1. An Introduction to Animal Diversity
Chapter 32
An Introduction to Animal Diversity

2. 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

3. 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

4. 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

5. 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.

6. 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

7. 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

8. 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

9. 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

10. 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

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

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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.

18. 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)

19. 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

20. 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

21. 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.

22. 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

23. 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

24. 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.

25. 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

26. 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.

27. 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

28. 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

29. 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

30. 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!

31. 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

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

33. 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

34. 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

35. 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