In the previous tutorial, we learned about how animals develop, from a small blastula
whose cells then undergo differentiation, allowing for all the complex tissues and organ
systems possessed by that organism.
But how is all of this organized, and how does this organization differ from one group
of animals to another?
Among the different domains of life, we recognize five grades of organization.
First, there is protoplasmic organization.
This layer of organization is exhibited by all unicellular organisms, such as Paramecium.
All life functions are confined within the boundaries of a single cell.
Cellular organization refers to an aggregation of differentiated cells that work together,
such as with Volvox, a motile green algal colony composed of a few thousand cells arranged
along the periphery of a hollow sphere.
Cell-tissue organization refers to the organization of cells into similar layers that perform
a common function, known as tissue.
Organisms with cell-tissue organization, like the hydra, have tissues, but lack organs and
organ systems.
Tissue-organ organization refers to organisms with aggregations of tissues that form organs
and carry out a specific function.
Organisms that exhibit tissue-organ organization, like the Planarians, have tissues and organs,
but lack organ systems.
Finally, organ-system organization refers to animals with organs that work together
to perform functions and become organ systems.
Most animals, like insects, earthworms, mollusks, and certainly all vertebrates, exhibit organ-system
organization.
Organisms within the different grades also have varying types of biological symmetry,
which refers to the balanced proportions of living things.
Spherical symmetry means that any plane passing through the center of an organism divides
it into mirrored halves.
True spherical symmetry is not found in kingdom animalia, but it does exist in some unicellular
organisms, like certain types of bacteria Radial symmetry applies to body forms that
can be divided into mirrored halves by more than two planes, so we are talking about symmetry
around a central axis.
Radial symmetry is found in many cnidarians and echinoderms, such as the hydra, sea anemone,
and starfish shown here, as well as certain sponges.
A variation of radial symmetry is biradial symmetry, where only two planes can produce
mirrored halves.
The comb jellies, for example, look radial, but they have a pair of tentacles and branched
organs which prevent radial symmetry, so these are an excellent example of this type of biradial
symmetry.
Beyond these comparatively more rare types of symmetry, the majority of all animals exhibit
bilateral symmetry.
This means that their bodies are arranged in such a way that a single plane, known as
the sagittal plane, passing through the longitudinal axis, would divide the animal into left and
right mirror images.
Most bilaterally symmetrical animals also show cephalization, meaning the formation
of the head, at the anterior end where most of the complex sensory, nervous, and feeding
structures are located.
Since we are already throwing around terms like sagittal and anterior, let’s go ahead
and review all the planes and directional terms pertaining to animals, since we’ll
need this information moving forward.
It will be a good idea to memorize this important terminology.
First, the posterior end, or back end of an animal, is opposite the anterior end, or front
end, and the two segments are separated by the transverse plane.
Meanwhile, the frontal plane separates the dorsal, or top end, and ventral, or bottom
end of the animal.
And remember that the sagittal plane separates the animal into two halves which are mirror
images.
In vertebrates, the term pectoral refers to the chest region associated with the anterior
pair of appendages, and the term pelvic refers to the hip region associated with the posterior
pair of appendages.
Finally, some animals, like many sponges and placozoans, show no symmetry as adults and
can therefore be called asymmetrical.
Their body parts are not arranged in any discernible pattern.
Sponge bodies are complicated, as we’ll soon see, but as adults many sponges show
no clear planes of symmetry.
And with that, we are finished covering some basic information on cladistics, animal development,
and organization.
These are important zoological concepts that needed to be understood before diving into
the minute details of all the different animal phyla, which is the primary focus of this
series.
So with this prerequisite information complete, let’s move forward and start learning about
different types of animals.