1. INTRODUCTION TO ANIMAL EVOLUTION
CHAPTER 32
INTRODUCTION TO ANIMAL EVOLUTION

2. I. WHAT IS AN ANIMAL?
ANIMALS ARE MULTICELLULAR, HETEROTROPHIC EUKARYOTES THAT INGEST THEIR FOOD.
ANIMAL CELLS LACK WALLS.
THE MAIN STRUCTURAL PROTEIN IS COLLAGEN.
NERVOUS TISSUE AND MUSCLE TISSUE ARE UNIQUE TO ANIMALS.
REPRODUCTION IS PRIMARILY SEXUAL, AND THE LIFE CYCLE IS DOMINATED BY THE DIPLOID STAGE, GAMETES GENERALLY BEING THE ONLY HAPLOID CELLS.
FERTILIZATION INITIATES CLEAVAGE (MITOTIC DIVISION) OF THE ZYGOTE AND THE FORMATION OF A BLASTULA (A HOLLOW BALL OF CELLS IN SPECIES).
GASTRULATION FOLLOWS THE BLASTULA, RESULTING IN FORMATION OF EMBRYONIC TISSUE LAYERS

3. II. AN OVERVIEW OF ANIMAL PHYLOGENY AND DIVERSITY
ANIMALS PROBABLY EVOLVED FROM COLONIAL FLAGELLATES
FOUR KEY EVOLUTIONARY BRANCH POINTS ACCOUNT FOR MUCH OF ANIMAL DIVERSITY

5. A. PARAZOANS LACK TRUE TISSUES
THE FIRST MAJOR BRANCHING IN OF ANIMALS PRODUCED THE PARAZOANS, REPRESENTED BY THE (PHYLUM PORIFERA).
PARAZOANS LACK TRUE TISSUES, WHEREAS ALL OTHER ANIMALS (THE EUMETAZOA) HAVE TISSUES.

6. B. RADIATA AND BILATERIA ARE THE MAJOR BRANCHES OF EUMETAZOANS
IN THE SECOND MAJOR BRANCH POINT IN ANIMAL PHYLOGENY, EUMETAZOA DIVERGED EARLY INTO TWO MAJOR BRANCHES, THE RADIATA AND THE BILATERIA.
MEMBERS OF THE BRANCH RADIATA ARE JELLIES, MONES, AND THEIR RELATIVES, SEDENTARY AND PLANKTONIC FORMS WITH RADIAL SYMMETRY.
BILATERIANS ARE CHARACTERIZED BY BILATERAL SYMMETRY AND CEPHALIZATION.
ANOTHER IMPORTANT DIFFERENCE IS THAT ANIMALS ARE DIPLOBLASTIC (TWO-LAYERED: ECTODERM AND ENDO),WHILE BILATERIAN ANIMALS ARE TRIPLOBLASTIC (THREE-LAYERED: ECTODERM, MESODERM, AND ENDODERM).

8. C. EVOLUTION OF BODY CAVITIES LED TO MORE COMPLEX ANIMALS
THE THIRD MAJOR BRANCH POINT ON THE ANIMAL TREE SPLITS FLATERIANS INTO ANIMALS LACKING A BODY CAVITY (ACOELOMATES) ANIMALS HAVING A BODY CAVITY.
IN PSEUDOCOELOMATES, THE CAVITY IS LINED BY EMBRYONIC MESODERM.
COELOMATES HAVE A TRUE COELEM, A BODY CAVITY COMPLETELY LINED BY MESODERM.

10. D. COELOMATES BRANCHED INTO PROTOSTOMES AND DEUTEROSTOMES
COELOMATE PHYLA ARE DIVIDED INTO TWO MAIN GROUPS: PROTOSTOMES, INCLUDING THE ANNELIDS, MOLLUSKS, AND ARTHROPODS AND DEUTEROSTOMES, INCLUDING ECHINODERMS AND CHORDATES.

12. RELATED WEBPAGES Introduction to Porifera Introduction to Cnidaria
Introduction to the Platyhelminthes
Introduction to the Nematoda
Introduction to the Rotifera
Introduction to the Mollusca
Introduction to the Annelida
Introduction to the Arthropoda
Introduction to the Echinodermata
Introduction to the Chordata