Chapter 24 - Introduction to Animals
Sponges, Cnidarians, Flatworms, Roundworms
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.
The cells are held together by protein structures called junctions that extend from one cell
to another. An abundance of extracellular proteins also support the cells.
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.
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. Gametes
are produced by meiosis. A typical animal life cycle is shown below.
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
The evolutionary sequence progressed from asymmetrical animals to radial and then to
bilaterally symmetrical animals.
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 major organs derived from
it.
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 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.
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.
A complete gut has two openings, a mouth and an anus. It is sometimes referred to as a
tube-within-a-tube.
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.
Major Trends in Evolution
There are approximately 34 phyla of animals. We will consider the 9 phyla shown below.
1. multicellularity
2. development of tissues, first none (sponges), then 2 (cnidarians), then 3
3. development of symmetry, first none (sponges), then radial (cnidarians), then
bilateral
4. development of a gut, first none (sponges), then sac-like (cnidarians, flatworms),
then complete
5. development of a body cavity, first none (flatworms), then a pseudocoelom
(roundworms), then a coelom
6. development of segmentation; segmentation evolved in protostomes (annelids and
arthropods) independently of that which evolved in deuterostomes.
Summary of Evolutionary Trends
Sponges
Symmetry
Gut
asymmetrical no gut
Cnidarians
radial
saclike
Flatworms
bilateral
saclike
Roundworms
bilateral
Coelom
Embryonic Germ Layers
none
2, (tissues, no organs)
Acoelomate
complete Pseudocoelomate
3, (tissues and organs)
Mollusks
bilateral
complete
Coelomate
Annelids
Arthropods
Echinoderms*
Chordates
*Although echinoderm adults are radial, they are grouped with bilateral animals because
their larvae are bilaterally symmetrical and they evolved from bilaterally symmetrical
ancestors.
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 bluish-colored 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.
Some Anatomical Terms
You will not be required to learn these terms for an exam, however, this list can serve as
a reference.
Ventral - the underside
Dorsal - the back of the animal; the side opposite the ventral side. The vertebral column
of vertebrates is no the dorsal side of the animal.
Lateral - toward the side
Median - toward the middle
Anterior - the head end
Posterior - the end opposite the head end
Caudal - toward the tail
Cranial - toward the head
Longitudinal - along a line from the head to the tail
Transverse - along a line that is 90° to the longitudinal axis (see above)
Superficial - shallow
Pectoral - toward the forelimbs
Pelvic - toward the rear limbs
Distal - far from
Proximal - near
Sponges (Phylum Porifera)
Click here to view a diagram of evolutionary trends in the animal kingdom.
Sponges are multicellular but are thought to have evolved from unicellular protists.
Sponges are sessile (immobile) filter feeders.
Click on the photographs to view enlargements. Click "Back" to return here.
Structure of Sponges
Although sponges have cells with specialized functions they do not have tissues. There is
no endoderm, mesoderm, or ectoderm.
The cells of a sponge are arranged into an inner and an outer layer with amoeboid cells
crawling between the two.
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 color cells
and is digested within the cell (intracellular digestion) or is passed to amoeboid cells for
digestion.
Choanocytes (collar cells) are similar to protists called choanoflagellates. These
unicellular protists are thought to be the ancestors of sponges.
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.
Amoeboid cells also produce gametes (by meiosis).
The photograph below is a cross section of a sponge (Grantia) magnified 400 times.
Notice the outer epidermal cells, pores, and choanocytes.
Reproduction
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.
Comments
Sponges resemble a colony of protozoans more than multicellular animals; they have no
true tissues.
Cnidarians (Phylum Cnidaria)
Some examples of Cnidarians are hydra, jellyfishes, corals, sea anemones, and
Portuguese man-of-wars.
Click here to view a diagram of evolutionary trends in the animal kingdom.
The photograph below is a hydra (40X).
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.
Below: Hydra cross section (X100). Note the two cell layers in both photographs below.
Below: Hydra longitudinal section (X100).
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.
VIDEO - Hydra movement, X40, X100 (3.59 MB)
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.
Examples
Hydra and Relatives (Class Hydrozoa)
This group includes freshwater and marine species.
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 and produces a larval (juvenile) form called a planula
(plural planulae). The planula is ciliated and capable of swimming to a new location.
Planulae settle down and develop into a polyps.
Obelia is a colony of polyps enclosed by a protective layer composed of chitin. New
polyps are produced asexually by budding. Budding from the polyp also produces
Medusae. The medusae function in sexual reproduction by producing sperm and eggs.
The zygote develops into a ciliated larvae (planula) which settles down and develops into
a polyp colony. The life cycle of obelia is shown below.
Below: Obelia
Below: Obelia (X 40)
Below (2 photographs): Obelia medussae (X100)
Below: hydrozoan jellyfishes (preserved specimens); Gonionemus; Polyorchis
The Portuguese man-of-war (below) is a colony. The original polyp becomes a float.
Other polyps become specialized for feeding or reproduction. The feeding polyps each
contain a single long tentacle.
Sea Anemones and Corals (class Anthozoa)
Sea anemones and corals have polyps and no medusae.
Below: Sea anemone (preserved specimen)
Corals are colonial and secrete calcium carbonate skeletons. Coral reefs are the
accumulation of these skeletons.
Below: Close-up of the calcium carbonate skeleton secreted by the northern coral
(astrangia).
Many corals have photosynthetic algae living within their tissues. Photosynthesis
provides these corals with an additional source of food.
Jellyfishes (Class Scyphozoa)
The polyp stage of jellyfishes is small; the medusa stage is dominant.
Practice Exercises
1. Draw a cross-section of the following kinds of animals:
acoelomate
pseudocoelomate
coelomate
(click here to review notes on these body plans)
Label the following on your drawings:
ectoderm
mesoderm
endoderm
pseudocoelom
Coelom
2. Draw an evolutionary tree of animals using the 9 phyla listed below:
annelids, arthropods, chordates, cnidarians, echinoderms, flatworms, mollusks,
roundworms, sponges
Identify the type of symmetry, body cavity, and embryological development that each
phylum of animal exhibits. Use the list below.
Symmetry
radial
bilateral
asymmetry
Body Cavity
acoelomate
pseudocoelomate
coelomate
Embryonic Development
protostomes
deuterostomes
Flatworms (Phylum Platyhelminthes)
Click here to view a diagram of evolutionary trends in the animal kingdom.
Characteristics
Flatworms are flattened, bilaterally symmetrical animals.
They have 3 embryonic tissue layers (ectoderm, mesoderm, 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)
Example: Dugesia - a freshwater planarian
Planarians have a branching sac-like gut (one opening).
The main function of the excretory system is for water regulation. It consists of two
structures called protonephridia. Each protonephridium contains flame cells that move
excess water into tubes that open to the outside.
Planarians have a head region with sense organs. The nervous system of Dugesia is
somewhat more complex than the nerve net of Cnidarians. It consists of a brain and nerve
cords arranged in a ladder-like configuration.
Planarians have ocelli (eyespots) allow the presence and intensity of light to be
determined. These structures are covered but have an opening to one side and forward.
They can tell the direction of light because shadows fall on some of the receptor cells
while others are illuminated. 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.
VIDEO - Planarian movment (Dugesia) (1.83 MB)
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 host.
Liver flukes are found in vertebrate livers.
Nearly half of people in the tropics have blood flukes. Schistosomiasis is a blood fluke
that afflicts one out of twenty 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 scolex X 40
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 secondary hosts are usually pigs or cattle. Tapeworms can be passed to humans in
undercooked meat, especially pork.
The photographs below show the scolex and proglottids at increasing distances from the
scolex. Those segments closest to the scolex (on the right) are the smallest. Those furthest
away (on the left) become filled with zygotes, break away, and pass out with the feces.
Roundworms (Phylum Nematoda)
Click here to view a diagram of evolutionary trends in the animal kingdom.
Characteristics
Roundworms have bilateral symmetry with a pseudocoelom (tube within a tube).
They are very abundant and occur almost everywhere.
They are not segmented. Do not confuse them with earthworms.
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.
Rotifers (Phylum: Rotifera)