An introduction to the
diversity of animal life
Peter Shaw
RU
This is the jpg-free version to save space
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
Dendrogram
the classification of life
forms.
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:
Eukaryotes:
Animals
Plants
Fungi
Prokaryotes
Eubacteria
Archaebacteria
(Viruses would count as a 6th, if
you regard them as alive).
Phyla
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
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!
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”.
1st name has a capital letter, 2nd does not
Homo sapiens OR Homo sapiens
On a PC make the font italic When writing by hand
underline the name.
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
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
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
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)
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
Medusoid
Budding
phase
A cnidocyte
This phylum feeds by capturing planktonic (=nematocyst)
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 Bilateral symmetry
2 cell layers in embrya 3 cell layers in embryo
Phyla cnidaria and ctenophora Remaining animal Phyla
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).
5m
long, with another 10m of tentacles.
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!
Classes
Asteroidea - starfish
Echinoidea - sea urchins
Ophiuroidea - brittle stars
Holothuridae - sea cucumbers
Crinoidea - feather stars
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 Sub-phylum Sub-phylum
Mandibulata Chelicerata Crustacea
Mouthparts are Mouthparts are Mouthparts are
mandibles, 1 pair claw-like mandibles, 2 pairs
antennae. (chelicera), no antennae.
Insects, millepedes, antennae. Crabs, shrimps,
centipedes etc Spiders, mites, and lobsters, woodlice
Insects have 3 pairs horseshoe crabs. etc.
of legs All have calcified
cuticle.
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
Vertebrates
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!