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:
- Multi-cellular: Animals are composed of eukaryotic cells that have a plasma membrane
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.
- Heterotrophic: Animals consume their food, feeding on pre-made organic materials to
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.
- Exhibit movement: Most animals are capable of motion due to the presence of nervous
and muscle tissue.
- Exhibit embryonic development: Animals go through a process of development or
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
- Cellular organization
- Number of tissue layers in the embryo
- The body plan symmetry
- Type of digestive tract
- Type of body cavity
- The type of skeleton (if any)
- The presence of jointed appendages
- Presence of notochord, dorsal hollow nerve chord and post-anal tail
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