The Metazoa(Differentiated Multicellular Animals)

An Overview of the Major Groups

How Do We Classify Life ?

Note: Kingdoms Eubacteria and Archaea formerly classified as Kingdom Monera

Three Domains

Archaea

Eukarya

Bacteria

Now Three Domains and 6 Kingdoms

Conventional Classification of Major Phyla

Protista amoebas, foraminifera, radiolaria(presumed ancestors of Metazoa)

MetazoaPorifera spongesCnidaria/Coelenterata sea anemones, corals, sea pens, etc.Several “worm” phyla flatworms, annelids, etc.Brachiopoda brachiopods (“lamp shells”)Bryozoa bryozoans (“moss animals”)Mollusca snails, clams, cephalopods, etc.Arthropoda shrimps, crabs, insects, spiders, etc.Echinodermata starfish, sea urchins, crinoids, etc.Hemichordata acorn worms, graptolites, etc.Chordata lancelet, vertebrates, etc.

Pound QuartziteEdiacara Hills, north of Sydney, Australia

Metazoan Fossils Found in 1946

Discovery of classic “Ediacaran Fauna”(about 543 Ma)

Classification: lumped together as “medusoids”

Dickinsonia

Mawsonites Spriggina

Details originally overlooked:A diverse assemblages of fossils

Mistaken Point, Newfoundland565 Ma

“spindles”

Portugal Cove NewfoundlandUp to about 575 Ma

Charniodiscus (fronds)

Latest Proterozoic (Ediacaran Period)Oxygenated atmosphere and seas

Complex, soft-bodied metazoa

Ostrich plume hydroid

NudibranchSea Pen

Were Vendian organismsEarly representatives of modern phyla ?

Adolph SeilacherConcept of “Vendozoa:

(Extinct phylum ? Probably not.)

soft bodied“quilted” structure (fluid-filled bags ?”Dependent on microbial mats“mat stickers”: fixed to seafloor, photosynthesizers“mat scratchers”: grazed on microbial mats

No carnivores !

Single celled Protista

Amoebas, Foraminifera, Radiolaria, etc.

How do You Make a Metazoan ?You Have to Start Out Simple.

Phylum Protista: the importance of choanoflagellates

A choanoflagellate is a protist with a collared cell and a flagellum

Some choanoflagellates form colonies

In such colonies, all individuals cooperate in moving their flagella, generating a current from which food particles can be extracted

On to the Metazoa…

Phylum Porifera (Sponges)Most Basic Metazoan Plan of Cowen

Single layer of tissue(collared cells)

Sponges also have collared cells, but these form a larger, integrated structure supported by rigid spicules or organic tissue. The differentiation of cells required the evolution of Hox Genes (genes that dictate differing

functions of cells)

Similar to some of the Ediacaran animals (remember the frond-like

creatures), sponges show a fractal organization

Ascon-grade sponge

Sycon-grade sponge(contains multiple “ascon” elements)

Leucon-grade sponge(contains multiple “sycon” elements)

Phylum Cnidaria / Coelenterata(Second Metazoan Body Plan of Cowen)

2 layers of tissue:ectoderm, endoderm

(probably resulted from invagination of ectoderm)

Phylum Cnidaria / Coelenterata

hard coralsHydra sea pens

soft corals

2 tissue layers: ectoderm, endoderm

sea anemonesjellyfish

Again, in the more complex forms of these simple organismsfractal geometry is apparent

Natural coral Computer-generatedfractal

triploblastic - 3 principal cell layers ectoderm, mesoderm, endoderm

Basic bilateral symmetry: fractal geometry breaks down, but tissue differentiation is incredible !

“Worms” or “Bilaterans”Most Complex Metazoan Body Plan of Cowen

The Coelom

The Ectoderm and Endoderm can be viewed as essentially solid, continuous layers.The Mesoderm is a little more complicated in that it actually lines a fluid-filled body cavity called the coelom. It is within the coelom that internal organs other than the gut develop (e.g. respiratory organs)

Coelom and Orifice Development;Protostomes

In the Protostomes (including molluscs, annelid worms and arthropods), the coelom develops directly from mesodermal tissue.

Another distinguishing characteristic to the protostomes is the development of the mouth before the anus in the young embryo

In the Deuterostomes (including echinoderms and chordates), the coelom develops from outpockets of the gut (endoderm)

Another distinguishing characteristic to the protostomes is the development of the anus before the mouth in the young embryo (blastophore)

Coelom and Orifice Development;Deuterostomes

The Evolution of the Coelom

The coelom may have initially evolved as a hydraulic device.

A bilateran with a coelom can squeeze its internal fluids with body muscles.

This squeezing bulges the body wall at the weakest point, and can be used as a “power drill” for burrowing (think about how a worm gets around).

The Evolution of the Coelom

In addition, this pumping could facilitate the transport of oxygen through the body without relying on the bathing of tissues in oxygenated water by diffusion through a thin ectoderm.

This means that animals could efficiently deliver oxygen throughout their bodies without compromising the effectiveness of their outer skins (ectoderm) or size.

This also meant that animals could evolve exoskeletons.

The Protostomes can be subdivided in two smaller groups(clades):

1. Lophotrochozoa2. Ecdysozoa

Lophotrochozoa: This group gets its confusing name from two related subgroups (linked by molecular phylogenetic studies):

1. The trochozoa - animals with distinctive, fuzzy, trochophore larvae, which include the phyla Platyhelminthes and the Mollusca.

2. The lophophora – animals which feed via a fringe of hollow tentacles, called a lophophore), which include the phyla Brachiopoda and Bryozoa.

trochophore larva

Lophophore (in brachiopod)

Important Lophotrochozoans

Flatworms do not have a coelom, and it is likely that something like a flatworm gave rise to more advanced coelomate bilaterans.

Lophotrochozoa: Phylum Platyhelminthes (flatworms)

Each class derived from HAM(hypothetical ancestral mollusc)

Key Features:gut

mantle cavityradula (rasping organ)

gillsfoot

Lophotrochozoa: Phylum Mollusca

Lophotrochozoa: Phylum Mollusca

Gastropods Bivalves

Cephalopods (squids, octopuses, cuttlefish, ammonoids)

Lophotrochozoa: Phylum Brachiopoda (“arm foot”)

Key Features: pedicle, gut, muscles, lophophore

Lophotrochozoa: Phylum Bryozoa (“moss animals”)

Key Features:colonial habit,lophophore

Ecdysozoa: This group includes animals that moult their outer covering as they grow. Phylum Arthropoda is the primary phylum of this group.

Important Ecdysozoa

Ecdysozoa: Phylum Arthropoda

scorpions

trilobites

InsectsSpidersCrabs

LobstersBarnacles

Etc.

Key Features:Jointed appendages

3-fold division of body(head, thorax, abdomen)

eurypterids

shrimps

Important Deuterostomes (Deuterostomia)

Deuterostomia: Phylum Echinodermata (“spiny skin”)

Sea urchins Brittlestars

Starfish

Crinoids

Sea cucumbers

Key Features: 5-fold symmetry, calcite plates(but embryos are bilateral, suggesting a

bilateral ancestor)

Deuterostomia: Phylum Hemichordata

Pterobranchs

Acorn wormsGraptolites

Key Characteristics:3-part division of body

(preoral lobe, collar, trunk)PharynxGill slits

Stomochord

Deuterostomia: Phylum Chordata

Sea squirts and salps (Urochordates)

Amphioxus (lancelet)(Cephalochordates)

Key Features:notochord

dorsal nerve cord, pharynxgills slits

post-anal tail

END OF LECTURE