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Chapter 34: Introduction to Animals
34-1 The Nature of Animals
34-2 Animal Bodies
34-3 Comparison of Invertebrates and Vertebrates
34-1 The Nature of Animals
I. The Nature of Animals (Kingdom Animalia)
• Eleven major phyla based on phylogenetic relationships. (animals are for
simplicity divided into invertebrates and vertebrates)
(1) Invertebrate
• Animal lacking a backbone; 95% of ALL animals are invertebrates.
(2) Vertebrate
• Animals with a backbone; represent ONLY 5% of all animal species.
II. Characteristics
• Multicellular eukaryotes that are heterotrophic, no cell walls, locomotion.
(A) Multicellular Organization (enables adaptability)
• Each animal cell depends on the presence and functioning of other cells
(interdependency exist at the cellular level)
(1) Specialization
• Is the adaptation of a cell for a particular function (i.e., there is a
division of labor among animal cells)
Critical Thinking
(1) From the perspective of a SINGLE cell, what may be one ADVANTAGE
of cell specialization and one DISADVANTAGE of cell specialization?
(2) Cell Junctions (NOT found in unicellular organisms)
• Connections between cells that hold the cells together as a unit (tissue).
(B) Heterotrophy
• Nutrients are obtained from outside organic material and are ingested by
all animals.
(1) Ingestion (Intracellular)
• Digestion occurs WITHIN the animal’s body, allowing proteins, lipids, and
carbohydrates to be extracted for cellular use (metabolic activity).
(C) Sexual Reproduction and Development
• Most animals can reproduce sexually (unison of gametes), however some
can reproduce asexually as well.
(1) Zygote (2N—a diploid cell, feature of sexual reproduction)
• First cell of a new individual; undergoes mitotic divisions as development
takes place, paving the way for cell specialization.
(2) Differentiation (a.k.a. the cellular pathway to specialization)
• As new cells are yielded from a dividing zygote, genes become activated
or deactivated leading to cell differentiation.
(D) Movement (e.g., locomotion is a common animal behavior)
• Facilitated by the interrelationship of two types of tissue found ONLY in
animals:
(1) Nervous Tissue (2) Muscle Tissue.
Critical Thinking
(2) Consider that an endoskeleton can support MORE weight than an
exoskeleton, would a large-bodied animal with an exoskeleton be more likely
to live in the water OR on land? Explain your reasoning.
(1) Neurons (Nervous Tissue Cells)
• Conduct electrical signals working in a circuit between nervous tissue and
muscles. (Detecting environmental stimuli can RESULT in movement)
III. Origin and Classification
• The first animal ancestors were believed to be from the seas, and
possibly arose from COLONIAL protists (of which were heterotrophic and
eukaryotic).
NOTE: Colonialism brought similar unicellular organisms together as a unit,
possibly encouraging specialization of different cells.
34-2 Animal Bodies
I. Body Structure (morphology is BOTH external AND internal)
• Those that LACK true tissues and an organized body shape (sponges) to
highly organized tissues and a consistent body shape (the other ten phyla)
Critical Thinking
(3) Observe the body of the animal pictured below.
(a) What kind of symmetry does the animal display?
(b) Is the animal cephalized?
(c) How many germ layers are present?
(d) How many openings does its digestive system possess?
(e) Does the animal posses neurons?
(1) Symmetry
• Refers to a consistent OVERALL pattern of structure in an animal body
plan. (e.g., Sponges of Phylum Porifera display NO symmetry)
(A) Patterns of Symmetry (e.g., sponges excluded)
• Symmetry shows ONE of TWO patterns depending on body plan.
(1) Radial Symmetry (e.g., Sea anemone, jellyfish, and hydra)
• Similar parts BRANCH OUT in all directions from a central line; NO head
region.
(2) Bilateral Symmetry (e.g., Moth, planarian, human)
• Two SIMILAR halves on either side of a central plane; includes BOTH
anterior/posterior ends AND a head region.
(3) Dorsal-Ventral & Anterior-Posterior (i.e., anatomical POSITIONS)
• Top (dorsal), Bottom (ventral), Head (anterior), and Tail (posterior)
(4) Cephalization (evolution of a “head region” in an animal)
• Concentration of sensory and brain structures in the ANTERIOR end of
the animal (i.e., a “cephalized” animal has a head)
(B) Germ Layers (present in developing animal zygote; i.e., embryo)
• Fundamental tissues found in embryos of ALL animals—give rise to EVERY
body feature—tissues and organs.
• Sponges are the ONLY animals with have NO GERM LAYERS.
• Cnidarian and Ctenophore embryos have TWO GERM LAYERS.
• ALL other animals have evolved THREE GERM LAYERS.
(C) Body Cavities (most animals have SOME type of body cavity)
• A body cavity is a fluid-filled SPACE that forms between the digestive
tract AND the outer wall of the body during development.
NOTE: TWO functions:
(1) Provides a firm base against which muscles can contract.
(2) Acts as a reservoir and transport medium for body chemicals.
II. Animal Diversity
• There are features zoologists consider: (1) Presence/Absence of Tissue
Layers, (2) Body Cavity Type, and (3) Presence/Absence of Backbone.
(A) Invertebrates (10 invertebrate PHYLA of Kingdom Animalia)
• Highest VARIATION among the ANIMALS, including body symmetry,
tissue organization, and cell specialization.
(B) Chordates (last phyla of Kingdom Animala, includes Vertebrates)
• Characterized by PRESENCE of a notochord, dorsal nerve cord,
pharyngeal pouches, and a postanal tail.
(1) Notochord (develops into the BACKBONE)
• A firm, flexible rod of tissue located in the dorsal part of the body.
(2) Dorsal Nerve Cord (develops into the BRAIN and SPINAL CORD)
• A hollow tube lying just ABOVE (dorsal to) the notochord.
(3) Pharyngeal Pouches (develop into GILLS—aquatic species)
• Small outpockets of the ANTERIOR part of the digestive tract.
(4) Postanal Tail (develops in a tail in some, vestigial in others)
• Muscle tissue located BEHIND the posterior opening of the digestive
tract.
(C) Vertebrates
• A subphylum of Phylum Chordata, includes fishes, amphibians, reptiles,
birds, and mammals.
34-3 Comparison of Invertebrates and Vertebrates
I. Invertebrate Characteristics (classification of phyla)
• Variation in SYMMETRY, SEGMENTATION, SUPPORT, and the major
BODY SYSTEMS.
(A) Symmetry
• Radial (receive stimuli from ALL directions); Bilateral (allowed for
CEPHALIZATION to occur). [NOTE: Most invertebrates are bilateral]
(B) Segmentation
• A body of REPEATING segments or SIMILAR units (annelids); segments
can FUSED and SPECIALIZED (arthropods).
(C) Support of the Body (internal OR external skeleton)
• Influenced by AQUATIC or TERRESTRIAL inhabitation (water pressure,
gravity, etc…)
Critical Thinking
(4) On mammals and birds, the head is positioned higher with respect to
the body than it is on amphibians and reptiles. Why do you suppose it may
be evolutionary advantageous to have a head positioned OVER the body?
(1) Exoskeleton (does NOT grow  stretched, shed and replaced)
• Rigid outer covering  PROTECTS the soft tissues, including arthropods
and mollusks.
(D) Respiratory and Circulatory Systems
• Evolved to EXCHANGE gases, nutrients, and wastes from CELL activity.
(1) Gas Exchange (CO2, a metabolic waste product of respiration)
• Exchange of gases (CO2 and O2) among cells occurs by DIFFUSION.
(2) Gills (aquatic arthropods and mollusks)
• Specialized organs adapted for exchanging gases in AQUATIC habitats.
(3) Open-Circulatory System (e.g., aquatic arthropods and early mollusks)
• Blood-like fluid is pumped from the body vessels to the BODY CAVITY,
and returned to the body vessels. (i.e., a primitive, LESS efficient system)
(4) Closed-Circulatory System (e.g., annelids and later mollusks)
• Blood SEPARATED in tubular vessels; exchange between cells and
capillaries that line each cell. (i.e., MORE efficient, faster)
(E) Digestive and Excretory Systems
• Digestion AND excretion result from actions of METABOLIC activity.
(1) Gut (digestive tract RUNS through the body)
• Food is broken down AND nutrients are absorbed by cells that LINE gut.
(F) Nervous System (sensory data)
• High variation (sponges—NO neurons, primitive responses; octopus—one
of the most evolved invertebrate brains, capable of emotion)
(G) Reproduction and Development
• Asexual and sexual; TWO types of DEVELOPMENT are observed.
(1) Hermaphrodite (e.g., ALWAYS have a mate vs. REDUCED variability)
• Capable of BOTH types of gametes  allowing an individual to behave
sexually as male OR female.
(2) Indirect Development (e.g., MOST invertebrates, including beetles)
• Invertebrates have an intermediate LARVAL stage during the life cycle
(3) Larva (INDIRECT development; TWO niches)
• Free-living, IMMATURE form of an organism.
(4) Direct Development (e.g., fewer invertebrates, grasshoppers)
• Born or hatched with the SAME appearance and niche it will have as an
ADULT (i.e., no larval stage)
II. Vertebrate Characteristics (a subphylum of Chordata)
• ALL vertebrates (fish aside) spent PART of their life on land; SUPPORT
of body and RETAINING water are two adaptations.
(A) Support of the Body (vertebrates)
• HABITAT and LOCOMOTION  influence ENDOSKELETON.
(1) Endoskeleton (supports a backbone)
• An INTERNAL skeleton that can support a LARGE, heavy body; GROWS
as the VERTEBRATE grows.
(2) Vertebrae
• Repeating BONY UNITS of the backbone, positioning of limbs and skull
evolved for FLEXIBILITY.
(B) Body Coverings (over endoskeleton)
• Protection, Insulation, Watertight, Gas Exchange…
(1) Integument (provides a BARRIER against the environment)
• Outer COVERING of an animal (e.g., fish, amphibian, reptile, bird, and
mammal integuments)
(C) Respiratory and Circulatory Systems (vertebrates)
• Closed-circulatory system with chambered heart with either gills or
lungs.
(1) Lungs (terrestrial AND aquatic vertebrates)
• Moist-membranous surfaces deep inside the animal’s body, require a
muscular diaphragm.
(D) Digestive and Excretory Systems (vertebrates)
• Occurs in elongated GUT  deals with expelling wastes WHILE
conserving water.
(1) Kidneys (ammonia is a PROBLEM  MUST be detoxified by body)
• Filter wastes from the BLOOD while regulating water levels in the body.
(E) Nervous System (more advanced in vertebrates)
• Highly organized brain and neural circuits; specific REGIONS of the
brain have evolved certain duties.
(F) Reproduction and Development
• Internal AND external fertilization methods are observed.
Chapter 34: Introduction to Animals
34-1 The Nature of Animals
34-2 Animal Bodies
34-3 Comparison of Invertebrates and Vertebrates
34-4 Fertilization and Development
I. Fertilization and Early Development
• Embryology—dividing zygote and the formation of the three primordial
germ layers (embryonic tissues).
(A) Gametes
• Sperm (specialized for movement); Ovum (larger, size dependent on how
long the food supply in the yolk must last—compare fish vs. bird eggs)
(B) Fertilization
• Sperm nucleus merges with ovum nucleus (restores diploid number) and
replication of DNA begins as the first cell division takes place.
(C) Cleavage (immediate zygote divisions) and Blastula Formation
• As cleavage ensues, the number of cells increases (2-4-8) but the cells do
not increase in size (i.e., they get smaller to fit)
(1) Blastula (hollow ball of embryonic cells) and Blastocoel (cavity)
• As the number of dividing cells increases, the mass becomes a hollow ball.
II. Gastrulation (stage following blastula formation)
• An area of the blastula collapses inward, forming the blastopore, which
ultimately transforms the blastula to a gastrula (multilayered embryo).
(1) Archenteron (“primitive gut;” surrounded by endoderm)
• Blastopore folding results in a cup-shaped embryo and a deep-cavity
which behaves like a gut (i.e., the archenteron).
(2) Ectoderm (outermost germ layer of gastrula)
• Develops into the outer layer of the skin, the hair, nails and nervous
system.
(3) Endoderm (innermost germ layer of gastrula, surrounds archenteron)
• Develops into the throat passage, gills/lungs, and digestive organs.
(4) Mesoderm (middle germ layer of gastrula; most versatile germ layer)
• Develops into the skeleton, muscles, inner layer of the skin, circulatory
system, and the lining of the body cavity.
III. Patterns of Development
• Body symmetry, number of germ layers, and body cavity—three types
(1) Coelom
• A body cavity that is COMPLETELY LINED by mesoderm.
(A) Blastopore Fate and Cleavage
• Two phylogenetic pathways in TRUE Coelomates:
(1) Mollusks, Arthropods, and Annelids—blastopore develops into a
MOUTH, and a second opening arises to become an anus.
(2) Echinoderms and Chordates—blastopore develops into an ANUS, and a
second opening arises to become a mouth.
(1) Protosomes (“mouth-FIRST”) & Spiral Cleavage
• Cells of protostomes divide in a spiral arrangement (determinate cleavage).
(2) Deuterostomes (“mouth-SECOND”) & Radial Cleavage
• Cell divisions are parallel to or are at right angles (indeterminate cleavage).
(3) Determinate Cleavage (Protostome Development)
• If the cells are separated in this embryo, each one will develop into its
part and the overall organism will die.
(4) Indeterminate Cleavage (Deuterostome Development)
• If the cells are separated in this embryo, each cell will embark on its own
path to become a separate organism.
NOTE: I.C. very early in embryo development in humans can result in
identical twins.
(B) Coelom Formation (variation as embryos)
• Protostome vs. Deuterostome; in both types, the mesoderm (red) lines
the interior of the outer body wall and surrounds the gut (mouth-anus).
(2) Enterocoely (“gut-body cavity;” Deuterostome coelom formation)
• Mesoderm forms by rapid division of cells lining the dorsal part of the
blastopore.
(1) Schizocoely (“split-body cavity;” Protostome coelom formation)
• Mesoderm forms by rapid division of cells at the boundary of the
endoderm and ectoderm.
(C) Types of Body Cavities (3 Types)
• Relationship between: Ectoderm, Mesderm, and Endoderm.
(1) Acoelomates (“without a coelom;” flatworms)
• NO body cavity present; the endodermic gut and the outer covering of
the animal are connected by the solid tissue of the mesoderm.
(2) Pseudocoelomates (“false-body cavity;” roundworms)
• Mesoderm lines the interior of the coelom BUT does NOT surround the
exterior of the endodermic gut.
(i.e., the mesoderm lines the fluid-filled coelom and the endodermic gut is
suspended in the fluid of the coelom.)
(3) Coelomates (“true coelom;” mollusks, annelids, arthropods,
echinoderms, and chordates)
• Mesoderm lines the body cavity and surrounds and supports the
endodermic gut.