Can you help me understand this Biology question?
Exercise 5: The Animal Kingdom:
or Even if it looks like blob of jelly…..it may still be an animal!
If someone were to ask you what a giraffe looks like, you would most likely be able to bring an image to
mind, and then describe it to the person asking the question. Your mental image of the giraffe would most
likely be one that was visual; we human tend to rely on our eyes and the things we can see directly. But
these direct visual observations are not the only ones that are available to us when we describe an
organism. We could also describe an organism in terms of the way it feels, the way it smells or the
sounds that it makes. But in the end, we all think we know what an animal looks like and how we would
It turns out, that identification of animals, and many other organisms is not necessarily straightforward. In
the last exercise you spent time finding and identifying organisms that are invisible to our naked eyes and
which fell into a variety of different classification categories. In this lab exercise, you will find that even
when visible, and when called by a name that we think of as familiar, it is not just how something looks on
the outside that determines how it is classified.
Look at the two organisms in the pictures below. As a biologist, it would be your job to identify
and classify organisms such as these. But where would you start? Could you tell simply based
on their looks whether these would be classified as plant, animal, fungus or protist?
Many of the organisms that we call animals are obvious to most of us. Many of the species that
biologists have named belong to the animal group because they are usually easy to see, and
they have been studied extensively over the past several hundred years. However, there are
some organisms that defy classification simply by the way that they look. To classify some
organisms we must look beyond the surface.
What is an animal?
The word “animal” comes from the Latin word animal, (for which animalia is the plural) and
means “vital breath or soul”. Animals form a major group within the domain Eukarya, the
kingdom Animalia. Most animals are defined by sharing the same group of characteristics:
surrounding the cells, but no cell walls. Most animals (although not all) are multi-cellular,
that is, they are made up from more than one cell, and in most cases, the different cells
carry out different functions for the organism.
obtain nutrients for growth and development. Most animals feed by ingestion; they take
in whole parts of other organisms and digest them inside their bodies.
and muscle tissue.
change from their early life as an embryo, to their final adult form.
In order to identify and classify animals, we need to look at characteristics that would permit us
to organize them into groups. Ideally, we would like to look for characteristics that are common
to the largest number of organisms, and then to find characteristics which fit progressively fewer
and fewer of the organisms in question. These sets of characteristics could then be used to
construct a dichotomous key (similar in concept to the one used in last week’s exercise) that
could be used to identify newly discovered animals. In this exercise, you will be looking at
certain features of body organization that provide clues to the evolutionary ancestry of the
animals. These features include
Animals differ in their degree of organization and complexity. The very simplest animals, those
from phylum Porifera, are composed of cells that are only loosely interconnected, but can exist
and function independently even if they serve a specific function within the organism. In other
animals, the cells come together to form tissues, groups of cells that are similar in structure and
function and which work together to perform a specific activity. All animals except those in
phylum Porifera have defined tissues.
All animals except the Porifera and the Cnidarians have tissues that work together to perform
specific activities forming structures called organs. Organs consist of two or more different
types of tissues organized into these units that have a characteristic size and shape. Example of
organs in humans would be heart, brain, lung and skin.
Embryonic Tissue Layers
The embryonic tissues layers of an animal are also known as germ layers and give rise to all of
the tissues and organs in the adult animal. During development, the zygote (the first cell formed
from the union of an egg and sperm cell) develops in such a way that different layers of tissues
are formed. The innermost layer is the endoderm that gives rise to organs such as the digestive
tract and lungs; the mesoderm is a middle layer that gives rise to muscles, the circulatory and
skeletal system; the ectoderm is the outer layer which forms the nervous system and the
epidermis of the organism.
The Porifera do not have tissues, and thus do not have these layers. The Cnidarians have only
two germ layers, ectoderm and endoderm. All other animals have all three germ layers.
An organism’s body plan is the blueprint for the way the body of the organism is laid out. An
organism’s symmetry, the number of body segments and the number of limbs are all part of its
Symmetry describes the layout of the body parts on either side of a dividing line or plane.
Asymmetrical body plan: the
animal body cannot be divided
into two equal halves along any
Radial body plan: any plane
passing through the center of the
body divides the animal into two
Bilateral body plan: the animal
body is arranged around a single
plane that is parallel to the length
of the animal and divides the
body into right and left halves that
are mirror images externally.
Figure 1: Types of Animal Body Symmetry
The simplest animals do not show any regular symmetry to their body plan and are therefore
described as being asymmetrical. These animals are irregular in their shape and size. The only
phylum of animals that is asymmetrical are the sponges, phylum Porifera.
Organisms with radial symmetry have body parts that are arranged in a regular, repeating
pattern around a central axis (think wagon wheel!), or are completely symmetrical about a
central axis (like a frisbee). These organisms resemble a pie where several planes can be cut
through the organism to produce roughly identical pieces. These organisms do not have right or
left sides, but only have a top and bottom surface. Radial symmetry allows an organism to
detect things coming towards it from any direction. Organisms that are radially symmetrical are
those from phylum Cnidaria and phylum Echinodermata
In bilateral symmetry, only a single plane will divide an organism into roughly mirror images
(this is with respect to external appearance only, not internal structure). Most animals are
bilaterally symmetric, including humans. The bilateral symmetry supports the formation of a
central nerve center that contributes to cephalization, the organization of the organism around
a head region that contains sensory organs.
Body plans in most species appear to be controlled by a set of genes known as homeobox
genes. A particular group of these genes called Hox genes, function in patterning the body axis
of the organism. The genes determine where limbs and other body segments will grow in the
developing organism. The most commonly seen body plan is the tetrapod, organisms that have
four feet, legs or leg-like appendages. Tetrapods include all mammals, birds, amphibians and
reptiles (yes, even snakes are tetrapods by descent). Some of the groups such as the
cetaceans (whales) and bats have been modified (the front legs are now flippers or wings) but
they are still tetrapods.
Cephalization refers to the concentration of nervous tissue at one end of an organism. Over
many generations, the evolutionary process results in the production of a head region with
sensory organs. Cephalization is generally associated with the bilateral symmetry. The
association of cephalization and bilateral symmetry resulted in animals having sensory organs
facing the direction of movement, allowing the animal to better assess the conditions in the
direction of movement. These bilaterally symmetrical animals are consequently more active and
efficient in seeking food and mates, and also in avoiding predators.
Segmentation is the division of the body along its length into a series of semi-repetitive
segments. Segmentation results in more effective body movement, and the ability of
differentiation of individual segments into specialized structures that can perform different
functions. Annelids, arthropods and chordates are all segmented. In the annelids and
arthropods, the segmentation can be seen externally, while in the chordates the segmentation is
reflected internally in the structure of vertebrae, muscles and nerves.
All animals have to perform some type of digestion process in order to use food to obtain
energy. In the simplest cases, the digestion is completely intracellular (inside of individual
cells). Single celled organisms rely on intracellular digestion, as do some multi-cellular
organisms such as sponges. Sponges obtain tiny particles of food from water that passes
through its body; cells catch and engulf the food particles, and then digest it inside the cell.
Intracellular digestion works well for simple organisms, but does not meet the needs of more
complex ones. Animals such as jellyfish and flatworms have simple sac-like cavities in which
digestion can occur outside of the cells, but inside of a confined area. These animals have a
definite mouth that leads to this sac-like cavity. The sac is lined with cells that secrete enzymes
that break down food within the cavity. This extracellular digestion starts the process, and the
digested foods are then engulfed by cells where the digestion process is completed. Waste
products are excreted through the mouth, the same opening where the food entered.
The most complex digestive systems are one-way digestion tubes with an opening at each
end. In these cases, the food is completely digested through extracellular processes. Food
moves into the organism through the mouth, through a series of organs in the digestive tube
that may be specialized to increase the efficiency of the digestive process. Food moves through
this one-way tube and its various organs until it has been digested and nutrients have been
absorbed. Wastes left over from the digestion process are released at the opposite end of the
tube from where they entered (the anus).
In general, a body cavity is the fluid-filled space between the digestive tract and the outer body
wall of an animal that has tissues and organs. The body cavity is also known as the coelom.
Depending upon the type of body cavity, animals can be placed into one of three groups.
Aceolomate animals, such as the flatworms, do not have a body cavity. Their organs have
direct contact with the body wall. Semi-solid tissues between the gut and the body wall hold
their organs in place.
Figure 2: Body Cavity Types
Pseudocoelomate animals have a “false body cavity”. A pseudocoelom is not completely lined
with tissue derived from the mesoderm. Organs are held in place, but not as well organized as
in animals with a true body cavity.
Coelomate animals have a “true coelom” or body cavity that is filled with fluid and completely
lined by tissues derived from the mesoderm. The complete mesoderm lining allows organs to be
attached to each other, suspending both the gut and the organs. Most bilateral animals are
Looking at the three groups of worms shown in the diagram above, one can see the differences
between the three types. The flatworms of phylum Platyhelminthes provide an example of the
acoelomate animals. The body is filled with tissue that contains the organs. Roundworms
(phylum Nematoda) are the major group of pseudocoelomate animals. All other animals
including the segmented worms of phylum Annelida possess true coeloms.
Skeletal systems in animals are generally divided into three types: external or exoskeleton,
internal or endoskeleton, and fluid based or hydrostatic. The skeleton provides physical
support and allows the organism to move. In some cases the skeleton may also provide
physical protection for the organism.
A hydrostatic skeleton is a structure found in many soft-bodied animals that consists of a fluidfilled
cavity surrounded by muscles. The pressure of the fluid and action of the surrounding
muscles can be used to produce movement. Animals such as worms use their hydrostatic
skeletons to change their body shape from long and thin to shorter and wider as they move
forward. Hydrostatic skeletons are found in cnidarians, echinoderms, annelids, nematodes,
octopuses, and crabs when they have recently molted and lost their external shell.
Exoskeletons are found in many organisms of the phylum Arthropoda. The exoskeleton is
composed of primarily of a polysaccharide called chitin that forms a hard, shell-like covering
on the exterior of the organism. The exoskeleton is not living tissue, and thus cannot grow with
the organism. The organism must shed it skeleton to grow, in a process known as molting or
ecdysis. The arthropod secretes a new exoskeleton which hardens around the organism. The
exoskeleton provides structural support and protection on land, and prevents water loss by
evaporation. Muscles are attached to internal projections of the exoskeleton that allow the
animal to move.
Endoskeletons consist of rigid structures made of either cartilage or bone. These structures
are found inside the muscles and are connected to the muscles by tendons that allow the
muscles to move the organism. In some organisms a component of the endoskeleton is a
backbone composed of a series of segments called vertebrae that enclose the spinal cord. The
presence or absence of a backbone forms the basis of dividing animals into two groups, the
invertebrates (which lack a backbone) and the vertebrates (which have a backbone)
Appendages are seen in a number of phyla, but the most significant advancement of
appendage structure appears in the arthropods where the appendages have distinct segments
or joints that allow them to be used for movement, feeding, reproduction, sensory organs and
flight. Much of the success of insects and other phyla with jointed appendages can be attributed
to the presence of these appendages that have evolved into specialized structures that allow a
wide diversity of tasks to be accomplished. Jointed appendages first appear in the arthropods,
and are also present in the chordates.
Notochord, Dorsal Hollow Nerve Chord, Pharyngeal Gill Slits and Post-Anal Tail
The most complex group of animals, the chordates, is characterized by the presence of four
major characteristics at some time during their lifecycle. The characteristics may only be present
during the larval or embryonic stage of the organism’s life cycle. The characteristics include the
to the body cavity
digestive tract and the outside of the body
the longitudinal axis of the body, dorsal to the notochord
in humans as the coccyx (tailbone) or in most other mammals as a wagging tail.
Figure 3: Lancelet Structure
Members of phylum Chordata may be either invertebrates (such as the tunicate in the left
picture on the first page of this exercise) or vertebrates (possessing a vertebral column) such as
humans. The lancelet in Figure 3 is a member of a group of primitive chordates that provide
information for tracing how vertebrates have evolved and adapted. They display all of the basic
characteristics of the chordates, with nerve cord running along the back, pharyngeal slits and a
tail that runs past the anus. Unlike the vertebrates, the dorsal nerve cord is not enclosed in
bone, but is protected by a simpler notochord made up by a cylinder of cells that are closely
packed to form a tough rod.
(By the way, going back to the first page of this exercise, the photo on the left is of an animal
called a tunicate that is part of the animal kingdom, and the phylum Chordata, the same phylum
where we humans are grouped. The photo on the right is of a red alga that belongs to the
On the Skill Check worksheet, there are two different activities you must complete for
Activity 1: You will find a table that will help you organize the information found so far in
this exercise. Each of the major animal phyla is listed, along with the different scientific
characteristics that are used to describe animals. For each of the characteristic
categories in the left hand column, use the information presented in this exercise to
determine how the characteristic applies to each of the animal phyla. This chart will be
turned in as part of the lab exercise. All of the information you need to complete the chart
is in the reading for this exercise. So read carefully in order to complete the chart.
Activity 2: Animal Kingdom representatives.
In the lab, you will find groups of organisms that represent each of the different phyla.
You are going to need to do some research to figure out what animals fit into each
phylum. Your goal will be to find representatives of the different animal phyla. You can
use a textbook to do this, or you can do some research online to find the information. For
each of the phyla named in the table, you should identify an organism that fits into the
1. In the table, under the phylum name, write the full scientific name (genus and
species) of the organism you have identified.
2. Then fill in the remainder of the table on the Skill Check sheet for activity 2. In the
second column of the table, list any characteristics of the organism that could be
used to aid in identification of the organism. This means that you want to talk
about the characteristic information in Activity 1. Then describe the organism in
terms of obvious characteristics that you can see.
3. In the third column, you may either insert a picture from online of the organism, or
you may make your own drawing of the organism based on a photo or photos you
can see online. If you paste in a photo from an online resource, you must include
the link to the website where you obtained it.
Although this exercise may seem simple on the surface, you will have to spend some
time finding the information necessary to thoroughly complete it.