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An introduction to the diversity of animal life. Peter Shaw RU. This is the jpg-free version to save space. Aim for today.

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an introduction to the diversity of animal life

An introduction to the diversity of animal life

Peter Shaw


This is the jpg-free version to save space

aim for today
Aim for today

To introduce you to the range of animal life on the planet. In one lecture I can do no more than scrape the surface, but want to give you a basic structure to carry in your head into which any animal may be fitted. Forgive me for throwing a lot at you in one lecture!

This framework has a hierarchical structure (meaning it can be shown as a dendrogram) founded in taxonomy.

Taxonomy – the study of the classification of life forms.


taxonomic hierarchies
Taxonomic hierarchies

These are about seeking common features unifying all the organisms in a named group. The deepest split of all is between two ways of organising cells – the eukaryotic cell (with a nucleus and organelles) and prokaryotic cells (with DNA loops floating free in the cytoplasm). These are divided into 5 kingdoms in modern systems:








(Viruses would count as a 6th, if you regard them as alive).


In this course we will concentrate on just one kingdom, the animals. Luckily there are few hidden catches here – it is usually pretty obvious if a life form is an animal or not, though at the single celled level things can get rather blurred. (Volvox is a single celled green, photosynthetic entity which can ingest particulate food. It has good claims to be both animal and plant).

The next level down from kingdom is the one that REALLY matters for classifying animals. It is called Phylum, plural phyla.

(NOT fila, as a student once wrote in a failed exam paper…)

There are about 30 phyla, each with a deep underlying similarity of body form. Once you can place an animal in its phylum you have made an excellent start towards understanding its anatomy.

the full hierarchy
The full hierarchy

Kingdom - animalia

Phylum - mandibulata

Class - Insecta

Order - Collembola

Family - Entomobryidae

Genus Entomobrya

Species Entomobrya nivalis

 Species - the basis of taxonomy, dignified by a Latinised binomial = the scientific name: Homo sapiens, Apodemus sylvaticus, Lumbricus terrestris.

(I dislike the term “latin name”, since it is not Latin but merely latinised. Others find it acceptable, but I would encourage ‘Scientific name’)

how to write a scientific name
How to write a scientific name!

So many students get this wrong that I want to tell you now, at the start of your careers, how to write these names. Remember that getting it wrong is equivalent to saying “I have not been formally trained in biology”.

Homo sapiens OR Homo sapiens

1st name has a capital letter, 2nd does not

When writing by hand underline the name.

On a PC make the font italic

one cell or many
One cell or many?

We start dividing up animals here.

Some animals have just one cell – many others have large numbers of differentiated cells.

1 cell - Protozoa

Many cells – parazoa and metazoa

the protozoa the single celled animals
The Protozoa – the single celled animals

In fact many of these are photosynthetic and are claimed as plants by botanists, while some are both photosynthetic and carnivorous! The animal -plant - fungus split does not make sense at this level.

Old system: exclude green species, lump the rest in Phylum protozoa, which has 4 classes:

ciliates (Paramecium caudatum) – many small cilia

flagellates (Euglena, Trypanosoma) – one big cilium (flagellum)

Rhizopoda (Amoeba proteus) – no cilia

+ a less well known class of parasitic species: Sporozoa (Plasmodium vivax)

new version kingdom protozoa
New version – kingdom Protozoa

Instead of the drastic shoe-horning described above, the current version is to regard all single-celled organisms as belonging to the kingdom Protozoa with many phyla (27 at last count!)

This is probably more realistic, but much harder to remember.

sponges phylum parazoa
Sponges – Phylum parazoa

 These are essentially colonial protozoa, whose colonies are reinforced with solid spicules of various shapes and composition. Silica SiO2 and Calcite CaCO3 are the commonest.


Parazoa are exclusively aquatic, mainly marine, and live by filter feeding. The feeding cells are called choanocytes, which incorporate a central flagellum pumping water through the sponge, and the water passes through a collar of cilia-like filtering projections. The other main cell type is ameoba-like, making the supporting tissues and moving nutrients around.

Typically sponges suck water in from around their bodies and exhale it from a common central siphon. Due to their diffuse form, and often variable colour, identifying them is often difficult / impossible in the field and relies on microscopic examination of spicules.


Metazoa: These are animals with fully differentiated tissues, including muscles and nerves.

Many cells

1 cell - Protozoa

No clear tissues: parazoa

Tissues: metazoa

The next level up in organisation takes us to the group of animals that used to be classed as phylum coelenterata (jellyfish, anemones and sea gooseberries). These are now split into 2 phyla, based on deep differences in design of their their stinging cells:

Cnidaria – jellyfish and anemones

Ctenophora – sea gooseberries.

phylum cnidaria radially symmetric 2 cell layers in body
Phylum Cnidaria (radially symmetric, 2 cell layers in body)

Jellyfish and allies. These alternate 2 phases in their life cycle: the free-living medusoid phase (“jellyfish”), and a sessile hydroid phase.

Eggs and sperm

Hydroid phase





A cnidocyte


This phylum feeds by capturing planktonic food using tentacles armed with a cnidarian speciality, the class of stinging cell called nematocysts. Some are entangling, some inject barbed points to anchor, some inject toxins.

A few a lethal to humans - NEVER EVER swim with box jellies (sea wasps, class Cubomedusae).

Bilateria: this comprises c. 25 phyla all with bilateral symmetry (at least as larvae) and 3 layers of cells in the embryo.

Many cells

1 cell - Protozoa

No clear tissues: parazoa

Tissues: metazoa

Radial symmetry

2 cell layers in embrya

Phyla cnidaria and ctenophora

Bilateral symmetry

3 cell layers in embryo

Remaining animal Phyla

phylum platyhelminths
Phylum Platyhelminths

The simplest of these phyla are the flatworms, platyhelminths. These have no body cavity (acoelomate), and a “bottle gut” (ie mouth and anus are the same orifice).

<1mm deep

Combined mouth and anus, leading into gut

Many are free living, the planaria, and are active hunters. One recently introduced species from New Zealand is a serious earthworm predator - Arthiopostioa triangulata.



A few are internal parasites, ie liver fluke Fasciola hepatica. Bilharzia is caused by a flatworm Schistosoma that lives inside blood vessels - a serious medical problem.

body cavities
Body cavities

None of the phyla mentioned so far have any internal fluid-filled body cavities. In fact most animal phyla do – these turn out to be highly important for making sense of phyla.

Bilateral symmetry

3 cell layers in embryo

No body cavity


Phylum platyhelminths

(and the closely related phylum nemertini, bootlace worms.)

Has body cavity

Lined with cells

Coelomate phyla

Not lined with cells

Pseudocoelomate phyla

pseudocoelomates especially phylum nematoda the roundworms
Pseudocoelomates, especially phylum nematoda, the roundworms

There are quite a few rather obscure phyla here, mainly of tiny (<2mm) and unfamiliar creatures that live in the water between grains of sand, in sediments etc – Phyla rotifera, gastrotricha and others (look up “minor pseudocoelomate phyla”). There is only one of these phyla that is really significant in terms of species richness.

These are the roundworms, phylum nematoda.


Phylum nematoda – the roundworms

Nematodes: Almost all have the same body shape - round, pointy at both ends. (A very few plant parasitic species look like balloons, being immobile and full of eggs).

All have a thick collagen body wall retaining a high internal hydrostatic pressure - they are almost impossible to squash under normal circumstances.

Most of you here will have been infected with nematodes. Luckily the commonest nematode in humans is tiny and harmless - the pinworm Enterobius vermicularis.

Nematode eggs are very tough (collagen wall again) and stay viable for months or years.

the big 5 coelomate phyla
The big 5 coelomate phyla

There are about 10 phyla in which the basic body design involves a body cavity lined with cells (called a coelom), but of these I will only cover 4 today – these are the important common ones. One grouping is probably 3 distantly related phyla.

Phylum annelida – the segmented worms

Phylum mollusca: snails and allies

Phylum echinodermata – starfish and allies

Phylum (superphylum?) arthropoda – insects, spiders and crustaceans.

Phylum chordata – everything with a backbone (including us)

phylum annelida the segmented worms
Phylum Annelida – the segmented worms.

The most familiar of these is the common earthworm, Lumbricus terrestris.

(In fact, ecologically, this is one of the oddest annelids!) 

All have true metameric segmentation, with each segment carrying gut, musculature and part of the nerve cord. There is often some differentiation of segments, ie the collar (clitellum) of earthworms.

Lumbricid showing clitellum


The classes are:

Class chaetopoda - annelids with chaetae:

order Polychaetes - marine worms, often very spiky with chaetae on lateral projections called parapodia (These are the great majority of annelids).

order oligochaeta - freshwater / terrestrial, small chaetae, earthworms etc.

Class hirudine - leeches; predators / ectoparasites with anterior + posterior suckers

phylum mollusca snails and allies
Phylum Mollusca – snails and allies

These have a soft, mucus-covered body with a muscular foot, often with a calcareous shell.

Class Lamellibranchs (=Bivalves) - aquatic filter feeders, using their gills to capture suspended food particles.


Phylum Mollusca – snails and allies

Class gastropoda - limpets, slugs and snails. Originally marine grazers, have emerged to become major terrestrial herbivores.


Phylum Mollusca – snails and allies

Class Cephalopoda - octopuses, squids, ammonites, nautilus (ie common octopus; Octopus vulgaris). Very different to other molluscs, with the muscular foot becoming 8-10 tentacles for food capture. They have independently evolved an eye almost identical to vertebrates, and seem to be the most highly intelligent invertebrates. They also include the largest invertebrates - a giant squid can be >5m long, with another 10m of tentacles.

phylum echinodermata starfish and allies
Phylum Echinodermata – starfish and allies

All have an unexplained pentagonal symmetry, and a calcite exoskeleton supporting a complex system of tube feet used for slow locomotion. Any fossil – if it is pentagonal, it’s an echinoderm!


Asteroidea - starfish

Echinoidea - sea urchins

Ophiuroidea - brittle stars

Holothuridae - sea cucumbers

Crinoidea - feather stars

phylum arthropoda insects spiders and crustaceans
phylum Arthropoda – insects, spiders and crustaceans

This is the biggest phylum in existence.

All these animals have a hard external skeleton and jointed legs. (‘Arthropod’ means jointed foot or limb). For many years these were treated as one huge phylum with three clear subphyla. More recently various lines of work, notably DNA analyses, suggest that the differences in these 3 subphyla are so great that they probably evolved the ‘armoured’ body form independently, and should be seen as 3 distinct phyla.

Note that there is continuing disagreement about whether arthropods are a phylum or a super-phylum. The recent merge with nematodes into ecdysozoa strengthens the case for ‘phylum’.


phylum Arthropoda

(all have exoskeleton)

Sub-phylum Crustacea

Mouthparts are mandibles, 2 pairs antennae.

Crabs, shrimps, lobsters, woodlice etc.

All have calcified cuticle.

Sub-phylum Chelicerata

Mouthparts are claw-like (chelicera), no antennae.

Spiders, mites, and horseshoe crabs.

Sub-phylum Mandibulata

Mouthparts are mandibles, 1 pair antennae.

Insects, millepedes, centipedes etc

Insects have 3 pairs of legs

our phylum the chordates
Our phylum – the chordates

All chordates have a dorsal nerve cord running along the body. There is an anterior swelling (‘brain’), and segmentalised body with segmented blocks of muscle. Unlike the arthropods and molluscs the brain does not encircle the gut – happens to be a good design for large body sizes.

Most chordates have bones along their nerve cord, making them vertebrates. Not all – some of our phylum are invertebrates!

A hagfish – a boneless chordate, here tying

A knot in itself.


Our phylum – the chordates

Sea squirts (subphylum urochordates) have a larval form that is built much like a tadpole, barring a lack of bone, and are clearly from the chordate mould. But the adults forsake this for a sedentary life filtering sea water through a mucus net. There are a few other less well known invertebrate chordates.

Adult sea squirt – Ciona intestinalis

Larval tunicate, showing same notochord anatomy as mouse


The bony animals divide neatly into 5 classes, all of which you will recognise:

Pisces (fishes)

Amphibia – frogs newts etc (smooth skin)

Reptiles – lizards etc (scales)

Birds (feathers)

Mammals (us, whales and everything else warm and furry)

Inevitably, the harder one looks at the fossil record, the less clear-cut these boundaries become!