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Kingdom Animalia
With over 2 million different kinds of animals, Kingdom Animalia is
the largest of the kingdoms. But when you think of an "animal", what
image comes to mind? While animals like bears, fish, and birds are
the most familiar to us, over half of all the animals on Earth belong
to a group of animals known as "arthropods". Arthropods include
animals such as centipedes, crabs, insects, and spiders. This
means that the majority of animals come from a group of critters that
give most folks the creeps!
So, what exactly is an "animal"? With so many different kinds of
animals, it's hard to imagine what they all might have in common.
First, animals are multicellular. This means they are made of many
cells, unlike bacteria, which are made of only one cell.
Second, all animals are heterotrophs; they must get their
food by eating other organisms, such as plants, fungi, and
other animals. In addition, all animals need oxygen to
survive. Did you know that fish can breathe oxygen from
the water that passes through their gills and earthworms
get their oxygen through their skin? Most animals (with the
exception of sponges) have bodies that are differentiated
into tissues, which are organized into organs, which are in
turn organized into organ systems.
Animals range in size from microscopic animals known as plankton to the massive blue whale.
They inhabit virtually every habitat on the planet – from the poles to the
tropics, mountain tops to ocean depths.
All animals have cells that lack rigid cell walls (like those found in plant
cells) and have nuclei and membrane-bound organelles (eukaryotic).
Most animals reproduce sexually and are diploid (adults have cells that
include two copies of their genetic material). There are between 3 and 30
million species of animals, though this remains a rough estimate. The
largest subgroup of animals by far is the insects.
Characteristics of Animals
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Animals are eukaryotic, multicellular, ingestive heterotrophs, most of which are mobile.
Animals may be herbivores, predators, saprophytes, or parasites.
Most animals reproduce sexually, while some reproduce asexually.
Animals exhibit the following features:
A. levels of organization
B. symmetry
C. cephalization
D. digestive tract
A. Animals show levels of organization within them.
This Means that similar cell types are organized into specialized tissues, such as skin or
muscle, and these tissues in turn are organized into organs, such as a heart or liver. Organs
may be organized into organ systems, such as the circulatory system.
B. Many animals show symmetry.
Symmetry is the quality of having one part of the body be a mirror reflection of another part. If
it is theoretically possible to place a mirror across an organism and have the reflection
coincide with the blocked half, then that animal shows symmetry.
For example: the two sides of a human body are a familiar example of symmetry because
each half is roughly a reflection of the other.
Animals exhibit two types of symmetry :
1. Radial symmetry:
Certain animals have a body with a number of similar parts radiating out from a central
axis. Their body therefore can be divided into equal halves by any number of planes
passing through the center. Such a form of symmetry is known as radial symmetry
Example: Hydra
2. Bilateral symmetry:
Most of these animals have paired structures, arranged on either side of a central axis
connecting the head with the tail-like the arms, eyes, ears and so forth on either side of a
human. The body can be divided into two similar halves by one plane going down the
center. The head end is called the anterior end and the "Lower" or tail end is called the
posterior end. The side of the body facing forward is called the ventral side, and the side
facing backwards is called the dorsal side. Most of the animals that immediately come to
mind show bilateral symmetry. For example: dogs, ants, horses, humans, etc.
Example: Lobster
C. Most animals show cephalization.
Cephalization is the differentiation of a definite head, most obvious on animals with bilateral
symmetry. Since the animals move with the head or anterior end forward, it is helpful to have
the sense organs here. A concentration of nerve tissue, often a full brain, is also often seen in
the head.
D. Most animals contain a digestive tract.
Digestive tract refers to a tube or gut into which food is taken to be digested and absorbed
into the body. This is in contrast to each cell of an organism absorbing food from either a
neighbouring cell or directly from the environment. Some animals, such as flatworms, have a
digestive tract with a single external opening that both ingests food and expels waste, thus
serving as both mouth and anus. Most animals have a digestive tract with two external
openings: the mouth for the intake of food and the anus for elimination of waste.
Classification Within Animalia
Animals are divided into two main categories:
A. Invertebrates or nonchordata - those without backbones
B. Chordata - most of which have backbones and are vertebrates
A. Nonchordata
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The invertebrates include over twelve phyla.
Below are listed the main phyla of invertebrates.
1. Invertebrate Animalia Phylum Porifera
Porifera consists of sponges.
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Sponges resemble plants in their "Branched" appearance
as well as their inability to move (instead they attach to
surfaces).
Sponges consist of hollow tubes, with openings called
osculum at one end.
The sides of the tubes are composed of a double layer of
cells separated by a jelly-like substance
Water is expelled through the osculum.
Habitat: sponges are all aquatic animals. While most of them are marine, i.e. salt water, a
few are found in fresh water.
Locomotion: the larvae of sponges can swim using flagella. These larvae eventually attach
themselves to a surface and grow into an adult organism, which then remains attached to
one place, or sessile.
Support: sponges require structural support to prevent collapse. A protein fiber called
spongin serves as a skeletal system in some sponges, while others have an internal
skeleton composed of crystalline spicules, made of calcium carbonate or siliceous
material.
Reproduction: sponges reproduce both asexually and sexually.
Asexual methods: buds May break off from an organism and grow into a separate
organism.
Sexual method: Sperm from one sponge may be taken in and carried to eggs. The
zygote develops into a larva that swims using its flagella until it attaches itself
somewhere and grows into a new organism.
Poriferans lack a nervous system. Sponge cells are unusual in that they function on their
own, without any coordination with one another.
2. Invertebrate Animalia Phylum Cnidaria
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This phylum includes hydra, corals, sea anemones and jellyfish.
Cnidarians are characterized by stinging cells called cnidocytes.
They also have tentacles.
Cnidarians are animals that exhibit radial symmetry.
A cnidarian's body shows some CELL SPECIALIZATION AND DIVISION OF LABOR. The
outer epidermis contains sensory nerve cells, gland cells, small interstitial cells, epitheliomuscular cells and cnidocytes. Each of these cells performs different functions.
Cnidarians have a hollow central cavity with one opening called the mouth, which actually
serves as mouth and anus, taking in food and expelling waste.
Habitat: Cnidarians are found in both fresh and salt water.
Locomotion: Some cnidarians are free-floating medusas, while others live anchored to a
surface as polyps. Some, like jellyfish, go through a polyp stage, but live most of their lives
as medusae.
Feeding: Cnidarians capture food by stinging barbs called Nematocysts, Which paralyze
or poison their prey. The prey is then taken into the mouth with tentacles
Reproduction: Most cnidarians can reproduce both sexually and asexually. Asexual
reproduction is by budding. Sexual reproduction is by production of eggs and sperm.
3. Invertebrate Animalia Phylum Platyhelminthes
• This phylum includes the most common flatworms.
• They are the most primitive representatives of bilateral symmetry.
• They have a single opening to their digestive tract that serves as mouth and anus.
• Excretory and reproductive systems are well developed in platyhelminthes. The
presence of these two systems indicates that flatworms have advanced beyond the
tissue level of organization to an organ level of organization. However platyhelminthes
lack respiratory and circulatory systems.
• They also have nervous systems, able to sense light, and simple brains.
Examples of platyhelminthes include planarians, flukes and tapeworms
4. Invertebrate Animalia Phylum Nematoda
This phylum includes the roundworms.
• A tough cuticle covers their body.
• Nematodes are found in almost all habitats. Some of them are free-living forms found in
water and soil, while some live as parasites in plants and animals. You may have heard
of the human parasites hookworms, pinworms, or filarial worms. The nematode
Trichinella spiralis generally enters the human body through improperly cooked pork,
causing trichinosis.
• Nematodes have a digestive tract with two openings, a mouth to take in food and an
anus for excreting the wastes.
• Sexes are separate in nematodes and look different from each other, A phenomenon
known as sexual dimorphism. Males are smaller than females and have curved tails.
5. Invertebrate Animalia Phylum Annelida
Annelida includes the segmented worms, including the earthworm.
• Most are free-living, while a few are parasitic.
• Annelids contain well-developed organ systems, such as circulatory, digestive,
nervous, and excretory.
• The circulatory system consists of sealed vessels that deliver blood to all parts of
the body.
• The digestive system has two openings. The digestive tube is differentiated into
specialized regions such as a crop for storage, and a gizzard for grinding.
• The most familiar representatives of this phylum are the earthworms, which
benefit plant growth. Their burrowing aerates the soil while their feces enrich it.
6. Invertebrate Animalia Phylum Arthropoda
Arthropoda include animals with jointed appendages and exoskeletons, such as insects and
spiders, lobsters and crabs, and millipedes and centipedes.
• The phylum arthropoda contains the greatest number of species of any animal phyla, with
about 75% of the species.
• The exoskeleton-the hard outer covering-protects the organism and lends structural
support. It must be shed periodically in a process called molting, to allow growth.
• Arthropods have an open circulatory system, meaning blood is released from the vessels of
the circulatory system to bathe body cells. The blood is then collected back into ducts and
returned to the system. Blood in arthropods carries food and waste products but plays no
role in respiration.
• Arthropoda contain well-developed organ systems, such as circulatory, digestive, nervous,
and excretory.
• The digestive system has two openings.
• Reproduction is generally sexual, and sexes separate, the fertilization of the egg taking
place within the female's body
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7. Invertebrate Animalia Phylum Mollusca
Mollusca include snails, slugs, clams, squids, and octopus.
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Mollusks are the second largest phylum of invertebrates.
• Mollusks have soft bodies and often an inner or outer shell.
• Some mollusks are sedentary filter feeders, like clams, which strain their food from the
water as it passes by. Others are highly mobile predators, such as squid and octupi.
• Mollusks have organ systems for circulation, which is open, respiration, digestion,
excretion, nerve conduction, and reproduction.
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9. Invertebrate Animalia Phylum Echinodermata
Phylum echinodermata includes starfish, brittle stars, sea cucumbers, sea urchins and
sand dollars.
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Echinoderms are spiny-skinned animals, exclusively marine bottom dwellers.
Adults show radial symmetry
They possess an internal skeleton of calcareous plates which may bear spines.
They lack a circulatory, respiratory, or excretory system.
They have a nervous system but no brain.
They have a digestive tract with a mouth and anus.
Numerous small protrusions called tube feet aid in locomotion, feeding,
respiration, and excretion.
Echinoderms have both sexes and can reproduce asexually or sexually.
B. Chordata
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Chordata include the subphylum vertebrata, Or Vertebrates, which contains most of the animals
that we commonly think of when we think of an animal.
Some of the animals in this subphylum are fishes, frogs, snakes, lizards, turtles, birds, bats, rats,
cats, dogs, lions, and humans.
Chordates are bilaterally symmetrical
The subphylum vertebrata includes these classes:
A) 3 classes of fish
B) amphibia
C) reptilia
D) aves (birds)
E) mammals
A) Vertebrate classes of fish
Fish, of course, are aquatic vertebrates.
In order to move efficiently through water, which is 800 times denser than air, fish developed several key
adaptations, such as:
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A streamlined body
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A powerful tail for propulsion
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Fins for stabilization and movement
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The capacity to regulate the amount of gas in their body and thus the ability to move up or down
in water
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Highly developed senses of touch and smell
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A row of sensory cells along their body that detect chemicals and vibrations
• Gills for breathing
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Fish have well-developed systems of circulation, digestion, respiration, excretion, and nerve
conduction.
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Sexes are separate and fertilization is external.
B) vertebrate class Amphibia
Amphibians live both in water and on land, and may be considered halfway between fish and
reptiles.
They are considered to be the first land vertebrates to have evolved.
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Amphibians change form, through metamorphosis, from an aquatic larval form to an adult
terrestrial form, which may spend much of its time in water.
Larval amphibians breathe through gills while adults breathe with lungs and through their
skin.
Eggs lack a protective shell and must be laid in water to avoid being dried out
(desiccation). Fertilization is external, when the male releases sperm to cover the eggs.
Skeletons are bony.
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The skin is thin, kept moist by numerous glands, and lacks scales.
Amphibians have well-developed organ systems.
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Examples: frogs, salamanders, toads.
C) vertebrate class Reptilia
Reptiles were the first vertebrates to evolve that were fully terrestrial, i.e. Lived entirely on land.
Reptiles in the form of dinosaurs were the dominant land animals from 225 to 65 million years
ago.
Reptiles developed several key adaptations that allowed them to live on land, completely apart
from water:
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Their skin is waterproof and protected by scales or plates.
They have more powerful limbs equipped with claws, or suction cups, adapted for
movement, digging, and/or climbing on land.
They have a more efficient circulatory system than amphibians, meaning that blood
reaching the cells has more oxygen in it than in amphibians.
Their excretory system is adapted for conservation of water.
Their respiratory system, which includes lungs, is completely internal where moistness can
be maintained.
Fertilization is internal, providing a moist environment in which sperm can travel to eggs.
Fertilized eggs are known as amniote eggs. They can be laid on land because the embryo
is protected by a membrane called an amnion, and by a tough outer shell, both of which
protect the egg and prevent desiccation. Within the shell, the egg is provided with food in
the form of a yolk.
Other features of reptiles:
Reptiles, like fish and amphibians, are cold-blooded, meaning their internal temperature
depends on the surrounding temperature.
Examples: Turtles, crocodiles, snakes, lizards and even the dinosaurs!
D) vertebrate class Aves
The members of the aves class are commonly called birds.
Birds are warm-blooded, egg-laying vertebrates that are covered with feathers.
Birds are thought to have evolved from reptiles.
Key features of birds:
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The feathers are modified scales that provide lift for flight and insulation.
Birds have toothless beaks.
Their forelimbs are modified into wings, usually used for flight.
Bones are hollow, spongy and have air spaces in them, making them very light. This is an
adaptation for decreasing the weight of the body.
Birds have extensions of the lungs, called air sacs, which connect to air spaces in the
bones and increase buoyancy for flight.
They have a four-chambered heart, which is seen in mammals too.
Birds are warm-blooded, able to maintain a constant body temperature.
Eggs are protected by an amnion and hard outer shell.
Usually parents incubate eggs.
Birds may feed on insects, fruits, seeds, worms, nuts, nectar, small reptiles, amphibians,
fish, or mammals, as well as dead animals.
Courtship, artful nest building, parental care of young, and migration during winters are
some interesting features of bird behaviour.
Some birds have lost their ability to fly. The wings of these birds may be small compared to their
body size. Such flightless birds have specialized as runners, such as the ostrich, rhea, and emu,
or as swimmers, such as the penguin.
E) vertebrate class Mammalia
Mammals are warm-blooded vertebrates that have hair or fur and feed their young ones with milk
produced from mammary glands.
Mammals are the dominant land vertebrates, with over 4,000 species.
Key features of mammals:
Mammals are warm-blooded and hence can maintain a constant body temperature.
Mammals breath through lungs aided by a muscle called a diaphragm.
They have a four-chambered heart for efficient blood circulation.
Mammals have well-developed organ systems, particularly the nervous system which often
includes a highly developed brain.
Fertilization in mammals is internal.
Mammals are generally viviparous, meaning development of embryos occurs in the female's
uterus and the young are born live.
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Mammals may be herbivores, carnivores, omnivores, or scavengers.
Some mammals can fly, their forelimbs being modified into wings, which include extensions
of the skin between the body and fingers. examples: bats, flying squirrels.
Some mammals are aquatic, though they still breathe air. Examples: whales, dolphins,
manatees.
Types of mammals
A. some mammals, called prototheria, lay eggs. Examples: echidna, duck-billed platypus.
B. a few species, called marsupials, give birth to very immature offspring, which are then kept in
internal pouches to complete development. Examples: kangaroos, koala bears.
C. the third and largest group of mammals is called placental mammals. In these mammals, the
young complete their development in the uterus of the mother and are born in a fairly advanced
stage of their development. examples: most of the mammals that commonly come to mind such as
lions, horses, mice, rabbits, apes, and humans.
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