Introduction to Animal Diversity: Characteristics, Evolution, and Body Plans

1. Chapter 32 An Introduction to Animal Diversity

2. Figure 32.1 • Overview: Welcome to Your Kingdom • The animal kingdom • Extends far beyond humans and other animals we may encounter

3. Concept 32.1: Animal are multicellular, heterotrophic eukaryotes with tissues that develop from embryonic layers • Several characteristics of animals • Sufficiently define the group

4. Nutritional Mode • Animals are heterotrophs • That ingest their food

5. Cell Structure and Specialization • Animals are multicellular eukaryotes • Their cells lack cell walls

6. Their bodies are held together • By structural proteins such as collagen • Nervous tissue and muscle tissue • Are unique to animals

7. Reproduction and Development • Most animals reproduce sexually • With the diploid stage usually dominating the life cycle

8. 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

9. 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 Figure 32.2

10. All animals, and only animals • Have Hox genes that regulate the development of body form • Although the Hox family of genes has been highly conserved • It can produce a wide diversity of animal morphology

11. Concept 32.2: 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

12. 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 Figure 32.3

13. 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” • Was probably itself a colonial, flagellated protist Figure 32.4

14. (b) (a) Neoproterozoic Era (1 Billion–524 Million Years Ago) • Early members of the animal fossil record • Include the Ediacaran fauna Figure 32.5a, b

15. Paleozoic Era (542–251 Million Years Ago) • The Cambrian explosion • Marks the earliest fossil appearance of many major groups of living animals • Is described by several current hypotheses Figure 32.6

16. 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

17. 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

18. Concept 32.3: 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

19. The set of morphological and developmental traits that define a grade • Are generally integrated into a functional whole referred to as a body plan

20. Symmetry • Animals can be categorized • According to the symmetry of their bodies, or lack of it

21. (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. • Some animals have radial symmetry • Like in a flower pot Figure 32.7a

22. (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. • Some animals exhibit bilateral symmetry • Or two-sided symmetry Figure 32.7b

23. 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

24. 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

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

26. Body Cavities • In triploblastic animals • A body cavity may be present or absent

27. (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) Figure 32.8a • A true body cavity • Is called a coelom and is derived from mesoderm

28. 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) • A pseudocoelom • Is a body cavity derived from the blastocoel, rather than from mesoderm Figure 32.8b

29. 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. • Organisms without body cavities • Are considered acoelomates Figure 32.8c

30. 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

31. 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 • In protostome development • Cleavage is spiral and determinate • In deuterostome development • Cleavage is radial and indeterminate Figure 32.9a

32. (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 Figure 32.9b 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

33. Mouth Anus Digestive tube Anus Mouth Mouth develops from blastopore Anus develops from blastopore Figure 32.9c Fate of the Blastopore • In protostome development • The blastopore becomes the mouth • In deuterostome development • The blastopore becomes the anus

34. Concept 32.4: Leading hypotheses agree on major features of the 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

35. Rotifera Cnidaria Porifera Annelida Mollusca Chordata Phoronida Nemertea Ctenophora Nematoda Arthropoda Ectoprocta Brachiopoda Echinodermata Platyhelminthes “Radiata” Deuterostomia Protostomia Bilateria Eumetazoa Metazoa Ancestral colonial flagellate • One hypothesis of animal phylogeny based mainly on morphological and developmental comparisons Figure 32.10

36. 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 • One hypothesis of animal phylogeny based mainly on molecular data Figure 32.11

37. Points of Agreement • All animals share a common ancestor • Sponges are basal animals • Eumetazoa is a clade of animals with true tissues

38. Most animal phyla belong to the clade Bilateria • Vertebrates and some other phyla belong to the clade Deuterostomia

39. 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

40. Ecdysozoans share a common characteristic • They shed their exoskeletons through a process called ecdysis Figure 32.12

41. Apical tuft of cilia (a) An ectoproct, a lophophorate Mouth (b) Structure of trochophore larva Figure 32.13a, b Anus • Lophotrochozoans share a common characteristic • Called the lophophore, a feeding structure • Other phyla • Go through a distinct larval stage called a trochophore larva

42. Future Directions in Animal Systematics • Phylogenetic studies based on larger databases • Will likely provide further insights into animal evolutionary history