The Origin of Animal Diversity

What is an animal? Animals are: Multicellular Heterotrophic Eukaryotic Animals store energy as glycogen (not starch, as in plants) Animals have nervous and muscular tissue Animal cells do not have cell walls

Animal Development: A Brief Overview Most animals reproduce sexually Flagellated sperm unites with an egg to form a diploid zygote This zygote undergoes cleavage, wherein the zygote divides by mitosis

Animal Development: A Brief Overview Cleavage leads to the formation of a blastula A blastula is a hollow ball of cells The internal cavity of the blastula is called a blastocoel.

Gastrulation then occurs, leading to the formation of a gastrula A gastrula has specialized tissues that will develop into adult tissues This then leads to the development of two primordial tissues: the endoderm and the ectoderm

The Origin of Animals Most scientists believe that animals originated from a colonial protist (Choanoflagellate) which developed into a “proto-animal”

Phylogeny of the Animal Kingdom Animals are divided into phyla (DKPCOFGS) based on evolutionary relationships and anatomical similarities Organisms that split apart further back in the evolutionary tree (phylogeny) are the least related

Phylogeny of the Animal Kingdom Division 1: Presence or Absence of Tissues Parazoa vs. Eumetazoa Parazoa do not have true tissues (specialized groups of cells) Phylum Porifera (sponges) Eumetazoa have true tissues All other animals

Phylogeny of the Animal Kingdom Division 2: Body Symmetry & Germ Layers Radiata vs. Bilateria Radiata have radial symmetry top & bottom, but no left/right, no anterior (head) or posterior (tail) Phyla Cnidaria (jellyfish, corals, anemones) & Ctenophora (comb jellies) Radiata also have only 2 germ layers: - endoderm & ectoderm Bilateria have bilateral symmetry Top & bottom Left & right Anterior (head) & posterior (tail) end Bilateria have 3 germ layers: Endoderm, ectoderm, and mesoderm

Radial symmetry

Bilateral Symmetry

Phylogeny of the Animal Kingdom Division 3: Presence or Absence of a Coelom Acoelomates vs. Coelomates Acoelomates do not have a coelom (body cavity lined with mesoderm tissue) Phylum Platyhelminthes (flatworms) Pseudocoelomates Have a coelom (body cavity) that is not completely lined with mesoderm tissue Phyla Rotifera (rotifers) and Nematoda (roundworms) Coelomates Have a true coelom (body cavity lined with mesoderm tissue)

Phylogeny of the Animal Kingdom Division 4: Protostomes vs. Deuterostomes Protostomes The blastopore develops into the mouth Phyla Mollusca (clams, snails), Annelida (segmented worms), & Arthropoda (crustaceans, insects, spiders) Deuterostomes The blastopore develops into the anus Phyla Echinodermata (sea stars, sea urchins) & Chordata (lancelets, vertebrates)