Overview of Animal Diversity Chapter 32

General Features of Animals Animals are the consumers of the Earth They are a very diverse group -However, they share major characteristics -Are heterotrophs -Are multicellular -Have cells without cell walls -Most are able to move

General Features of Animals -Are very diverse in form and habitat -Most reproduce sexually -Have a characteristic pattern of embryonic development -Cells of all animals (except sponges) are organized into tissues

Evolution of the Animal Body Plan Five key transitions can be noted in animal evolution 1. Tissues 2. Symmetry 3. Body cavity 4. Development 5. Segmentation

Evolution of the Animal Body Plan 1. Evolution of tissues -Parazoa (Sponges - the simplest animals) lack defined tissues and organs -Have the ability to disaggregate and aggregate their cells -Eumetazoa (all other animals) have distinct and well-defined tissues -Have irreversible differentiation for most cell types

Evolution of the Animal Body Plan 2. Evolution of symmetry -Sponges also lack any definite symmetry -Eumetazoa have a symmetry defined along an imaginary axis drawn through the animal’s body -There are two main types of symmetry

Evolution of the Animal Body Plan -Radial symmetry -Body parts arranged around central axis -Can be bisected into two equal halves in any 2-D plane -Bilateral symmetry -Body has right and left halves that are mirror images -Only the sagittal plane bisects the animal into two equal halves

Top Back Front Bottom

Evolution of the Animal Body Plan Bilaterally symmetrical animals have two main advantages over radially symmetrical ones 1. Cephalization -Evolution of a definite brain area 2. Greater mobility

Evolution of the Animal Body Plan 3. Evolution of a body cavity -Eumetazoa produce two or three germ layers -Outer ectoderm -Middle mesoderm (not found in Cnidaria & Ctenophora) -Inner endoderm Body cavity = Space surrounded by mesoderm tissue that is formed during development

Evolution of the Animal Body Plan 3. Evolution of a body cavity -Three basic kinds of body plans -Acoelomates = No body cavity -Pseudocoelomates = Body cavity between mesoderm and endoderm -Called the pseudocoelom -Coelomates = Body cavity entirely within the mesoderm -Called the coelom

Evolution of the Animal Body Plan -The body cavity made possible the development of advanced organs systems -Coelomates developed a circulatory system to flow nutrients and remove wastes -Open circulatory system: blood passes from vessels into sinuses, mixes with body fluids and reenters the vessels -Closed circulatory system: blood moves continuously through vessels that are separated from body fluids

Evolution of the Animal Body Plan 4. Evolution of different patterns of development -The basic Bilaterian pattern of development: -Mitotic cell divisions of the egg form a hollow ball of cells, called the blastula -Blastula indents to form a two-layer- thick ball with: -Blastopore = Opening to outside -Archenteron = Primitive gut

Evolution of the Animal Body Plan Bilaterians can be divided into two groups: -Protostomes develop the mouth first from or near the blastopore -Anus (if present) develops either from blastopore or another region of embryo -Deuterostomes develop the anus first from the blastopore -Mouth develops later from another region of the embryo

Evolution of the Animal Body Plan Deuterostomes differ from protostomes in three other fundamental embryological features: -1. Cleaveage pattern of embryonic cells -Protostomes = Spiral cleavage -Deuterostomes = Radial cleavage -2. Developmental fate of cells -Protostomes = Determinate development -Deuterostomes = Indeterminate development

Evolution of the Animal Body Plan -3. Origination of coelom -Protostomes = schizocoely -Deuterostomes = enterocoely Deuterostomes evolved from protostomes more than 500 MYA

Evolution of the Animal Body Plan 5. Evolution of segmentation -Segmentation provides two advantages -1. Allows redundant organ systems in adults such as occurs in the annelids -2. Allows for more efficient and flexible movement because each segment can move independently Segmentation appeared several times in the evolution of animals

Traditional Classification of Animals Multicellular animals, or metazoans, are traditionally divided into 36 or so distinct phyla based on shared anatomy and embryology Metazoans are divided into two main branches: -Parazoa = Lack symmetry and tissues -Eumetazoa = Have symmetry and tissues -Diploblastic = Have two germ layers -Triploblastic = Have three germ layers

A New Look At Metazoans The traditional animal phylogeny is being reevaluated using molecular data Myzostomids are marine animals that are parasites of echinoderms -Have no body cavity and only incomplete segmentation -And so have been allied with annelids

A New Look At Metazoans Recent analysis of the translation machinery revealed that myzostomids have no close link to the annelids at all -Instead, they are more closely allied with the flatworms (planaria and tapeworms)

A New Look At Metazoans Therefore, key morphological characters used in traditional classification are not necessarily conservative Molecular systematics uses unique sequences within certain genes to identify clusters of related groups

A New Look At Metazoans Most new phylogenies agree on two revolutionary features: 1. Separation of annelids and arthropods into different clades 2. Division of the protostome group into Ecdysozoa and Spiralia -The latter is then broken down into Lophotrochozoa and Platyzoa

A New Look At Metazoans

Evolutionary Developmental Biology Most taxonomists agree that the animal kingdom is monophyletic Three prominent hypotheses have been proposed for the origin of metazoans from single-celled protists

Evolutionary Developmental Biology 1. The multinucleate hypothesis 2. The colonial flagellate hypothesis 3. The polyphyletic origin hypothesis Molecular systematics using rRNA sequences settles this argument in favor of the colonial flagellate hypothesis

Evolutionary Developmental Biology Molecular analysis may also explain the Cambrian explosion -The enormous expansion of animal diversity in the Cambrian period (543 to 525 MYA) -The homeobox (Hox) developmental gene complex evolved -Provided a tool that can produce rapid changes in body plan

Evolutionary Developmental Biology