Biomedical Engineering Reference
In-Depth Information
Chapter 25
Overview of Class Animalia
“If you think you are worth what you know, you are very wrong. Your knowledge
today does not have much value beyond a couple of years. Your value is what you
can learn and how easily you can adapt to the changes this profession brings so
often.”
Jose M. Aguilar
Here is the ancestral hierarchy leading to Class Animalia.
Eukaryota (nucleated cells)
Unikonta (one flagellum)
Opisthokonta (flagellum located at the posterior pole)
Animalia (individual organisms develop from a blastocyst)
The highest class, Class Eukaryota, contains organisms with nucleated cells,
and all the descendent subclasses will contain organisms that have nucleated
cells. Furthermore, all classes that are not subclasses of Class Eukaryota,
will not contain organisms that have nucleated cells. For example, Class
Bacteria, which is not a subclass of Class Eukaryota, contains no organisms
that have nuclei.
Class Unikonta, one of the two major subclasses of Class Eukaryota, is
characterized by organisms that have a single flagellum. Class Opisthokonta
is one of the subclasses of Class Unikonta. In Class Opisthokonta, the flagel-
lum is located at the posterior pole. Class Animalia is a subclass of Class
Opisthokonta. Animals have their own defining property (i.e., development
from a blastula), along with inherited features descending from their ances-
tral superclasses. Hence, members of Class Animalia have nucleated cells,
with one flagellum protruding from a posterior pole, as seen in sperm cells.
Animals are thought to have evolved from gallertoids; simple, spherical
organisms suspended in ancient seas. The living sphere was lined by a single
layer of cells enclosing a soft center in which fibrous cells floated in extracellular
matrix. The earliest fossil remnants (seabed burrows and tracks) of these early
animals date back to about 1 billion years ago. The classes of animals that we
would recognize today (corresponding roughly to the chapter divisions of animals
