INTRODUCTION TO ANIMAL EVOLUTION AND DIVERSITY Chapter 32

Kingdom: ANIMALIA Characteristics Multicellular Heterotrophic – most ingest organic “food” Lack cell walls, but structural proteins like collagen hold the multicellular bodies together along with intercellular junctions Nervous and Muscle tissue for impulse conduction and movement Mostly sexual reproduction (although Porifera and Cnidaria can “bud”) Diploid somatic cells and haploid gametes Parazoa lack true tissues (like porifera) or eumetozoa possess true tissues (Diploblastic or triploblastic) Digestive tracts (internal digestion) Radial / Bilateral symmetry Radial / Spiral Cleavage Protostomes / Deuterostomes Coelomates/Pseudocoelomates/Acoelomates Segmented,unsegmented Chordate characteristics (Notochord, nerve chord, postanal tail, pharyngeal slits)

Choanoflagellates Free-living and colonial choanoflagellates were probably the ancestors of basal animals such as sponges The colonies evolved into multicellular animals with “division of labor”

Animal Phylogeny

Metazoa(Parazoa) or Eumetazoa Early (basal) animals lacked true tissues. These are called metazoa or parazoa Sponges are metazoa or parazoa Animals who are composed of true tissues: ectoderm, endoderm and mesoderm, are called eumetazoa

Diploblastic or Triploblastic Animals with 2 tissue layers: Ectoderm and endoderm are called diploblastic Animals with all three germ layers are called triploblastic

How many germ layers? The Radiata are diploblastic, whereas All other eumetozoa are triploblastic

Animal Development The diploid zygote undergoes cleavage – a succession of mitotic divisions This forms a hollow, multicellular ball called the blastula The process of gastrulation follows next, during which layers of embryonic tissues begin to migrate and become germ layers: Ectoderm – covers the surface of the embryo and will give rise to the animal’s outer covering Mesoderm – the germ layer between the ectoderm and the deepest layer the endoderm. It gives rise to the musculature Endoderm – the innermost germ layer, it gives rise to the digestive tract. Animals that have no true germ layers are called parazoa (like Porifera or sponges) Animals that have true germ layers – either 2 (diploblastic) or 3 (triploblastic)layers, are called eumetozoa (all other animals)

Early Embryonic Development Archenteron

Radial vs. Bilateral symmetry Bilateral symmetry is a characteristic of multicellular organisms, particularly animals. A bilaterally symmetric organism is one that is symmetric about a plane running from its frontal end to its caudal end (head to tail), and has nearly identical right and left halves. Most animals are bilaterally symmetric, including humans. They are collectively called the bilateria. The exceptions are sponges (no symmetry), jellyfish and ctenophores or comb jellies (radial symmetry), and echinoderms (partial radial symmetry; starfish exhibit pentamerism). Radial Symmetry is exhibited by organisms that have a dorsal and ventral axis (top and bottom) or an oral and aboral side but no anterior or posterior axis. These animals are collectively called radiata. Pentamerism is a unique body symmetry exhibited primarily by starfish. The body arranged around the axis of the mouth with five equal sectors.

Radial symmetry In biology, radial symmetry is a property of some multicellular organisms. Any cut through the center of a radially symmetric organism, with the plane of the cut going from the top to the bottom (dorsal to ventral), results in roughly equal halves in terms of organs and body parts. For example, multi-layered circular pies exhibit radial symmetry. Organisms with radial symmetry only have a single orientation: dorsal-ventral (or anterior-posterior, there is no differentiation). Organisms with bilateral symmetry, on the other hand, have two orientations: dorsal-ventral, as well as anterior-posterior. Cnidaria and ctenophores are the only animals with true radial symmetry; echinoderms have partial radial symmetry.

Radial vs. Bilateral Symmetry

Spiral and Determinate Cleavage Spiral cleavage is a cleavage pattern characteristic of such invertebrates as annelids and mollusks, in which the cleavage planes are oriented off center to the polar axis of the oocyte. Determinate cleavage is when the fate of each embryonic cell is determined very early. If cells are removed during early embryogenesis, the embryo is inviable.

Radial and Indeterminate Cleavage Radial cleavage is a cleavage pattern characteristic of vertebrates and echinoderms, in which the spindle axes are parallel or at right angles to the polar axis of the oocyte. Indeterminate cleavage is when each cell produced by cleavage, retains the capacity to develop into a complete embryo.

Protostomes vs. Deuterostomes Protostomes have the formation of the "mouth first" (hence the name) during gastrulation, before the future anus. (The site of gastrulation initiation, the blastopore, becomes the mouth.) Major protostome phyla include molluscs, annelids and arthropods. Deuterostomes have the formation of the "mouth second" (hence the name) during gastrulation, after the future anus, which is comes from the blastopore, the site of gastrulation initiation. (Contrast with protostomes)

Acoelomates, Pseudocoelomates and Coelomates A Coelom is the cavity within the body of all “higher” animals and certain primitive worms, formed by the splitting of the embryonic mesoderm into two layers. In mammals it forms the peritoneal (abdomen), pleural (Lung), and pericardial (Heart) cavities. Also called body cavity. Animals can be acoelomates, pseudocoelomates or coelomates. Coelom formation can be schizocoelous or enterocoelous - in schizocoelous organisms, solid “chunks” of mesoderm split and form the coelom - in enterocoelous organisms, certain cells of the archenteron form the mesoderm which migrates and forms the coelom (chordates and echinoderms)

What is a Coelom? A body cavity that forms when the mesoderm layer splits in two – usually found only in echinoderms and chordates.

Summary of Evolutionary Trends   Symmetry Gut Coelom Embryonic Germ Layers Sponges asymmetrical no gut  N/A None (parazoa) Cnidarians radial saclike diploblastic (tissues, no organs) Flatworms bilateral Acoelomate triploblastic (tissues and organs) Roundworms complete Pseudocoelomate   triploblastic (tissues and organs) Mollusks Annelids Arthropods Echinoderms* Chordates Coelomate

Evolutionary Trends The animal kingdom is monophyletic, meaning if we could trace all animal lineages back to their origin, they would lead to a common ancestor.

Segmentation Another evolutionary trend, which first appears in the phylum annelida is apparent in other complex phyla, especially chordates

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