Overview of Animal Diversity Chapter 32 General Features

Overview of Animal Diversity Chapter 32

General Features of 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 2

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

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 4

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 5

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 perpendicular to that axis 6

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 7

Evolution of the Animal Body Plan • Bilaterally symmetrical animals have two main advantages over radially symmetrical animals 1. Cephalization • Evolution of a definite brain area 2. Greater mobility 8

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) 9

Evolution of the Animal Body Plan 3. Evolution of a body cavity • Three basic kinds of body plans a. Acoelomates have no body cavity 10

Evolution of the Animal Body Plan b. Pseudocoelomates have a body cavity between mesoderm and endoderm • Called the pseudocoel 11

Evolution of the Animal Body Plan c. Coelomates have a body cavity entirely within the mesoderm • Called the coelom 12

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 13

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 14

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 15

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 16

Evolution of the Animal Body Plan 17

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 18

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 19

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 20

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) 21

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 22

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 23

A New Look At Metazoans Examples can be found in Table 32. 2 of Raven et al. 24

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 25

Evolutionary Developmental Biology 1. The multinucleate hypothesis 2. The colonial flagellate hypothesis 3. The polyphyletic origin hypothesis • Molecular systematics using r. RNA sequences settles this argument in favor of the colonial flagellate hypothesis 26