An Introduction to Animal Diversity Chapter 32 An Introduction to Animal Diversity

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

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

Animals are heterotrophs Nutritional Mode Animals are heterotrophs That ingest their food

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

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

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

Cross section of blastula Early embryonic development in animals The zygote of an animal undergoes a succession of mitotic cell divisions called cleavage. 1 Only one cleavage stage–the eight-cell embryo–is shown here. 2 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 Zygote Cleavage Eight-cell stage Blastula Cross section of blastula Blastocoel Gastrula Gastrulation Endoderm Ectoderm Blastopore 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 Figure 32.2

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

The animal kingdom includes not only great diversity of living species 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

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 Figure 32.3

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 Figure 32.4

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

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

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

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

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

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

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) Figure 32.7a

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) Figure 32.7b

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

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

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

In triploblastic animals 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 Figure 32.8a 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)

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 Pseudocoelomate. Pseudocoelomates such as nematodes have a body cavity only partially lined by tissue derived from mesoderm. (b) Figure 32.8b

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) Figure 32.8c

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

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

Coelom Formation In protostome development In deuterostome 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 Figure 32.9b Archenteron Blastopore Mesoderm Coelom Schizocoelous: solid masses of mesoderm split and form coelom Enterocoelous: folds of archenteron form 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).

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

Zoologists currently recognize about 35 animal phyla 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

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

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 Figure 32.11

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

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

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

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

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