An introduction to the diversity of animal life
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An introduction to the diversity of animal life. 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.

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One cell or many l.jpg
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 l.jpg
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)

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Ciliates are covered in hundreds of tiny motile hairs = cilia (sing. cilium). Are common in freshwater, also benign gut inhabitants.

flagellates move by a small number of long motile hairs = flagellae (sing. flagellum). Free living, also rumen flora and some gut parasites.

Rhizopoda free living in sediments etc, moving by slow protrusion of pseudopodia. A few are nasty parasites (Entamoeba dysenterica, Naegleria spp.)

Sporozoa (Plasmodium vivax causes malaria, the biggest killer in human history)

New version kingdom protozoa l.jpg
New version – kingdom Protozoa cilia (sing. cilium). Are common in freshwater, also benign gut inhabitants.

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 l.jpg
Sponges – Phylum parazoa cilia (sing. cilium). Are common in freshwater, also benign gut inhabitants.

 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.

They 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.

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Metazoa: cilia (sing. cilium). Are common in freshwater, also benign gut inhabitants. 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.

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Bilateria: cilia (sing. cilium). Are common in freshwater, also benign gut inhabitants.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

The big 5 coelomate phyla l.jpg
The big 5 coelomate phyla cilia (sing. cilium). Are common in freshwater, also benign gut inhabitants.

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)

Protostome and deuterostome development l.jpg
Protostome and Deuterostome Development cilia (sing. cilium). Are common in freshwater, also benign gut inhabitants.

  • Based on certain features seen in early development

    • Many animals can be categorized as having one of two developmental modes: protostome development or deuterostome development

Cleavage l.jpg

Deuterostome development cilia (sing. cilium). Are common in freshwater, also benign gut inhabitants.

(examples: echinoderms,


Protostome development

(examples: molluscs, annelids,


(a) Cleavage. In general, protostomedevelopment begins with spiral, determinate cleavage.Deuterostome development is characterized by radial, indeterminate cleavage.

Eight-cell stage

Eight-cell stage

Spiral and determinate

Radial and indeterminate


  • In protostome development

    • Cleavage is spiral and determinate

  • In deuterostome development

    • Cleavage is radial and indeterminate

Figure 32.9a

Coelom formation l.jpg

(b) Coelom formation. cilia (sing. cilium). Are common in freshwater, also benign gut inhabitants. Coelom formation begins in the gastrula stage. In protostome development, the coelom forms from splits in the mesoderm (schizocoelous development). In deuterostome development, the coelom forms from mesodermal outpocketings of the archenteron (enterocoelous development).









folds of archenteron

form coelom

Schizocoelous: solid

masses of mesoderm

split and form coelom

Figure 32.9b

Coelom Formation

  • In protostome development

    • The splitting of the initially solid masses of mesoderm to form the coelomic cavity is called schizocoelous development

  • In deuterostome development

    • Formation of the body cavity is described as enterocoelous development

Fate of the blastopore l.jpg

Mouth cilia (sing. cilium). Are common in freshwater, also benign gut inhabitants.


Digestive tube



Mouth develops

from blastopore

Anus develops

from blastopore

Figure 32.9c

Fate of the Blastopore

  • In protostome development

    • The blastopore becomes the mouth

  • In deuterostome development

    • The blastopore becomes the anus

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Rotifera phylogenetic tree





















Ancestral colonial


  • One hypothesis of animal phylogeny based mainly on morphological and developmental comparisons

Figure 32.10

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Cnidaria phylogenetic tree
























Ancestral colonial


  • One hypothesis of animal phylogeny based mainly on molecular data

Figure 32.11

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Points of Agreement phylogenetic tree

  • All animals share a common ancestor

  • Sponges are basal animals

  • Eumetazoa is a clade of animals with true tissues

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Disagreement over the Bilaterians phylogenetic tree

  • The morphology-based tree

    • Divides the bilaterians into two clades: deuterostomes and protostomes

  • In contrast, several recent molecular studies

    • Generally assign two sister taxa to the protostomes rather than one: the ecdysozoans and the lophotrochozoans

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Figure 32.12

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Apical tuft phylogenetic tree

of cilia


An ectoproct, a lophophorate



Structure of trochophore larva

Figure 32.13a, b


  • Lophotrochozoans share a common characteristic

    • Called the lophophore, a feeding structure

  • Other phyla

    • Go through a distinct larval stage called a trochophore larva

Chapter 33 invertebrates sponges l.jpg
Chapter 33 Invertebrates- sponges phylogenetic tree

  • Overview: Life Without a Backbone

  • Invertebrates

    • Are animals that lack a backbone

    • Account for 95% of known animal species

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Porifera phylogenetic tree




Other bilaterians (including

Nematoda, Arthropoda,

Mollusca, and Annelida)




Ancestral colonial


Figure 33.2

  • A review of animal phylogeny

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CNIDARIA (10,000 species) phylogenetic tree

PORIFERA (5,500 species)

A sponge

A jelly

PLACOZOA (1 species)

KINORHYNCHA (150 species)

0.5 mm

250 µm

A placozoan (LM)

A kinorhynch (LM)

ROTIFERA (1,800 species)

PLATYHELMINTHES (20,000 species)

A marine flatworm

A rotifer (LM)

PHORONIDA (20 species)

ECTOPROCTA (4,500 species)



  • Exploring invertebrate diversity

Figure 33.3

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BRACHIOPODA (335 species) phylogenetic tree

NEMERTEA (900 species)

A brachiopod

A ribbon worm

ACANTHOCEPHALA (1,100 species)

CTENOPHORA (100 species)

5 mm

An acanthocephalan

A ctenophore, or comb jelly

MOLLUSCA (93,000 species)

ANNELIDA (16,500 species)

An octopus

A marine annelid

LORICIFERA (10 species)

PRIAPULA (16 species)

50 µm

A priapulan

A loriciferan (LM)

Figure 33.3

  • Exploring invertebrate diversity

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ARTHROPODA (1,000,000 + species) phylogenetic tree

NEMATODA (25,000 species)

A roundworm

A scorpion (an arachnid)

CYCLIOPHORA (1 species)

TARDIGRADA (800 species)

100 µm

100 µm

A cycliophoran (colorized SEM)

Tardigrades (colorized SEM)

HEMICHORDATA (85 species)

ONYCHOPHORA (110 species)

An onychophoran

An acorn worm

ECHINODERMATA (7,000 species)

CHORDATA (52,000 species)

Figure 33.3

A sea urchin

A tunicate

  • Exploring invertebrate diversity

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Choanocytes. phylogenetic tree The spongocoel

is lined with feeding cells called

choanocytes. By beating flagella,

the choanocytes create a current that

draws water in through the porocytes.



Food particles

in mucus



Azure vase sponge (Callyspongia



Spongocoel. Water

passing through porocytes

enters a cavity called the



Phagocytosis of

food particles


Porocytes. Water enters

the epidermis through

channels formed by

porocytes, doughnut-shaped

cells that span the body wall.


The movement of the choanocyte

flagella also draws water through its

collar of fingerlike projections. Food

particles are trapped in the mucus

coating the projections, engulfed by

phagocytosis, and either digested or

transferred to amoebocytes.



Epidermis. The outer

layer consists of tightly

packed epidermal cells.




Amoebocyte. Amoebocytes

transport nutrients to other cells of

the sponge body and also produce

materials for skeletal fibers (spicules).


Mesohyl. The wall of this

simple sponge consists of

two layers of cells separated

by a gelatinous matrix, the

mesohyl (“middle matter”).


  • Sponges are suspension feeders

    • Capturing food particles suspended in the water that passes through their body

Figure 33.4

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  • Choanocytes, flagellated collar cells phylogenetic tree

    • Generate a water current through the sponge and ingest suspended food

  • Most sponges are hermaphrodites

    • Meaning that each individual functions as both male and female

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Animal diversity and cnidocytes


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Phylum Cnidaria and cnidocytes(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. Both feed 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).

The main classes are:

Scyphozoa = jellyfish, Aurelia aurita in the common UK moon jelly (harmless to humans)

Anthozoa: sessile forms: sea anemones, corals, sea fans

Hydrozoa: various medusoid radiations, often with several body forms fused into one animal ie Physalia physalis, the infamous, portugese man o’war (avoid!).

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Phylum Platyhelminths and cnidocytes

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.

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Body cavities and cnidocytes

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

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Pseudocoelomates, especially phylum and cnidocytesnematoda, 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.

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Phylum nematoda – the roundworms and cnidocytes

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.

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Phylum Annelida – and cnidocytesthe 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.

The classes are:

Class chaetopoda - annelids with chaetae

order Polychaetes - marine worms, often very spiky with chaetae on lateral projections called parapodia (Beware: divers do not touch)

order oligochaeta - freshwater / terrestrial, small chaetae

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

Phylum mollusca snails and allies l.jpg
Phylum Mollusca – snails and allies and cnidocytes

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

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

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

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 l.jpg
Phylum Echinodermata – starfish and allies and cnidocytes

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

Starfish are predators, echinoids are herbivores, holothuridae are detritivores, the remainder filter feeders.

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Superphylum Arthropoda – insects, spiders and crustaceans and cnidocytes

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.

Forgive me if I still use the term ‘Arthropod’! It may yet come back, and if it doesn’t it remains a handy abbreviation.

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Superphylum Arthropoda and cnidocytes

(all have exoskeleton)

Phylum Crustacea

Mouthparts are mandibles, 2 pairs antennae.

Crabs, shrimps, lobsters, woodlice etc.

All have calcified cuticle.

Phylum Chelicerata

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

Spiders, mites, and horseshoe crabs.

Phylum Mandibulata

Mouthparts are mandibles, 1 pair antennae.

Insects, millepedes, centipedes etc

Insects have 3 pairs of legs

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Our phylum – the chordates and cnidocytes

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!

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.

Vertebrates l.jpg
Vertebrates and cnidocytes

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!