Chapter 32

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. Only one cleavage stage–the eight-cell embryo–is shown here. 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. The zygote of an animal undergoes a succession of mitotic cell divisions called cleavage. 2 3 1 Blastocoel Cleavage Cleavage The endoderm of the archenteron de- velops into the tissue lining the animal’s digestive tract. 6 Cross section of blastula Eight-cell stage Blastula Zygote Blastocoel Endoderm The blind pouch formed by gastru- lation, called the archenteron, opens to the outside via the blastopore. 5 Ectoderm Gastrulation Gastrula Blastopore 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 Early embryonic development in animals

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. Single cell Stalk 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

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

9. (b) (a) Neoproterozoic Era (1 Billion–524 Million Years Ago) • Early members of the animal fossil record • Include the Ediacaran fauna (From Ediacara Hills of Australia) Segmented body form Radial 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. (a) 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. Symmetry • Some animals have radial symmetry • Like in a flower pot

16. (b) 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. Symmetry • Some animals exhibit bilateral symmetry • Or two-sided symmetry

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. (a) Coelomate. Coelomates such as annelids have a true coelom, a body cavity completely lined by tissue derived from mesoderm. Body covering (from ectoderm) Coelom Tissue layer lining coelom and suspending internal organs (from mesoderm) Digestive tract (from endoderm) 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

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

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

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. Deuterostome development (examples: echinoderms, chordates) Protostome development (examples: molluscs, annelids, arthropods) (a) Cleavage. In general, protostomedevelopment begins with spiral, determinate cleavage.Deuterostome development is characterized by radial, indeterminate cleavage. Eight-cell stage Eight-cell stage Spiral and determinate Radial and indeterminate 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)

25. Deuterostome development (examples: echinoderms, chordates) Protostome development (examples: molluscs, annelids, arthropods) (a) Cleavage. In general, protostomedevelopment begins with spiral, determinate cleavage.Deuterostome development is characterized by radial, indeterminate cleavage. Eight-cell stage Eight-cell stage Spiral and determinate Radial and indeterminate 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)

26. (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). Coelom Archenteron Coelom Mesoderm Blastopore Mesoderm Blastopore Enterocoelous: folds of archenteron form coelom Schizocoelous: solid masses of mesoderm split and form coelom 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

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

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

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

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