Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings An introduction to the diversity of animal life

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

An introduction to the diversity of animal life

Aim for today

To introduce you to several characteristics found in animals and 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. This framework has a hierarchical structure (meaning it can be

shown as a dendrogram) founded in taxonomy.

Dendrogram Taxonomy – the study of 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:AnimalsPlantsFungi

ProkaryotesEubacteriaArchaebacteria(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.

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 - animaliaPhylum - mandibulataClass - InsectaOrder - CollembolaFamily - EntomobryidaeGenus EntomobryaSpecies Entomobrya nivalis

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

How to write a scientific name!

Homo sapiens OR Homo sapiens

When writing by hand underline the name.

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

On a PC make the font italic


• Concept 32.1: Animal are multicellular, heterotrophic eukaryotes with tissues that develop from embryonic layers

• Several characteristics of animals

– Sufficiently define the group


• Their bodies are held together

– By structural proteins such as collagen

• Nervous tissue and muscle tissue

– Are unique to animals


Reproduction and Development

• Most animals reproduce sexually

– With the diploid stage usually dominating the life cycle


• After a sperm fertilizes an egg

– The zygote undergoes cleavage, leading to the formation of a blastula

• The blastula undergoes gastrulation

– Resulting in the formation of embryonic tissue layers and a gastrula


Zygote

Cleavage

Eight-cell stage

Cleavage

Blastula Cross section of blastula

Blastocoel

Blastocoel

Gastrula Gastrulation

Endoderm

Ectoderm

Blastopore

• Early embryonic development in animals

Figure 32.2

In most animals, cleavage results in theformation of a multicellular stage called a blastula.The blastula of many animals is a hollow ball of cells.

3

The endoderm ofthe archenteron de-

velops into the tissuelining the animal’s

digestive tract.

6

The blind pouchformed by gastru-

lation, calledthe archenteron,

opens to the outsidevia the blastopore.

5

Most animals also undergo gastrulation, a rearrangement of the embryo in which one end of the embryo folds inward, expands, and eventually fills the blastocoel, producing layers of embryonic tissues: the ectoderm (outer layer) and the endoderm (inner layer).

4

Only one cleavagestage–the eight-cellembryo–is shown here.

2 The zygote of an animal undergoes a succession of mitotic cell divisions called cleavage.

1


• All animals, and only animals

– Have Hox genes that regulate the development of body form

• Although the Hox family of genes has been highly conserved

– It can produce a wide diversity of animal morphology

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 ciliaflagellates (Euglena, Trypanosoma) – one big cilium (flagellum)Rhizopoda (Amoeba proteus) – no cilia + a less well known class of parasitic species: Sporozoa (Plasmodium vivax)

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

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.

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.

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.

1 cell - Protozoa

No clear tissues: parazoa

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

Tissues: metazoa

Many cells

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

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

1 cell - Protozoa

No clear tissues: parazoa Tissues: metazoa

Radial symmetry2 cell layers in embryaPhyla cnidaria and ctenophora

Bilateral symmetry3 cell layers in embryoRemaining animal Phyla

Many cells

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).

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.

<1mm deep

Combined mouth and anus, leading into gut

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 symmetry3 cell layers in embryo

No body cavityFlatwormsPhylum platyhelminths(and the closely related phylum nemertini, bootlace worms.)

Has body cavity

Lined with cellsCoelomate phyla

Not lined with cellsPseudocoelomate phyla

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.

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.

Phylum nematoda – the roundworms

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.

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 chaetaeorder 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

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

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! ClassesAsteroidea - starfishEchinoidea - sea urchinsOphiuroidea - brittle starsHolothuridae - sea cucumbersCrinoidea - feather stars Starfish are predators, echinoids are herbivores, holothuridae are detritivores, the remainder filter feeders.

Superphylum 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.

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

Superphylum Arthropoda(all have exoskeleton)

Phylum MandibulataMouthparts are mandibles, 1 pair antennae.Insects, millepedes, centipedes etcInsects have 3 pairs of legs

Phylum ChelicerataMouthparts are claw-like (chelicera), no antennae.Spiders, mites, and horseshoe crabs.

Phylum CrustaceaMouthparts are mandibles, 2 pairs antennae.Crabs, shrimps, lobsters, woodlice etc.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!

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

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!

• Concept 32.2: The history of animals may span more than a billion years

• The animal kingdom includes not only great diversity of living species

– But the even greater diversity of extinct ones as well

• The common ancestor of living animals

– May have lived 1.2 billion–800 million years ago

– May have resembled modern choanoflagellates, protists that are the closest living relatives of animals

Figure 32.3

Single cell

Stalk

– Was probably itself a colonial, flagellated protist

Figure 32.4

Colonial protist,an aggregate ofidentical cells

Hollow sphere of unspecialized cells (shown in cross section)

Beginning of cell specialization

Infolding Gastrula-like “protoanimal”

Somatic cells Digestivecavity

Reproductive cells

• Concept 32.3: Animals can be characterized by “body plans”

• One way in which zoologists categorize the diversity of animals

– Is according to general features of morphology and development

• A group of animal species

– That share the same level of organizational complexity is known as a grade

• The set of morphological and developmental traits that define a grade

– Are generally integrated into a functional whole referred to as a body plan

Symmetry

• Animals can be categorized

– According to the symmetry of their bodies, or lack of it

• Some animals have radial symmetry

– Like in a flower pot

Figure 32.7a

Radial symmetry. The parts of a radial animal, such as a sea anemone (phylum Cnidaria), radiate from the center. Any imaginary slice through the central axis divides the animal into mirror images.

(a)

• Some animals exhibit bilateral symmetry

– Or two-sided symmetry

Figure 32.7b

Bilateral symmetry. A bilateral animal, such as a lobster (phylum Arthropoda), has a left side and a right side. Only one imaginary cut divides the animal into mirror-image halves.

(b)

• Bilaterally symmetrical animals have

– A dorsal (top) side and a ventral (bottom) side

– A right and left side

– Anterior (head) and posterior (tail) ends

– Cephalization, the development of a head

Tissues

• Animal body plans

– Also vary according to the organization of the animal’s tissues

• Tissues

– Are collections of specialized cells isolated from other tissues by membranous layers

• Animal embryos

– Form germ layers, embryonic tissues, including ectoderm, endoderm, and mesoderm

• Diploblastic animals

– Have two germ layers

• Triploblastic animals

– Have three germ layers

Body Cavities

• In triploblastic animals

– A body cavity may be present or absent

• A true body cavity

– Is called a coelom and is derived from mesoderm

Figure 32.8a

Coelom

Body covering(from ectoderm)

Digestive tract(from endoderm)

Tissue layerlining coelomand suspendinginternal organs(from mesoderm)

Coelomate. Coelomates such as annelids have a true coelom, a body cavity completely lined by tissue derived from mesoderm.

(a)

• A pseudocoelom

– Is a body cavity derived from the blastocoel, rather than from mesoderm

Figure 32.8b

PseudocoelomMuscle layer(from mesoderm)

Body covering(from ectoderm)

Digestive tract(from ectoderm)

Pseudocoelomate. Pseudocoelomates such as nematodes have a body cavity only partially lined by tissue derived from mesoderm.

(b)

• Organisms without body cavities

– Are considered acoelomates

Figure 32.8c

Body covering(from ectoderm) Tissue-

filled region(from mesoderm)

Digestive tract(from endoderm)

Acoelomate. Acoelomates such as flatworms lack a body cavity between the digestive tract and outer body wall.

(c)

Protostome and Deuterostome Development

• 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

• In protostome development

– Cleavage is spiral and determinate

• In deuterostome development

– Cleavage is radial and indeterminate

Figure 32.9a

Protostome development(examples: molluscs, annelids,

arthropods)

Deuterostome development(examples: echinoderms,

chordates)

Eight-cell stage Eight-cell stage

Spiral and determinate Radial and indeterminate

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

Coelom Formation


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


– Formation of the body cavity is described as enterocoelous development

Figure 32.9b

Archenteron

Blastopore MesodermCoelom

BlastoporeMesoderm

Schizocoelous: solidmasses of mesodermsplit and form coelom

Enterocoelous:folds of archenteronform coelom

Coelom (b) Coelom formation. 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).

Fate of the Blastopore


– The blastopore becomes the mouth


– The blastopore becomes the anus

Figure 32.9c

Anus

Anus

Mouth

Mouth

Mouth developsfrom blastopore

Anus developsfrom blastopore

Digestive tube

• Concept 32.4: Leading hypotheses agree on major features of the animal phylogenetic tree

• Zoologists currently recognize about 35 animal phyla

• The current debate in animal systematics

– Has led to the development of two phylogenetic hypotheses, but others exist as well

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

Figure 32.10

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“Radiata” Deuterostomia Protostomia

Bilateria

Eumetazoa

Metazoa

Ancestral colonialflagellate

• One hypothesis of animal phylogeny based mainly on molecular data

Figure 32.11

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“Radiata”

“Porifera” Deuterostomia Lophotrochozoa Ecdysozoa

Bilateria

Eumetazoa

Metazoa

Ancestral colonialflagellate

Points of Agreement

• All animals share a common ancestor

• Sponges are basal animals

• Eumetazoa is a clade of animals with true tissues

• Most animal phyla belong to the clade Bilateria

• Vertebrates and some other phyla belong to the clade Deuterostomia

Disagreement over the Bilaterians

• 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

• Ecdysozoans share a common characteristic

– They shed their exoskeletons through a process called ecdysis

Figure 32.12

• Lophotrochozoans share a common characteristic

– Called the lophophore, a feeding structure

• Other phyla

– Go through a distinct larval stage called a trochophore larva

Figure 32.13a, b

Apical tuftof cilia

Mouth

Anus(a) An ectoproct, a lophophorate (b) Structure of trochophore larva


Chapter 33 Invertebrates- sponges

• Overview: Life Without a Backbone

• Invertebrates

– Are animals that lack a backbone

– Account for 95% of known animal species


• A review of animal phylogeny

Ancestral colonialchoanoflagellate

Eumetazoa

Bilateria

Deuterostomia

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Figure 33.2


• Exploring invertebrate diversity

PORIFERA (5,500 species)

A sponge

CNIDARIA (10,000 species)

A jelly

PLACOZOA (1 species) KINORHYNCHA (150 species)0.5 mm

A placozoan (LM) A kinorhynch (LM)250 µm

PLATYHELMINTHES (20,000 species) ROTIFERA (1,800 species)

A marine flatworm A rotifer (LM)

ECTOPROCTA (4,500 species) PHORONIDA (20 species)

Ectoprocts PhoronidsFigure 33.3


• Exploring invertebrate diversityBRACHIOPODA (335 species) NEMERTEA (900 species)

A brachiopod A ribbon wormACANTHOCEPHALA (1,100 species) CTENOPHORA (100 species)

An acanthocephalan A ctenophore, or comb jelly

MOLLUSCA (93,000 species) ANNELIDA (16,500 species)

An octopus A marine annelidLORICIFERA (10 species) PRIAPULA (16 species)

5 mm

50 µm

A loriciferan (LM) A priapulanFigure 33.3


• Exploring invertebrate diversityNEMATODA (25,000 species) ARTHROPODA (1,000,000 + species)

A roundworm A scorpion (an arachnid)

CYCLIOPHORA (1 species) TARDIGRADA (800 species)100 µm

100 µm

A cycliophoran (colorized SEM) Tardigrades (colorized SEM)

ONYCHOPHORA (110 species) HEMICHORDATA (85 species)

An onychophoran An acorn worm

ECHINODERMATA (7,000 species) CHORDATA (52,000 species)

A sea urchin A tunicateFigure 33.3


• Sponges are sessile and have a porous body and choanocytes

• Sponges, phylum Porifera

– Live in both fresh and marine waters

– Lack true tissues and organs


• Sponges are suspension feeders

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

Azure vase sponge (Callyspongia plicifera)

Osculum

Spicules

Waterflow

Flagellum

CollarFood particlesin mucus

Choanocyte

Phagocytosis offood particles Amoebocyte

Choanocytes. The spongocoel is lined with feeding cells called choanocytes. By beating flagella, the choanocytes create a current that draws water in through the porocytes.

Spongocoel. Water passing through porocytes

enters a cavity called the spongocoel.

Porocytes. Water enters the epidermis through

channels formed by porocytes, doughnut-shaped cells that span the body wall.

Epidermis. The outer layer consists of tightly

packed epidermal cells.

Mesohyl. The wall of this simple sponge consists of

two layers of cells separatedby a gelatinous matrix, themesohyl (“middle matter”).

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.

Amoebocyte. Amoebocytes transport nutrients to other cells ofthe sponge body and also produce materials for skeletal fibers (spicules).

5

6

7

4

3

2

1

Figure 33.4


• Choanocytes, flagellated collar cells

– 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


• Concept 33.2: Cnidarians have radial symmetry, a gastrovascular cavity, and cnidocytes

• All animals except sponges

– Belong to the clade Eumetazoa, the animals with true tissues

• Phylum Cnidaria

– Is one of the oldest groups in this clade

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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings An introduction to the diversity of animal life