Animal Diversity and the Evolution of Body Plans

• The following study notes are for Chapters 32, 33, and 34.
• Passages from the textbook that are directly related to content explained in class are important.
• Students are not responsible for textbook concepts that have not been covered in powerpoints or explained in class.
• Students are expected to know textbook information pertaining to concepts and notions explained in class, even if it is not written in PowerPoint slides.

Chapter 32: Animal Diversity and the Evolution of Body Plans

• Chapter Outline:
• Some General Features of Animals
• Evolution of the Animal Body Plan
• Animal Phylogeny
• Parazoa: Animals That Lack Specialized Tissues
• Eumetazoa: Animals with True Tissues
• The Bilateria

General Features of Animals

• Heterotrophy: Animals obtain energy and organic molecules by ingesting other organisms.
• Multicellularity: Many animals have complex bodies.
• No Cell Walls: Animal cells lack rigid cell walls and are usually flexible.
• Active Movement: Animals move more rapidly and in more complex ways.
• Diversity of Form: Animals vary greatly in form, from microscopic to enormous organisms.
• Diversity of Habitat: Animals are grouped into 35-40 phyla, with most occurring in the sea, but many in fresh water and on land.
• Sexual Reproduction: Most animals reproduce sexually via nonmobile animal eggs.
• Embryonic Development: The zygote undergoes mitotic divisions to form a ball of cells called a blastula.
• Tissues: Cells of most animals organize into structural and functional units.
• Most animals move, but not all.
• Only animals can fly.
• Most animals are invertebrates
• Development from zygote begins with cleavage, producing a multicellular structure called blastula

Evolution of the Animal Body Plan

• Five key innovations in animal evolution:
• Symmetry
• Tissues
• Body cavity
• Patterns of Development
• Segmentation

Symmetry

• Sponges lack any definite symmetry.
• Other animals exhibit symmetry along an imaginary axis through their body.
• Two main types of symmetry:
• Radial Symmetry: Body parts arranged around a central axis and can be divided into two equal halves by any plane passing through the center.
• Bilateral Symmetry: The body has right and left halves that are mirror images, with only the sagittal plane bisecting the animal into equal halves.

Advantages of Bilateral Symmetry

• Bilaterally symmetrical animals have advantages over radially symmetrical ones including:
• Cephalization: Evolution of a definite brain area.
• Directional Movement

Tissues

• Zygotes, or fertilized eggs, are totipotent, meaning they can give rise to all other body cells.
• As the embryo develops, cells specialize.
• Cell specialization is irreversible, except in sponges.
• Sponges, the simplest animals, lack defined tissues and organs and can disaggregate/aggregate cells.
• All other animals have distinct and well-defined tissues, along with irreversible differentiation for most cell types.

Body Cavity

• Most animals have embryos that produce three germ layers, making them triploblastic:
• Outer ectoderm: Body coverings and nervous system
• Middle mesoderm: Skeleton and muscles
• Inner endoderm: Digestive organs and intestines
• All triploblastic animals exhibit bilateral symmetry.
• Cnidarians are diploblastic with an endoderm and ectoderm.
• Sponges lack germ layers.

Body Plans

• Body cavity = space surrounded by mesoderm tissue that is formed during development
• Three basic body plans:
• Acoelomates: No body cavity.
• Pseudocoelomates: Body cavity between mesoderm and endoderm, called the pseudocoelom.
• Coelomates: Body cavity, called the coelom, entirely within the mesoderm.
• The body cavity enabled the development of advanced organ systems.
• Coelomates: Have circulatory system to flow nutrients and remove wastes
• Open Circulatory System: Blood mixes with body fluids, and reenters the vessels
• Closed Circulatory System: Blood moves continuously through separated vessels

Different Patterns of Development

• The basic Bilaterian pattern of development involves mitotic cell divisions, cleavage forming a blastula.
• The blastula indents to form a two-layer-thick ball, with:
• Blastopore: An opening to outside, creating an anus
• Archenteron: Primitive body cavity

Groups of Bilaterians

• Bilaterians can be divided into two groups:
• Protostomes: Develop the mouth first from or near the blastopore.
• Deuterostomes: Develop the anus first from the blastopore.
• Anus (if present) develops either from blastopore or another region of embryo.
• Protostomes are more evolved
• Sponges Acoela evolved to coelomates

Differences between Protostomes and Deuterostomes

• Cleavage Pattern of Embryonic Cells:
• Protostomes= spiral cleavage. : New cells form to the right or left of previous cells.
• Deuterostomes= radial cleavage: New cells form on top of previous cells.
• Developmental Fate of Cells:
• Protostomes= determinate development: Cell fate is determined early.
• Deuterostomes= indeterminate development: Cell fate is not determined until after several divisions.
• Formation of the Coelom:
• Protostomes - cells move apart to form the coelom.
• Deuterostomes - groups of cells pouch off to form the coelom.

Segmentation

• Segmentation provides two advantages:
• It allows for redundant organ systems, as seen in annelids.
• It enables more efficient and flexible movement, as each segment can move independently.
• Segmentation appeared several times in the evolution of animals.

Animal Phylogeny

• Multicellular animals are divided into distinct phyla.
• Animals (Metazoans) are divided into two main branches:
• Parazoa: Lacks true tissues and includes sponges, which are monophyletic.
• A monophyletic group includes the most recent common ancestor and all its descendants.
• Eumetazoa: Have true tissues and are animals other than sponges.
• Cnidaria branch off evolutionary tree before bilateria. Have radial symmetry (e.g., Jellyfish).
• Bilateria
• Phylogeny of deuterostomes has not changed much.
• Phylogeny of protostomes has changed due to molecular data.

Use of the Coelom in Morphology-Based Phylogeny

• Previous phylogenies were based on the evolution of the coelom.
• The state of the internal cavity has evolved many times.
• Not a reliable character to infer phylogenetic relationships.
• The coelom appears to have evolved once.
• Some coelomate clades have reverted to acoelomate or pseudocoelomate condition.
• All deuterostomes have coeloms.
• Protostomes are variable.

Protostome Divisions

• Protostomes are divided into:

• Lophotrochozoans: lophophore or trocophore .

• Grow by gradual addition to the body mass.

• Have spiral cleavage.

• Ecdysozoans: shedding

• Animals that molt=undergo ecdysis.

• Includes the arthropods and many other phyla.

Deuterostome Divisions

Deuterostomes include chordates and echinoderms

• They consist of fewer phyla and species than protostomes.
• They are more uniform in many ways, despite great differences.
• All animals have DNA in common.
• *The position of ctenophores is currently being debated.
• All bilaterians are triploblastic.
• Radially symmetrical animals are typically diploblastic.

Kingdom Animalia (Metazoa)

• Kingdom Animalia (the Metazoa) is divided into 2 branches:
• (1) Parazoa: (literal translation: "beside the animals"): Animals lacking tissues and therefore organs and a definite symmetry
• Phylum Porifera (Sponges).
• (2) Eumetazoa: (literal translation: "well-after-animals"): Animals with a definite shape and symmetry, tissues, and possibly organs and organ systems.

Parazoa: Animals That Lack Specialized Tissues

• Within the branch Parazoa, the phylum Porifera Sponges
• 26,000 marine species; 150 freshwater species
• Among the most abundant animals in the deep ocean

Sponge Characteristics

• Most members lack symmetry.
• Various growth forms.
• Larval sponges are free-swimming.
• Adults remain attached (sessile).
• Cell types:
• Truly multicellular.
• 3 functional layers in vase-shaped body.
• Various key elements of sponges
• Choanocytes, flagellated collar cells, generate a water current through the sponge and ingest suspended food.
• Sponges consist of a gelatinous noncellular mesohyl layer between two cell layers.
• Amoebocytes are totipotent cells found in the mesohyl that play roles in the manufacture of skeletal fibers.

Eumetazoa: Animals with True Tissues

• Animals with true tissues.
• Embryos have distinct layers.
• Inner endoderm forms the gastrodermis (digestive tissue).
• Outer ectoderm forms the epidermis and nervous system.
• Middle mesoderm (only in bilateral animals) forms the muscles.
• Cnidarians and Ctenophores are diploblasts: have endo & ectoderm
• Bilaterians are triploblasts: have endo, ecto & mesoderm
• True body symmetry
• Radial symmetry
• Bilateral symmetry

Branch Eumetazoa, Phylum Ctenophora

• Known as comb jellies, sea walnuts, or sea gooseberries
• 8 rows of comblike plates of fused cilia that beat in a coordinated fashion
• Many bioluminescent
• 2 tentacles covered with colloblasts
• Discharge strong adhesive used to capture prey. Phylogenetic position unclear, formerly considered closely related to Cnidaria because of the gelatinous form of most species
• These Ctenophores lack nematocysts and are structurally more complex than cnidarians.

Branch Eumetazoa, Phylum Cnidaria

• Most marine, few fresh water species
• Diploblastic
• Bodies have distinct tissues but no organs
• No reproductive, circulatory, or excretory systems.
• No concentrated nervous system
• Latticework of nerve cells.
• Touch, gravity, light receptors.
• Capture prey with nematocysts (specialized venomous cells present in the tentacles cells).
• Unique to this phylum.

2 basic body forms of Cnidarians

• Cnidarians exhibit considerable variation in life history:
• some occur only as polyps – cylindrical and sessile
• others exist only as medusae – umbrella-shaped and free-living
• many alternate between these two phases
• Polyp and Medusa
• Gastrodermis
• Gastrovascular cavity
• Mesoglea
• Epidermis
• Mouth
• Tentacles

Commonalities of the Body Plans

• Body plan has single opening leading to gastrovascular cavity
• Site of digestion.
• Most gas exchange.
• Waste discharge.
• Formation of gametes in many -Gastrovascular cavity -Mesoglea
• Gastrodermis -Epidermis -Mouth -Tentacles
• 2 layers to body wall
• Epidermis.
• Gastrodermis.
• Mesoglea occurs between layers (can be acellular/glue-like in Hydra or multicellular like in jelly fish).

Support for the Body

• Gastrovascular space also serves as hydrostatic skeleton
• Provides a rigid structure against which muscles can operate.
• Gives the animal shape.
• Many polyp species build an exoskeleton of chitin or calcium carbonate around themselves.
• Some build an internal skeleton.

The Bilateria

• Characterized by bilateral symmetry.
• Allowed for high levels of specialization.
• Divided into 2 clades:
• Protostomes and Deuterostomes (discussed in Chapters 33 and 34)
• Acoel flatworms (discussed in the remainder of this chapter)

Phylum Acoela

• Acoel flatworms were once considered basal members of the phylum Platyhelminthes
• Have a primitive nervous system and lack a digestive cavity, so the mouth leads to a solid digestive syncytium (a mass of cells that have no cell membranes separating them).

Key Terms for Animal Groups

• Monophyletic: Includes the most recent common ancestor of a group of organisms, and all of its descendants.
• Polyphyletic: Does not include the common ancestor of all members of the taxon.
• Paraphyletic: Includes the most recent common ancestor, but not all of its descendants.

Evolutionary Patterns

• Divergent evolution: Populations from common ancestor evolve different phenotype.
• Convergent evolution: Populations with different ancestors evolve similar phenotypes.
• Homologous structure: Same evolutionary origin but now differ in structure and function (e.g. appendages of hippos and whales).
• Analogous structure: Structures of different origin used for the same purpose (e.g. appendages of shark and whales).