1. Chapter 32 An Introduction to Animal Diversity Estimates of the number of animal species range from 10-20 million to 100-200 million.

2. What is an Animal (some characteristics of animals) Multicellular eukaryotic heterotrophs that ingest their food. Lack the cell walls found in plants and fungi Have two unique cell types: nerve cells for impulse conduction and muscle cells for movement. Nervous and muscular tissues are unique to animals. Sexual reproduction (mostly), with diploid stage that dominates the life cycle All animals and only animals have Homeobox- containing family of genes (Hox genes) which control development of body form. Produce a wide diversity of animal morphology

3. Reproduction and Development Most animals reproduce sexually – With the diploid stage usually dominating the life cycle After a sperm fertilizes an egg – The zygote undergoes cleavage, leading to the formation of a blastula The blastula undergoes gastrulation – Resulting in the formation of embryonic tissue layers and a gastrula

4. Zygote Cleavage Eight-cell stage Cleavage Blastula Cross section of blastula Blastocoel Gastrula Gastrulation Endoderm Ectoderm Blastopore Early embryonic development in animals In most animals, cleavage results in the formation of a multicellular stage called a blastula. The blastula of many animals is a hollow ball of cells. 3 The endoderm of the archenteron de- velops into the tissue lining the animal’s digestive tract. 6 The blind pouch formed by gastru- lation, called the archenteron, opens to the outside via the blastopore. 5 Most animals also undergo gastrulation, a rearrangement of the embryo in which one end of the embryo folds inward, expands, and eventually fills the blastocoel, producing layers of embryonic tissues: the ectoderm (outer layer) and the endoderm (inner layer). 4 Only one cleavage stage–the eight-cell embryo–is shown here. 2 The zygote of an animal undergoes a succession of mitotic cell divisions called cleavage. 1

5. Reproduction and Development Some animals develop directly through transient stages into adults, but others have distinct larval stages. Larva is a sexually immature stage that is morphological distinct from the adult Larvae undergo metamorphosis, transforming the animal into an adult

6. Evolution of Animals The history of animals may span more than a billion years The animal kingdom includes not only great diversity of living species – But the even greater diversity of extinct ones as well

7. The common ancestor of living animals May have lived 1.2 billion–800 million years ago May have resembled modern choanoflagellates, protists that are the closest living relatives of animals Single cell Stalk

8. The common ancestor of living animals – Was probably itself a colonial, flagellated protist Colonial protist, an aggregate of identical cells Hollow sphere of unspecialized cells (shown in cross section) Beginning of cell specialization Infolding Gastrula-like “protoanimal” Somatic cells Digestive cavity Reproductive cells

9. (a) (b) Neoproterozoic Era (1 Billion–524 Million Years Ago) Early members of the animal fossil record – Include the Ediacaran fauna (From Ediacara Hills of Australia) Radial body form Segmented body form

10. Paleozoic Era (542–251 Million Years Ago) The Cambrian explosion – Marks the earliest fossil appearance of many major groups of living animals

11. Paleozoic Era (542–251 Million Years Ago) What caused the Cambrian explosion – New predator-prey relationships may have generated diversity through natural selection. – Predators acquired adaptations to catch prey – Prey acquired adaptations that helped them resist predation – Rise in atmospheric oxygen may have provided opportunities for animals with higher metabolic rates and larger body sizes – Evolution of the Hox complex provided developmental flexibility that resulted in variations in morphology

12. Mesozoic Era (251–65.5 Million Years Ago) During the Mesozoic era – Dinosaurs were the dominant terrestrial vertebrates – Coral reefs emerged, becoming important marine ecological niches for other organisms

13. Cenozoic Era (65.5 Million Years Ago to the Present) The beginning of this era – Followed mass extinctions of both terrestrial and marine animals Modern mammal orders and insects – Diversified during the Cenozoic

14. Animals can be characterized by “body plans” One way in which zoologists categorize the diversity of animals is according to general features of morphology and development A group of animal species that share the same level of organizational complexity is known as a grade The set of morphological and developmental traits that define a grade – Are generally integrated into a functional whole referred to as a body plan Animals can be categorized according to the symmetry of their bodies, or lack of it

15. Symmetry Some animals have radial symmetry – Like in a flower pot Radial symmetry. The parts of a radial animal, such as a sea anemone (phylum Cnidaria), radiate from the center. Any imaginary slice through the central axis divides the animal into mirror images. (a)

16. Symmetry Some animals exhibit bilateral symmetry – Or two-sided symmetry Bilateral symmetry. A bilateral animal, such as a lobster (phylum Arthropoda), has a left side and a right side. Only one imaginary cut divides the animal into mirror-image halves. (b)

17. Symmetry Bilaterally symmetrical animals have – A dorsal (top) side and a ventral (bottom) side – A right and left side – Anterior (head) and posterior (tail) ends – Cephalization, the development of a head

18. Tissues Animal body plans – Also vary according to the organization of the animal’s tissues Tissues – Are collections of specialized cells isolated from other tissues by membranous layers

19. Embryonic tissues Animal embryos – Form germ layers, embryonic tissues, including ectoderm, endoderm, and mesoderm Diploblastic animals – Have two germ layers Triploblastic animals – Have three germ layers

20. Body Cavities In triploblastic animals a body cavity may be present or absent A true body cavity is called a coelom and is derived from mesoderm Coelom Body covering (from ectoderm) Digestive tract (from endoderm) Tissue layer lining coelom and suspending internal organs (from mesoderm) Coelomate. Coelomates such as annelids have a true coelom, a body cavity completely lined by tissue derived from mesoderm. (a)

21. Body cavities A pseudocoelom – Is a body cavity derived from the blastocoel, rather than from mesoderm Pseudocoelom Muscle layer (from mesoderm) Body covering (from ectoderm) Digestive tract (from ectoderm) Pseudocoelomate. Pseudocoelomates such as nematodes have a body cavity only partially lined by tissue derived from mesoderm. (b)

22. Body Cavities Organisms without body cavities – Are considered acoelomates Body covering (from ectoderm) Tissue- filled region (from mesoderm) Digestive tract (from endoderm) Acoelomate. Acoelomates such as flatworms lack a body cavity between the digestive tract and outer body wall. (c)

23. Protostome and Deuterostome Development Based on certain features seen in early development – Many animals can be categorized as having one of two developmental modes: protostome development or deuterostome development

24. Cleavage (spiral versus radial) In protostome development – Cleavage is spiral and determinate (spiral = cell division diagonal to vertical axis of the embryo) In deuterostome development – Cleavage is radial and indeterminate (radial = the cleavage is either parallel or perpendicular to the vertical axis of the embryo) Protostome development (examples: molluscs, annelids, arthropods) Deuterostome development (examples: echinoderms, chordates) Eight-cell stage Spiral and determinate Radial and indeterminate (a) Cleavage. In general, protostome development begins with spiral, determinate cleavage. Deuterostome development is characterized by radial, indeterminate cleavage.

25. Cleavage(determinant versus indeterminate) Determinate: Ultimate fate of each cell is determined (locked-in) early. Indeterminate: Each early cell retains the potential to develop into a complete embryo! (this type of cleavage is what makes human identical twins possible) Protostome development (examples: molluscs, annelids, arthropods) Deuterostome development (examples: echinoderms, chordates) Eight-cell stage Spiral and determinate Radial and indeterminate (a) Cleavage. In general, protostome development begins with spiral, determinate cleavage. Deuterostome development is characterized by radial, indeterminate cleavage.

26. Coelom Formation In protostome development – The splitting of the initially solid masses of mesoderm to form the coelomic cavity is called schizocoelous development In deuterostome development – Formation of the body cavity is described as enterocoelous development Archenteron Blastopore Mesoderm Coelom Blastopore Mesoderm Schizocoelous: solid masses of mesoderm split and form coelom Enterocoelous: folds of archenteron form coelom Coelom (b) Coelom formation. Coelom formation begins in the gastrula stage. In protostome development, the coelom forms from splits in the mesoderm (schizocoelous development). In deuterostome development, the coelom forms from mesodermal outpocketings of the archenteron (enterocoelous development).

27. Fate of the Blastopore In protostome development – The blastopore becomes the mouth In deuterostome development – The blastopore becomes the anus Anus Mouth Mouth develops from blastopore Anus develops from blastopore Digestive tube

28. Animal phylogenetic tree Zoologists currently recognize about 35 animal phyla The current debate in animal systematics – Has led to the development of two phylogenetic hypotheses, but others exist as well

29. One hypothesis of animal phylogeny based mainly on morphological and developmental comparisons Animal phylogenetic tree Porifera Cnidaria Ctenophora Phoronida Ectoprocta Brachiopoda Echinodermata Chordata Platyhelminthes Mollusca Annelida Arthropoda Rotifera Nemertea Nematoda “ Radiata ” Deuterostomia Protostomia Bilateria Eumetazoa Metazoa Ancestral colonial flagellate

30. Animal phylogenetic tree One hypothesis of animal phylogeny based mainly on molecular data Calcarea Silicarea Ctenophora Cnidaria Echinodermata Chordata Brachiopoda Phoronida Ectoprocta Platyhelminthes Nemertea Mollusca Annelida Rotifera Nematoda Arthropoda “Radiata” “Porifera” Deuterostomia Lophotrochozoa Ecdysozoa Bilateria Eumetazoa Metazoa Ancestral colonial flagellate

31. Points of Agreement All animals share a common ancestor Sponges are basal animals Eumetazoa is a clade of animals with true tissues Most animal phyla belong to the clade Bilateria Vertebrates and some other phyla belong to the clade Deuterostomia

32. Disagreement over the Bilaterians The morphology-based tree – Divides the bilaterians into two clades: deuterostomes and protostomes In contrast, several recent molecular studies – Generally assign two sister taxa to the protostomes rather than one: the ecdysozoans and the lophotrochozoans

33. Ecdysozoans share a common characteristic – They shed their exoskeletons through a process called ecdysis

34. Lophotrochozoans – Have a lophophore, a ciliated feeding structure – OR… – Go through a distinct larval stage called a trochophore larva Apical tuft of cilia Mouth Anus (a) An ectoproct, a lophophorate (b)Structure of trochophore larva