Overview of Animal Diversity Chapter 32

General Features of Animals Animals are a diverse group of consumers that share major characteristics All are heterotrophs All are multicellular Cells do not have cell walls Most are able to move All are very diverse in form and habitat Most reproduce sexually Have a characteristic patterns of embryonic development Cells of all animals (except sponges) are organized into tissues

Evolution of the Animal Body Plan Five key transitions in animal evolution Tissues Symmetry Body cavity Development 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 Parazoa (sponges) lack any definite symmetry Eumetazoa have a symmetry defined along imaginary axes drawn through the animal’s body There are two main types of symmetry Radial symmetry Bilateral symmetry

Evolution of the Animal Body Plan Radial symmetry Body parts arranged around central axis Can be bisected into two equal halves in any 2D plane perpendicular to that axis

Evolution of the Animal Body Plan Bilateral symmetry Body has right and left halves that are mirror images Body has distinct anterior/posterior and dorsal/ventral divisions

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

Evolution of the Animal Body Plan 3. Evolution of a body cavity Eumetazoa produce three germ layers Outer ectoderm (body coverings and nervous system) Middle mesoderm (skeleton and muscles) Inner endoderm (digestive organs and intestines)

Evolution of the Animal Body Plan 3. Evolution of a body cavity Three basic kinds of body plans Acoelomates have no body cavity

Evolution of the Animal Body Plan Pseudocoelomates have a body cavity between mesoderm and endoderm Called the pseudocoel

Evolution of the Animal Body Plan Coelomates have a 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 2-layer-thick ball called a gastrula with: Blastopore - Opening to outside Archenteron - Primitive body cavity

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 embryological features: Cleaveage pattern of embryonic cells Protostomes - Spiral cleavage Deuterostomes - Radial cleavage Developmental fate of cells Protostomes - Determinate development Deuterostomes - Indeterminate development Origination of coelom Protostomes - Forms simply and directly from the mesoderm Deuterostomes - Forms indirectly from the archenteron

Evolution of the Animal Body Plan

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 It seems that key morphological characters used in traditional classification are not necessarily correct 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: Separation of annelids and arthropods into different clades Division of the protostome group into Ecdysozoa and Spiralia The latter is then broken down into Lophotrochozoa and Platyzoa

Examples can be found in Table 32.2 of Raven et al. A New Look At Metazoans Examples can be found in Table 32.2 of Raven et al.

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