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)

Metazoa

Porifera Cnidaria/Coelenterata Several “worm” phyla Brachiopoda Bryozoa Mollusca Arthropoda Echinodermata Hemichordata Chordata sponges sea anemones, corals, sea pens, etc.

flatworms, annelids, etc.

brachiopods (“lamp shells”) bryozoans (“moss animals”) snails, clams, cephalopods, etc.

shrimps, crabs, insects, spiders, etc.

starfish, sea urchins, crinoids, etc.

acorn worms, graptolites, etc.

lancelet, vertebrates, etc.

Discovery of classic “Ediacaran Fauna” (about 543 Ma) Pound Quartzite Ediacara Hills, north of Sydney, Australia Metazoan Fossils Found in 1946 Classification: lumped together as “medusoids”

Details originally overlooked: A diverse assemblages of fossils Mawsonites Spriggina Dickinsonia

Mistaken Point, Newfoundland 565 Ma “spindles”

Portugal Cove Newfoundland Up to about 575 Ma

Charniodiscus (fronds)

Latest Proterozoic (Ediacaran Period) Oxygenated atmosphere and seas Complex, soft-bodied metazoa

Were Vendian organisms Early representatives of modern phyla ?

Sea Pen Nudibranch Ostrich plume hydroid

Adolph Seilacher Concept 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 !

How do You Make a Metazoan ?

You Have to Start Out Simple.

Single celled Protista Amoebas, Foraminifera, Radiolaria, etc.

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

Hydra

hard corals sea pens soft corals jellyfish sea anemones 2 tissue layers: ectoderm, endoderm

Natural coral Computer-generated fractal Again, in the more complex forms of these simple organisms fractal geometry is apparent

“Worms” or “Bilaterans” Most Complex Metazoan Body Plan of Cowen triploblastic - 3 principal cell layers ectoderm, mesoderm, endoderm Basic bilateral symmetry: fractal geometry breaks down, but tissue differentiation is incredible !

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

Coelom and Orifice Development; Deuterostomes 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)

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. Lophotrochozoa 2. 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

Lophotrochozoa: Phylum Platyhelminthes (flatworms) Flatworms do not have a coelom, and it is likely that

something like

a flatworm gave rise to more advanced coelomate bilaterans.

Lophotrochozoa: Phylum Mollusca Each class derived from HAM (hypothetical ancestral mollusc)

Key Features:

gut mantle cavity radula (rasping organ) gills foot

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

trilobites Ecdysozoa: Phylum Arthropoda eurypterids Insects Spiders Crabs Lobsters Barnacles Etc.

Key Features: Jointed appendages 3-fold division of body (head, thorax, abdomen) scorpions shrimps

Important Deuterostomes (Deuterostomia)

Deuterostomia: Phylum Echinodermata (“spiny skin”) Sea urchins Brittlestars Starfish Sea cucumbers Key Features: 5-fold symmetry, calcite plates (but embryos are bilateral, suggesting a bilateral ancestor) Crinoids

Pterobranchs Deuterostomia: Phylum Hemichordata Key Characteristics: 3-part division of body (preoral lobe, collar, trunk) Pharynx Gill slits Stomochord Graptolites Acorn worms

Deuterostomia: Phylum Chordata Sea squirts and salps (Urochordates) Key Features: notochord dorsal nerve cord, pharynx gills slits post-anal tail Amphioxus (lancelet) (Cephalochordates)

END OF LECTURE