Introduction to Animals

To be an animal means

1. Multicellular – humans have ~ 50 - 100 trillion cells

2. Ingestive heterotroph

3. Lacking a cell wall

4. Specialization of cells (except Porifera)

5. May involve movement & sexual reproduction (formation of a zygote)

Phylogenetic tree of animal kingdomPhylogenetic tree of animal kingdom

• Based on:– Body symmetry– Patterns of embryo development– Comparing fossils– rRNA

Symmetry

• Overall pattern of structure– Asymmetric - No symmetry as with the Porifera

– Radial symmetry – even cut along a diameter. Echinodermata and Cnidaria

– Bilateral symmetry – even cut along a central plane. Worms, Arthropods, Mollusks, Echinoderms, and Vertebrates

Trends in Animal Evolution

• As you go up the phylogenetic tree, you will see a greater tendency towards bilateral symmetry.

• Also a concentration of nerves (sensory organs) towards the front of the organisms known as cephalization

Body views

1. Dorsal – top/back

2. Ventral – belly/bottom

3. Anterior – front/head

4. Posterior – tail/rear

Ventral (bottom)

Dorsal (top)

Posterior (tail end)

Anterior (front)

CleavageCleavage• A series of cell divisions that occurs

immediately following fertilization– Mitosis occurs but cells do not grow in size.

                                                                                                                

Stages in CleavageStages in Cleavage

Morula

Solid ball stage

Zygote

Blastula

Hollow ball stage

Gastrula

Indented ball stage

During gastrulation, an indentation occurs.

A multi-layer embryo is formed with 3 germ layers

Blastopore – opening of indentation

• Protostome– Blastopore developes into the oral opening

(mouth)– Mollusks, arthropods and annelids

• Deuterostomes– Blastopore developes into anus– Echinoderms and Chordates (that’s us)

Germ Layers

• Fundamental tissue types found in embryos of animals (but not sponges)

• Gives rise to every body feature, organs & tissues

• 2 germ layers in Cnidaria & 3 in all others– Endoderm – forms throat, gills, lungs and

digestive tract (with pancreas and liver)– Ectoderm – forms outer skin, hair, nails and

nervous system– Mesoderm – skeleton, muscles, inner skin layer,

circulatory system and lining of the body cavity

Body Cavities • Acoelomates – Body cavity is absent. As in Flatworms .

Endoderm is connected solidly to mesodermal tissue• Pseudocoelomates – Body cavity contains three distinct

layers but endoderm is not connected to mesoderm. Free floating in fluid. Found in roundworms.

• Coelomates – True body cavity. Mesoderm lines body cavity while supporting endodermic. Mollusks, annelids,

arthropods, chordates and echinoderms

Systems through the Phyla

Circulatory systems•

Closed Circulatory System– One in which blood is always contained within a

vessel. Heart serves as a pump. Found in annelids, chordates

Open Circulatory System– Pumps blood into spaces around the body tissues.

From there the blood makes its way back to vessels that will carry it back to the heart, or possibly to holes in the heart itself. Found in Arthropods and some mollusks

SkeletonsExoskeleton – Rigid, outer

covering protecting soft internal tissue. Found in Arthropods and mollusks. Needs to be shed (molt) as animal grows.

Endoskeleton – Internal skeleton. Best for larger organism. Found in echinoderms and vertebrates. Grows as animal grows.

Reproduction:• Asexually by budding

(Porifera, Cnidarians) or regeneration (Porifera, Planaria, Sea stars)

• Sexually – Hermaphroditic – contains

both sexes (but not self fertilization necessarily). Think of your earthworm

– Separate sexes – as you go up the phylogenetic tree

DevelopmentIncomplete

metamorphosis – Direct development – young animal is born or hatched looking like adult but smaller. No larval stage. Mantids, Grasshoppers, us

Complete metamorphosis – Indirect development – intermediate larval stages appearing different from adult.Butterfly, flies, moths