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Introduction to Animals
Characteristics of Animals
Animals are multicellular
Except for sponges, animal cells are arranged into tissues. Tissues are necessary to produce organs and
organ systems.
Tissues, organs, and organ systems enabled the evolution of large, multicellular bodies.
Animal cells lack cell walls
A skeleton supports the tissues of large animals.
Animals have a period of embryonic development
During embryonic development, cells become specialized and tissues form. The growth of tissues, organs,
and organ systems therefore requires a period of embryonic development.
Animals are heterotrophs
Heterotrophs consume their organic food. Except for sponges, they ingest food and digest it in a central
cavity.
Recall that fungi are also heterotrophs but fungi do not ingest their food. Fungi secrete enzymes into their
environment and absorb broken down organic food products.
Animals are motile
Heterotrophy often requires motility to capture prey. Animals have motility during at least some part of
their life cycle.
Animals have nervous and muscle tissue
Muscle tissue allows animals to move. Nervous tissue allows rapid intercellular communication and
enables coordinated movement and response to stimuli.
Animals are diploid (diplontic life cycle)
Their gametes are heterogametes (different sizes); eggs are larger than sperm.
The sperm are flagellated.
Gametes are produced by meiosis.
The development of some animals includes one or more larval stages. Larvae refers to immature
individuals of species in which the body form of the immature individuals (the larvae) is very different than
the body form of the adult. Because larvae and adults have different forms, they often eat different food and
may live in different habitats. Larvae are transformed into adults by a developmental process called
metamorphosis.
Symmetry
Types of Symmetry
Radial Symmetry
The body parts of a radially symmetrical animal are arranged around a central axis so that each part extends
from the center. The animal can be cut along the axis in more than one plane to produce identical halves.
Animals that exhibit radial symmetry tend to be sessile (immobile). Radial symmetry allows them to reach
out in all directions.
Bilateral Symmetry
Only one cut along the longitudinal axis will produce identical halves of a bilaterally symmetrical animal.
Bilateral symmetry is best for motile animals.
Asymmetry
Asymmetrical animals have no pattern of symmetry. The simplest animals (sponges) are asymmetrical.
Evolution of Symmetry
Sponges lack symmetry, and Cnidarians exhibit radial symmetry. The remainder of the phyla listed below
have bilateral symmetry.
Body Plans
Embryonic Development
A fertilized animal egg divides to produce a solid ball of cells. Then, cell migration results in a hollow ball
called a blastula.
Some cells of the blastula migrate inward producing a gastrula. The opening is the blastopore. The tube
produced by this process will become the gut (digestive tract) of the mature animal. In species that have a
separate mouth and anus, the tube will eventually extend through the length of the embryo and fuse with
the opposite side. One opening will become the mouth, the other will become the anus.
In the diagram below, a circle is used to represent a blastula.
Embryonic Germ Layers
The three layers of tissues that become established during early embryonic development are called germ
layers. They give rise to the body tissues. These layers are ectoderm, mesoderm, and endoderm.
The diagram below shows a cross section of an animal embryo
.
The ectoderm forms from the outer layer of cells. It gives rise to the skin and nervous system.
The cells that formed the tube-like structure in the gastrula (see the diagram above) are endoderm. These
cells will form the lining of the gut and the organs derived from the gut.
Mesoderm forms between the ectoderm and endoderm. It becomes the muscles, connective tissues,
skeleton, kidneys, circulatory and reproductive organs
Body Cavity
The body cavity is a fluid-filled space that separates the gut and internal organs from the rest of the body.
It isolates the internal organs from body-wall movements.
It also bathes the internal organs in a liquid through which nutrients and wastes can diffuse.
Arrangement of Ectoderm, Mesoderm, and Endoderm
An acoelomate animal does not have a body cavity.
A pseudocoelomate animal has a body cavity (called a pseudocoelom) located between endoderm and
mesoderm.
The body cavity of a coelomate animal (called a coelom) is located within the mesoderm.
The mesentery holds the gut in place.
The diagram below shows the body plans for nine major phyla of animals.
Gut
The gut is the digestive tract. It enables the animal to digest food outside of the cells (extracellular
digestion). In animals without a digestive tract, food items are brought into the cell for digestion
(intracellular digestion).
A sac-like gut has one opening. Food enters and leaves through the same opening. (two way)
A complete gut has two openings, a mouth and an anus. (one way)
This type of gut allows for the specialization of parts along the tube. For example, part of the gut can
become specialized for food storage, other parts can become specialized for secreting digestive enzymes
and other parts for absorbing nutrients.
Large, Active Animals
Small animals do not require any special means to distribute nutrients and gasses or to collect wastes
because every cell in the body is near a source of food. If the cells are in contact with the external
environment, it is not necessary to collect wastes for removal. As evolution proceeded toward larger forms
however, special structures evolved to facilitate these processes.
Circulatory System
Larger animals require a circulatory system to transport nutrients, gasses, and wastes. Fluid within the body
cavity can act like a circulatory system and distribute nutrients and gasses.
In an open circulatory system, blood leaves the blood and flows freely within the tissues. This system is
not very efficient because there is no blood pressure to move blood rapidly through the tissues. The oval
line in the diagram below represents an animals body.
Blood does not leave the blood vessels in a closed circulatory system. In this type of system, the heart can
pump blood through the tissues rapidly.
Gas Exchange (Respiration)
All animals need to take in O2 and eliminate CO2. Lungs are membranous structures designed for gas
exchange in a terrestrial environment. Gills are designed for gas exchange in an aquatic environment.
Oxygen must be dissolved in water before it can be absorbed by the respiratory structures (gills,lungs, etc.).
Therefore, the respiratory surfaces of animals (must always be moist. Oxygen is absorbed from the water
coating the surface.
Respiratory (Oxygen-Carrying) Pigments
The oxygen-carrying capacity of blood can be increased if the blood contains molecules that are capable of
binding to oxygen. These molecules are referred to as pigments because they are colored. For example,
hemoglobin is a red, iron-containing pigment found in the blood of vertebrates. Hemocyanin is a bluishcolored copper-containing pigment found in many mollusks and arthropods.
Nervous System
A nervous system receives information from sensory receptors and it is responsible for stimulating the
muscles and glands. Muscle movement requires stimulation by the nervous system.
Animals that are attached (sessile) do not need sophisticated sense organs and consequently do not need an
elaborate nervous system to service the sense organs. Animals that move, however, need sense organs
located on the anterior end (head region) so that they can sense the environment that they move into.
A concentration of sense organs requires a concentration of nervous tissue to receive the information and
decide what to do with it. The brain is a concentration of nervous tissue near the sense organs.
Cephalization refers to the degree of development of the brain. The evolutionary trend is more elaborate
and sensitive sense organs and increased cephalization.
Sponges (Phylum Porifera)
Sponges are multicellular but are thought to have evolved from unicellular protists.
Sponges are sessile (immobile) filter feeders.
Structure of Sponges
Although sponges have cells with specialized functions they do not have tissues. There is no endoderm,
mesoderm, or ectoderm.
Collar Cells (choanocytes)
The inner layer is composed of flagellated collar cells (choanocytes). The flagella beat to move water in
through the pores and out the osculum. Food is trapped by the collar cells and is digested within the cell
(intracellular digestion) or is passed to amoeboid cells for digestion.
Amoeboid Cells
Food particles trapped by collar cells are passed to amoeboid cells for digestion and circulation.
Amoeboid cells secrete hard mineral needle-like structures called spicules and skeletal fibers made of a
protein called spongin. Spongin gives a commercial sponge its elastic characteristics.
Reproduction
Most sponges exhibit sequential hermaphroditism; they function as one sex for a period of time and then
change to the other sex. This prevents self-fertilization.
Although the adult is stationary, the zygote develops into a ciliated larva and swims to a new location.
Asexual reproduction is by fragmentation and budding.
Skeletal Material
Two types of skeletal material support the cells of sponges. Spicules are needle-like structures composed of
either calcium carbonate or silica and offer support and protection. Spongin is a fibrous protein that
provides support and elasticity. Commercial sponges are composed of spongin.
Cnidarians (Phylum Cnidaria)
Some examples of Cnidarians are hydra, jellyfishes, corals, sea anemones, and Portuguese man-of-wars.
Characteristics
Radial symmetry
Cnidarians have two tissue layers. The outer layer is the epidermis. It is formed from ectoderm. The inner
layer, the gastrodermis, secretes digestive juices into the inner space called the gastrovascular cavity. The
gastrodermis is formed from endoderm.
Cnidarians do not have mesoderm and therefore do not have organs.
A nonliving gelatinous material called mesoglea separates the two tissue layers. A nerve net is located
between the epidermis and mesoglea.
The body contains long structures called tentacles that can be moved to capture prey.
The tentacles contain stinging cells called cnidocytes and within each one is a capsule called a nematocyst,
which discharges to either trap or sting the prey.
Contractile (muscle-like) fibers are found in both the epidermis and the gastrodermis. Their movements are
not complex because they do not have a brain.
Cnidarians have a hydrostatic skeleton. The contractile fibers act against the fluid-filled gastrovascular
cavity. The movements are like a balloon; the animal can be short and thick or long and thin.
Cnidarians have a saclike gut and extracellular digestion.
Two body forms are found among the Cnidarians, a polyp and a medusa. A polyp is attached and has the
tentacles and mouth directed upward. A medusa is free-floating and has the mouth and tentacles on the
ventral surface. It resembles an upside-down polyp. Some species have both a polyp and a medusa in their
life cycle, others have one or the other form dominant.
Most cnidarians are marine species but a few hydrozoans, including Hydra, live in freshwater.
Members of this group can reproduce by budding, an asexual process in which small polyps form on a
parent polyp and then break off.
Below: Budding in Hydra
Sexual reproduction also occurs .
Protostomes Vs Deuterostomes
Protostomes
Deuterostomes
Major animal groups
flatworms, mollusks, annelids,
roundworms, arthropods
echinoderms and chordates
Blastopore
becomes mouth
becomes anus
Flatworms (Phylum Platyhelminthes)
Characteristics
Flatworms are flattened and have bilateral symmetry.
They are triploblastic (have 3 embryonic tissue layers: ectoderm, mesoderm, and endoderm) and therefore
have organ-level of organization. There is no body cavity, so they are acoelomate.
Flatworms have a gastrovascular cavity with one opening (a sac-like gut).
Planarians (Class Turbellaria)
Most turbellarians are free-living (not parasites).
Example: Dugesia - a freshwater planarian
Planarians have a branching sac-like gut (one opening).
Planarians have cephalization with sense organs.. It consists of a brain and nerve cords arranged in a
ladder-like configuration.
Planarians have eyespots allow the presence and intensity of light to be determined. They move away from
light.
Planarians are hermaphroditic, that is, they contain both male and female sex organs. They can reproduce
asexually simply by pinching in half; each half grows a new half.
Movement is accomplished by the use of cilia and also by muscular contractions.
Flukes (class Trematoda)
Members of this class are primarily parasites (feed on a host species).
Parasitic forms lack cephalization.
The reproductive cycle typically involves two host species, a primary host and a secondary (or
intermediate) host. Adults live in the primary host and larvae develop in the secondary host. The life cycle
often alternates between sexual and asexual reproduction.
Liver flukes are found in vertebrate livers.
Nearly half of people in the tropics have blood flukes. Schistosomiasis is a blood fluke that afflicts 200
million people in the world. The secondary host is a snail.
Tapeworms (class Cestoda)
Tapeworms live in the intestines of vertebrates.
They may reach 10 m in length (>30 feet). They have no digestive or nervous tissue. Attachment to the
intestinal wall is by a scolex, a structure that contains hooks and suckers.
Below: Taenia, preserved specimens
Tapeworms are harmful because they excrete toxic wastes, absorb nutrients, and may block the intestine.
The segments (proglottids) each contain male and female reproductive organs. Eggs are fertilized from
sperm, which often come from other proglottids of the same individual. After fertilization, other organs
within the proglottid disintegrate and the proglottid becomes filled with eggs.
The intermediate hosts are usually pigs or cattle. They can become infected by drinking water contaminated
with human feces.
Tapeworms can be passed to humans in undercooked meat, especially pork.
Mollusks (Phylum Mollusca)
Mollusks include squids, octopuses, clams, scallops, oysters, snails, and slugs.
They are bilateral, coelomate, protostomes (see above).
Characteristics of Mollusks
All mollusks have a visceral mass, a mantle, and a foot. The visceral mass contains the digestive, excretory,
and reproductive organs. The mantle is a covering. It may secrete a shell. The foot is muscular and is used
for locomotion, attachment, and/or food capture.
There may be a radula, a structure that resembles a tongue but contains hard plates and is often used for
scraping food.
The coelom is reduced and limited to the region near the heart.
Most mollusks have an open circulatory system but cephalopods (squids, octopus) have a closed
circulatory system.
Most mollusks have separate sexes but most snails (gastropods) are hermaphrodites.
Snails and relatives (Class: Gastropoda)
Gastropods have an elongated, flattened foot and usually a head and shell although nudibranchs (sea slugs)
and terrestrial slugs lack a shell.
Most are marine but there are also numerous freshwater and terrestrial species.
Herbivorous gastropods use a radula to scrape food from surfaces. Carnivores may use a radula to bore a
hole through surfaces such as bivalve (clam) shells.
Some gastropods such as the slug (below) do not have a shell.
Gas Exchange (Respiration)
Some gastropod species are terrestrial and have lungs for gas exchange, others are aquatic and use gills.
Clams and relatives (Class: Bivalvia)
Bivalves have two shells (valves) held closed by powerful muscles. The shell is produced by the underlying
mantle; it grows along the outer margins.
They use their foot for burrowing. Mussels use their foot for the production of threads for attachment.
The gills are large because they are used for filter-feeding as well as respiration. Food is trapped by mucus
on the gills and moved by cilia. Water enters and exits through siphons.
Squid, Octopus, Nautilus, and Relatives (Class: Cephalopoda)
Cephalopods are predators and live in a marine environment.
A closed circulatory system allows them to move rapidly in pursuit of prey.
They move by jet propulsion; water in the mantle cavity is squirted rapidly through a siphon.
The foot has evolved into tentacles around head.
Cephalopods have a powerful beak-like structure to tear apart prey.
The sense organs of cephalopods are well developed.
Mollusks are the simplest animals with eyes. Some mollusks have lenses and therefore are capable of
forming clear images. The camera-type eyes of some cephalopods (squid, octopus) are capable of focusing
and forming clear images. Cephalopods are fast-moving predators and well-developed camera-type eyes
help them catch prey.
Well-developed brains (especially in octopuses) give them a high learning capacity.
Cephalopods can hide from enemies by releasing a dark colored fluid from ink sacs.
Segmented Worms (Phylum Annelida)
Annelids are bilateral, coelomate protostomes.
The fluid-filled coelom acts as a hydrostatic skeleton. When the circular muscles that surround each
segment contract, the segment becomes thinner and longer. When the longitudinal muscles that extend
from one end of the segment to the other contract, the segment becomes shorter but thicker.
Annelids exhibit specialization of the digestive tract. Some of these structures are the pharynx, crop,
gizzard, intestine, and accessory glands.
Annelids have a closed circulatory system.
Excretory System
The excretory system consists of two metanephridia most of the segments that function to remove excess
water and to rid the body of nitrogenous wastes.
Gas exchange occurs across the skin, so the body must remain moist.
Earthworms are hermaphroditic but cross-fertilize with another individual. Sperm are produced by testes
located within seminal vesicles. Sperm is stored within the seminal vesicles. Sperm from another individual
is stored in a seminal receptacles.
Earthworms feed by passing soil through their bodies and digesting organic matter in the soil. They are
important in soil ecology because their activities mix and aerate the soil.
Food enters the body through the pharynx (see the diagram above) and is stored in the crop. Muscular
action within the gizzard grinds food into smaller particles. Chemical digestion and absorption of digested
food occurs in the intestine.
Bloodsucking leeches produce hirudin, a powerful anticoagulant, in their saliva.
When surgeons reattach severed human fingers, they occasionally use laboratory-raised leeches during the
patient’s recovery. The leeches suck blood from the tissues around the reattachment sites, release
anesthetics and anticoagulants, and thereby relieve pressure and decrease pain. Removal of fluids from the
area enables fresh fluids to move into the area.
Roundworms (Phylum Nematoda)
Characteristics of Roundworms
Roundworms have bilateral symmetry with a pseudocoelom (tube within a tube).
Fluid within the pseudocoelom functions to transport nutrients.
They are very abundant and occur almost everywhere.
They are not segmented. Do not confuse them with earthworms.
They are covered by a tough outer layer called a cuticle. The cuticle does not grow; it must be shed as the
animal grows. After shedding, a new cuticle is secreted.
These feeding modes occur often among the various roundworm species: carnivores, herbivores,
saprotrophs, or parasites.
Examples of Parasitic Roundworms
Ascaris is a parasite that infects the digestive tracts of humans and other mammals.
Trichinella is passed to humans through poorly cooked pork.
Elephantiasis is a human disease caused by filaria worms. It is characterized by gross swelling and
malformations due to blockage of lymph vessels by the parasite. A mosquito is a secondary host.
Hookworms are tiny worms that are contracted by walking barefoot on soil that is contaminated with feces
from an infected host.
Pinworms are spread in children that put dirty fingers with eggs on them into their mouths.
Arthropods (Phylum Arthropoda)
Arthropods are bilateral, coelomate protostomes.
They exhibit a variety of specializations and adaptations that allow them to live in many different kinds of
habitats.
This phylum exhibits the greatest diversity of the animal phyla with more than 1,000,000 known species. It
has been estimated that there may be more than 30 million different species of insects alone! Arthropods
may be the most successful of the animals.
Reasons for Arthropod Success
Segmentation
The ancestral arthropods probably had many segments with a pair of appendages on each segment. The
evolutionary trend was for segments to become fused and appendages to become specialized.
The segments of arthropods today are often fused into regions such as the head, thorax, and abdomen. The
head and thorax may be fused to form a cephalothorax.
Exoskeleton
The external skeleton (exoskeleton) is composed of protein and chitin.
Chitin is hard and serves to protect the animal as well as provide an attachment site for muscles and prevent
desiccation. The thickness of the exoskeleton varies. It is very thin and flexible over the joints but thick
over other parts of the body.
Because chitin is hard and is not expandable; it requires periodic molting (shedding) to allow the animal to
grow.
The new skeleton must expand and harden when the old one is shed.
Jointed Appendages
Jointed appendages act like a system of levers and are ideal for any activity that requires movement.
Arthropods have evolved a variety of different kinds of specialized appendages for activities such as
walking, swimming, reproduction, feeding, sensory reception, and defense.
Well-developed sense organs
Well-developed sense organs are needed for active animals.
Arthropods have both compound and simple eyes.
Insects and crustaceans have compound eyes.
Well-developed nervous system
A well-developed nervous system is needed to integrate the sensory information and allow the animal to be
active.
The system enables arthropods to exhibit complex behavior.
The nervous system includes a dorsal brain and a ventral, solid nerve cord..
Internal Fertilization
Males deposit sperm directly into females where fertilization occurs. This type of breeding, called internal
fertilization, allows animals to breed on dry land. Another advantage is that fewer gametes are needed than
if both sexes deposit their gametes in water.
Coelom and Hemocoel
The coelom in arthropods is reduced; it consists of a small cavity surrounding the reproductive and
excretory organs.
Arthropods have an open circulatory system.
Overview of Arthropods (phylum: Arthropoda)
The five groups of arthropods are listed below.
Class Arachnida - (Spiders, Scorpions, Ticks and Mites)
These animals have four pairs of walking legs.
Only simple eyes are found on arachnids.
The body is fused into a cephalothorax and abdomen.
Class Diplopoda - Millipedes
Millipedes are cylindrical, slow-moving, nonaggressive scavengers.
millipedes have two pairs of walking legs per segment.
Class Chilopoda - Centipedes
Centipedes are flattened, fast-moving carnivores.
Each body segment has one pair of walking legs.
Class Insecta
750,000 species of insects have been described. It has been estimated that there may be 30,000,000 species.
Insects have three pairs of legs and many have one or two pairs of wings.
Insects have both simple and compound eyes.
Gas exchange is accomplished by a system of tubes that extend throughout the animal called tracheae.
They exhibit complex behavior. For example, bees, ants, and termites exhibit social behaviors such as
cooperative feeding, raising of young, and housekeeping.
Metamorphosis
The change in body form as an insect grows to adult is metamorphosis. In some species, the larvae look
like the adult; the animal simply gets larger as it matures. This type of change is incomplete
metamorphosis. Grasshoppers undergo incomplete metamorphosis.
Species that undergo a complete reorganization of the body tissues exhibit complete metamorphosis.
Worm-like larvae form a cocoon called a pupa and then become transformed into the adult form. Flies and
butterflies undergo complete metamorphosis.
Complete metamorphosis allows the animal to take advantage of two different ecological settings (niches).
For example, caterpillars are specialized for feeding on plant leaves. The butterfly of many species is
specialized for finding mates and reproduction.
The photograph below shows three stages in the complete metamorphosis of a fly. The larva is on the left, a
pupa is in the center and adult on the right.
.
Respiration
Insects have a network of tubules called tracheae that bring oxygen directly to the tissues and allow carbon
dioxide to escape. The openings to the outside, called spiracles, are located on the side of the abdomen. Air
sacs attached to the tubules pump air through the system.
Reproduction
Insects have internal fertilization. Sperm are delivered directly to the female by the male's penis. This
ability enables insects to breed in terrestrial environments.
The female's ovipositor deposits eggs in the ground.
Impact of Insects on Humans
Beneficial Insects
Pollination
Important in many food chains
Some species are important detritus feeders, breaking down dead organic material.
Useful products
Honey, beeswax (bees)
Silk (silkworms)
Some species destroy other species that are harmful to humans.
Harmful Insects
Some species carry diseases.
Tsetse flies carry Trypanosoma. (African sleeping sickness)
Mosquitoes carry Plasmodium (malaria).
Some species are parasites (mosquitoes, lice, fleas).
Some species have painful bites and stings (mosquitoes, fire ants).
Crop destruction by insect activities has a major economic impact.
Termites damage buildings.
Crustaceans (subphylum Crustacea) - Lobsters and relatives
lobsters, crayfish, crabs, shrimp, barnacles, isopods (often called pill bugs or sow bugs)
They have one pair of compound eyes.
The head and thorax are fused into a cephalothorax..
Deuterostomes: Echinoderms, Chordates
Phylum Echinodermata
Echinoderms are coelomate, and deuterostomes.
Echinoderms include sea stars (starfishes), sea urchins, sand dollars, sea cucumbers, and sea lilies.
There are 6,000 species of echinoderms; they are all marine.
Although echinoderm adults have radial symmetry, they evolved from ancestors that were bilaterally
symmetrical. They have free-swimming, bilateral larvae that metamorphose (change as they mature) into
adults with radial symmetry.
The adult body usually has five-part organization.
They possess an internal skeleton (endoskeleton) composed of calcium carbonate plates just beneath the
surface of the skin. The plates often bear spines that protrude through the skin.
Echinoderms have numerous tube feet underneath each arm. The tube feet are connected to a system of
pipes referred to as the water vascular system.
Large digestive glands produce enzymes necessary for digestion.
Sexes are separate and gametes are shed into the water. The gonads are large due to the necessity of
releasing large numbers of gametes into the marine environment.
Gas exchange is done with numerous tiny gills that extend from the surface of the skin.
Sea Stars feed on bivalves (clams) by pulling apart the shell and lowering their stomach into the mollusk,
releasing enzymes and digesting the mollusk, then absorbing the digested material into the body of the
starfish, where digestive glands in each arm continue the process of food breakdown.
Chordates (Phylum Chordata)
Chordates exhibit bilateral symmetry.
Chordates have the following characteristics at some point in their life history:
1.
2.
3.
4.
a dorsal, hollow nerve cord.
a dorsal supporting rod called a notochord. This is replaced by a vertebral column in vertebrates.
pharyngeal clefts (pouches). These develop into openings to the exterior (gill slits) in some
chordates. Gill slits functioned as a mechanism for filter-feeding in primitive vertebrates. The gills
of fish function in gas exchange.
a postanal tail. In most other kinds of animals, the digestive tract extends the entire length of the
animal.
Vertebrates (Subphylum Vertebrata)
The notochord of vertebrates is generally replaced by a vertebral column composed of numerous small
bones called vertebrae that are joined together to form a flexible supporting structure. In most vertebrates,
the vertebrae surround the spinal cord.
The vertebral column allows the body to flex and provides attachment sites for muscles. In addition, it
surrounds and protects the nerve cord.
They exhibit extreme cephalization and possess complex sense organs (ex: eyes, ears).
Lampreys (Class: Agnatha)
Lampreys do not have jaws.
Most species of lampreys are parasites. They attach to host fish and feed on the blood of the host.
The skeleton is cartilage and the notocord persists in the adult.
Cartilaginous Fish (Class: Chondrichthyes)
Chondrichthyes include the sharks, and rays.
The cartilage skeleton of sharks is partially hardened with calcium. This type of skeleton is strong and is
more flexible and lighter than a bony skeleton.
The bodies of cartilaginous fish are covered with small toothlike scales. Their teeth are larger versions of
these scales.
They do not have a swim bladder but the oil-storing capacity of their livers improves their buoyancy. The
shape of the head and caudal fin also lift the animal as it swims. Sharks must swim to keep from sinking.
Some sharks are fast-swimming predators; others are filter feeders.
Reproduction
Fertilization is internal. The pelvic fin is used to transfer sperm to the female.
Some species are oviparous- they lay eggs that hatch outside the mother's body; some are ovoviviparousthe eggs are retained within the body and young are born alive; and a few are viviparous- they receive some
nourishment via a placenta that develops from the yolk sac of the egg.
The reproductive and excretory system exit the body through a common opening called a cloaca.
Osteichthyans (Class: Osteichthyes)
Osteichthyans have an ossified (bony) skeleton. The skeleton is hardened with calcium phosphate.
Characteristics of Aquatic Osteichthyans (Bony Fish)
The gills are covered by an operculum so that the gill chamber is enclosed and protected.
A swim bladder and is used for buoyancy.
The gas content, and thus buoyancy, can be regulated by transfer to and from the blood.
Bony fish have broad, flat scales. The skin contains mucous secreting glands that reduce friction as the fish
swims through the water.
A lateral line system detects vibrations in the water.
Most species are oviparous.
Bony fish are the largest group of vertebrates. Approximately 49,000 species have been identified.
All fish have a two chambered heart.
Ray-finned Fish
The fins are supported by spinelike rays.
Lobe-finned Fish
Lobe-finned fish have fins located on fleshy appendages.
This group includes coelacanths (class Actinistia), Lungfishes (class Dipnoi) and tetrapods.
Coelacanths were thought to have been extinct for 75 million years until one was captured in 1938 off the
southeastern coast of Africa.
The first vertebrate animals to develop lungs were fish. The lungs developed from a sac-like pocket of
tissue that formed in the pharynx.
Lungfishes are a group of lobe-finned fish that inhabit stagnant fresh water ponds that dry up. Their lungs
allow them to gulp oxygen from the air when it has been depleted from the water. Their lobe fins enable
them to walk under water.
The ancestors of lungfish gave rise to amphibians.
Amphibians (Class Amphibia)
Frogs, salamanders and caecilians are amphibians.
Amphibians evolved from lobe-finned fish.
Amphibians have a three chambered heart.
Amphibians are tied to the water
Amphibians have small inefficient lungs and most species also use their skin for respiration. Because the
skin is used in gas exchange, it must remain moist, so amphibians can not venture far into very dry habitats.
Amphibians must reproduce in water because:
they have external fertilization and sperm require water to swim.
the eggs do not have an outer covering or shell and therefore are not protected against desiccation.
The larvae (tadpoles) are aquatic and use gills for respiration.
In many amphibian species, the larvae (tadpoles) metamorphose into adults. The larval form is aquatic but
the adult leaves the water and adopts an amphibious lifestyle.
Amphibian populations have been declining for several decades. Some reasons for the declines may be
habitat destruction, acid precipitation, increased levels of ultraviolet (U.V.) radiation, effects of introduced
species, global warming, and pollution.
MOVING ONTO LAND
Amniotic egg
An amniotic egg is well-suited for terrestrial environments due to the development of the extraembryonic
membranes listed below:
amnion - surrounds and protects the embryo
yolk sac - provides food
chorion - involved in gas exchange along with the allantois
allantois - stores wastes; involved in gas exchange along with the chorion
Reptiles (Class Reptilia)
This group includes the tuatara, lizards, snakes, turtles, crocodilians.
Adaptations for terrestrial living
Reptiles have more efficient lungs than amphibians.
Reptiles do not need to use their skin for gas exchange because the lungs are sufficient. They have a thick,
scaly skin with keratin that makes it impermeable to water.
Internal fertilization allows animals to mate without having to be in water.
The eggs are encased in leathery shells to protect them from desiccation. This eliminates the need for a
swimming larval stage. Reptiles have eggs with leathery shells. The eggs of birds and egg-laying mammals
have calcareous shells.
Most reptiles are ectothermic. They regulate body temperature by using heat sources in the environment
and by engaging in behaviors that reduce heat loss or heat gain. For example, sunning heats the body.
Remaining in an underground burrow may reduce the effect of extreme surface temperatures.
Snakes do not have legs. Some species of snakes have vestigial pelvic and leg bones, reflecting their
evolution from 4-legged ancestors.
Most reptiles have a three chambered heart.
Birds (Class Aves)
The oldest fossilized bird is Archaeopteryx. It had ancestral reptilian features such as teeth, clawed digits
on the wings, and a long reptilian tail, as well as wings and feathers.
Feathers are modified scales. They insulate against heat loss and allow birds to be endotherms (maintain a
constant body temperature). endothermy allows them to be active when the environmental temperature is
low.
Birds exhibit complex behavior including parental care.
Some anatomical features associated with flight are listed below.
Wings
Feathers
Hollow bones
They have an enlarged breastbone for attachment of flight muscles. The large breastbone can be
seen in the photographs below.
A light, strong beak replaces the teeth and jaws seen in their reptilian ancestors.
One ovary - The gonads are reduced in size during the nonbreeding seasons.
A high metabolic rate provides the necessary energy needed for flight.
A double circulatory system enables high activity levels needed for flight. Birds have a double
circulatory system with 2 atria and 2 ventricles. One atrium and ventricle provide a fast rate of
blood flow through the lungs; the other atrium and ventricle provide a fast flow rate for the rest of
the body, including the flight muscles.
Areas of the brain devoted to vision and motor coordination are well developed, enabling birds to
have keen vision and motor coordination needed for flight. The cerebellum, which functions in
motor coordination, is large in birds.
Mammals (Class Mammalia)
Characteristics of Mammals
Hair insulates the body. Like birds, mammals are endothermic and thus, are active in cold as well as warm
temperatures.
Mammary glands are modified sweat glands that produce milk. Young are born relatively helpless but are
fed milk and cared for by one or both parents. Lactation promotes parent-young bonding.
Mammals have a four-chambered heart and separate pulmonary (lung) and systemic (body) circulatory
systems.
Mammals walk more efficiently than reptiles because their legs are positioned further under their body.
Mammals have a diversity of teeth for different kinds of foods: incisors, canines, premolars, and molars.
Fish, amphibians, and reptiles have teeth that are all similar.
Mammals have an extended period of care for their offspring.
Three bones in the ear- the malleus, incus, and stapes- conduct sound from the tympanic membrane
(eardrum) to the inner ear.
Mammals have well-developed sense organs.
Mammals have a large brain.
Mammals have a high capacity for learning.
Monotremes
Monotremes are found only in Australia and New Guinea.
Unlike other mammals, monotremes lay eggs which the female incubates.
They do not have nipples; milk is secreted to the surface of the skin.
Only the duckbilled platypus and two species of echidnas (spiny anteaters) survive. Both of these species
live in Australia.
Marsupials
Marsupial young begin development in female’s body. They are born very immature and crawl into the
mother’s marsupium (pouch) and attach to nipples. Embryonic development is completed while the young
are in the marsupium.
Today, marsupials are found mostly Australia.
Eutherians (Placental Mammals)
Placental mammals are born relatively mature because a well-developed placenta for nutrient and waste
exchange enables them to remain within the mother for an extended period of development.
Evolution of Vertebrate Circulatory System
Chambers of the Heart
Vertebrate hearts contain muscular chambers called atria (sing. atrium) and ventricles. Contraction of the
chamber forces blood out. Blood flows in one direction due to valves that prevent backflow.
The atrium functions to receive blood that is returning to the heart. When it contracts, blood is pumped into
the ventricle.
The ventricle is the main pumping chamber of the heart. When it contracts, blood is pumped away from
the heart to the body, lungs, or gills.
In the diagrams that follow, arrows represent the direction of blood flow in blood vessels (arteries and
veins). Blood pressure is represented by the thickness of the arrows. Thick arrows indicate high blood
pressure. Blood that is rich in oxygen is represented by red arrows. Blue arrows represent blood that is low
in oxygen after it has passed through the body tissues.
Fish have a two-chambered heart with one atrium (A) and one ventricle (V).
The gills contain many capillaries for gas exchange, so the blood pressure is low after going through the
gills. Low-pressure blood from the gills then goes directly to the body, which also has a large number of
capillaries. The activity level of fish is limited due to the low rate of blood flow to the body.
Circulatory System of Amphibians
Amphibians have a 3-chambered heart with two atria and one ventricle.
Blood from the lungs (pulmonary flow) goes to the left atrium. Blood from the body (systemic flow) goes
to the right atrium.
Both atria empty into the ventricle where some mixing occurs.
The advantage of this system is that there is high pressure in vessels that lead to both the lungs and body.
Circulatory System of Some Reptiles
In most reptiles except crocodilians and birds, the ventricle is partially divided. This reduces mixing of
oxygenated and unoxygenated blood in the ventricle. The partial division of the ventricle is represented by
a dashed line below.
Circulatory System of Crocodilians, Birds, and Mammals
Birds and mammals (also crocodilians) have a four-chambered heart which acts as two separate pumps.
After passing through the body, blood is pumped under high pressure to the lungs. Upon returning from the
lungs, it is pumped under high pressure to the body. The high rate of oxygen-rich blood flow through the
body enables birds and mammals to maintain high activity levels.