30 animal phyla, six are studied here: Porifera, Cnidaria, Platyhelminthes, Annelida, Arthropoda, and Chordata.

Comparing body plans

Four key features let us compare body plans across phyla:

Symmetry and cephalisation — whether the body is symmetrical (and how), and whether there's a concentration of nerve tissue forming a head.
Number of tissue layers developed from the embryo (e.g. two layers vs three).
Number of openings in the gut — one opening serving as both mouth and anus, or two separate openings.
Coelom and blood systems — whether a fluid-filled body cavity (coelom) and a circulatory system are present.

Try the Animal Phylum Explorer in the Explore tab to compare all six phyla side-by-side.

Invertebrates matter — in agriculture and ecosystems

Invertebrates (animals without a backbone, which includes five of the six phyla studied here) play essential roles that are easy to overlook:

Pollination — many arthropods (like bees) pollinate crops and wild plants.
Decomposition — annelids (like earthworms) and other invertebrates break down organic matter.
Soil aeration — burrowing invertebrates like earthworms improve soil structure and aeration.

Answer in your exercise book.

Question 1 · (2 marks)

Define the term "phylum".

Question 2 · (4 marks)

Name FOUR key features used to compare the body plans of different animal phyla.

Question 3 · (6 marks)

Construct a comparative table of the six phyla studied, showing symmetry, number of tissue layers, and number of gut openings for each.

Question 4 · (3 marks)

Give THREE roles that invertebrates play in agriculture and ecosystems.

Question 5 · (2 marks)

Name the phylum that includes animals with a backbone (vertebrates), and give ONE example.

Question 6 · (6 marks)

The table shows the approximate number of described (named) species in each of the six phyla studied:

Phylum	Approx. described species
Porifera	8 000
Cnidaria	11 000
Platyhelminthes	20 000
Annelida	17 000
Arthropoda	1 200 000
Chordata	65 000

(a) Calculate the total number of described species represented by these six phyla. (1 mark)
(b) Calculate the percentage of this total made up by Arthropoda. (2 marks)
(c) Calculate the percentage of this total made up by Chordata (the phylum that includes all vertebrates). (2 marks)
(d) Vertebrates (Chordata) receive most of the attention in everyday life and media. Using your answers to (b) and (c), explain why this gives a misleading impression of where most of animal biodiversity actually lies. (1 mark)