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

What is an animal? Animals are multicellular, heterotrophic eukaryotes with tissues that develop from embryonic layers. Several characteristics of animals sufficiently define the group: Animals are heterotrophs that ingest their food Animals are multicellular eukaryotes The cell wall of animals LACKS a cell wall Animal bodies are held together by structural proteins such as collagen Nervous tissue and muscle tissue are unique to animals Most animals reproduce sexually with the diploid stage usually dominating the life cycle

Cleavage & Gastrulation in Animals After a sperm fertilizes an egg the zygote undergoes cleavage, leading to the formation of a blastula Cleavage is the process of cytokinesis in animal cells, characterized by pinching of the plasma membrane – and the succession of rapid cell divisions without growth during early embryonic development – CONVERTS ZYGOTE INTO A BALL OF CELLS A blastula is a hollow ball of cells marking the end stage of cleavage during early embryonic development The blastula undergoes gastrulation resulting in the formation of embryonic tissue layers and a gastrula

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

Hox Genes All animals, and only animals have Hox genes that regulate the development of body form Hox genes are special regulatory genes that control the transformation of a zygote to an animal of specific form Hox genes are responsible for the development of “body parts” in animals – they specify the position of the body part on the developing embryo. Mutations in hox genes result in the conversion of one body part to another.

Common Ancestor of Living Animals 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 Figure 32.4

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

Animals can be characterized by “body plans” Animal Body Plans 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

Body Symmetry in Animals Animals can be categorized According to the symmetry of their bodies, or lack of it Radial v/s Bilateral

Some animals have radial symmetry Like in a flower pot – these organisms are typically sessile in their environment 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 Bilateral Animals Bilaterally symmetrical animals have A dorsal (top) side and a ventral (bottom) side A right and left side Anterior (head) and posterior (tail) ends Bilateral Symmetry facilitates cephalization, the development of a head – an evolutionary trend toward the concentration of sensory equipment on the anterior end (toward the head) Bilateral organisms are typically motile in their environment

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

Germ Layer Formation during Gastrulation Animal embryos form germ layers (embryonic tissues), including ectoderm, endoderm, and mesoderm Ectoderm covers the surface of the embryo and gives rise to the outer covering of the animal Endoderm is the innermost germ layer which lines the digestive tract and organs Mesoderm lies between the endoderm and the ectoderm – it forms muscles and most other organs between the digestive tube and outer covering of the animal Diploblastic animals have two germ layers Triploblastic animals have three germ layers

In triploblastic animals a body cavity may be present or absent Body Cavities In triploblastic animals a body cavity may be present or absent A body cavity is a “tube-within-a-tube” body plan consisting of a fluid-filled space separating the digestive tract from the outer body wall.

Coelomates A true body cavity is called a coelom and is derived from and completely lined with mesoderm Annelids, Mollusks, Arthropods, Echinoderms, and Chordates 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)

Pseudocoelomates A pseudocoelom is a body cavity derived from the blastocoel, rather than from mesoderm – this body cavity is NOT completely lined with mesoderm Nematodes & Rotifers 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

Acoelomates Organisms without body cavities are considered acoelomates – this is a “solid” body plan Platyhelminthes (flatworms) 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

Feeding Systems in Coelomates and Acoelomates Acoelomates, such as flatworms, have a gastrovascular cavity with only one opening – which serves as a dual role of mouth and anus. This means that they lack a digestive tract altogether and absorb nutrients across their body surface. There is no specialization of compartments in this system. Digestion is extracellular – meaning that the breakdown of food is outside the cells. Coelomates, such as annelids, have a complete digestive tract that includes specialization along the tract. This means that the digestive tube extends between two openings, the mouth and an anus. Having an extracellular cavity for digestion enables an animal to devour much larger prey than can be ingested by phagocytosis and digested intracellularly.

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. During spiral cleavage, planes of cell division are diagonal to the vertical axis of the embryo. Determinate cleavage casts the developmental fate of each embryonic cell early – if one cell is separated from the rest, it will NOT develop into a complete embryo. During radial cleavage, planes are either parallel or perpendicular to the vertical axis of the egg. Indeterminate cleavage means that each cell produced by early cleavage divisions retains the capacity to develop into a complete embryo. 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). The archenteron is a “primitive” gut.

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

Embryonic Develop: Overview Embryonic development consists of three stages: cleavage, gastrulation, and organogenisis. Cleavage: rapid mitotic division of zygote occurring immediately after fertilization Two Patterns: protostomes (spiral and determinate) and deuterostomes (radial and indeterminate) In both groups, cleavage produces a fluid-filled ball of cells called a blastula. Cells of blastula are called blastomeres, and the fluid filled center is called the blastocoel.

Embryonic Develop: Overview Gastrulation is a process that involves rearrangement of the blastula and begins with the formation of the blastopore (an opening into the blastula). Blastopore becomes mouth in protostomes and anus in deuterostomes Some cells on the surface of the embryo may migrate into the blastopore to form a new cavity called the archenteron or “primitive gut” As a result of this cell movement, a three-layered embryo called a gastrula is formed

Embryonic Develop: Overview In most animals, the gastrula consists of three differentiated layers called the embryonic germ layers, each of which develops into all parts of the adult animal: Endoderm – forms viscera including lungs, liver, and digestive organs Ectoderm – becomes skin and nervous system Mesoderm – gives rise to muscles, blood, and bones

Embryonic Develop: Overview Organogenesis is the process by which cells continue to differentiate, producing organs from the three embryonic germ layers Once all organ systems have been developed, the embryo simply increases in size http://bcs.whfreeman.com/thelifewire/content/chp20/2002001.html

Order of Operations As far as the AP Biology exam is concerned, the order of the stages and the various events is extremely important: Zygote→Cleavage →Blastula →Gastrula →Organogenesis You should know the order of the events and a description of each phase!

Classifying Animals Most animal phyla belong to the clade Bilateria Vertebrates and some other phyla belong to the clade Deuterostomia 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 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

Morphological Phylogenetic Tree 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

Molecular 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 Figure 32.11