Biomedical Engineering Reference
In-Depth Information
in this topic) were all living during the Cambrian period, about 540 million years
ago.
As the gallertoids evolved to extract food from the seabed floor, they flat-
tened out. The modern animals most like the gallertoids are the placozoans,
discovered in 1833, plastered against the wall of a seawater aquarium. These
organisms are just under a millimeter in length and are composed of about
1000 epithelial cells. With the exception of being flat, rather than round,
they resemble the gallertoids, with an outer lining of cuboidal cells, and an
inner gelatinous matrix holding a suspension of fibrous cells.
What are the characteristic properties that set animals apart from non-
animals? Animals are multicellular and form specialized tissues and organs.
The most characteristic organ in animals is the gastrointestinal tract. A
gastrointestinal tract is present in all eumetazoans (i.e., the true animals; see
Glossary item, Metazoa). The gastrointestinal tract is only absent from the
most primitive animals, Class Parazoa, comprising the placozoans and the
porifera (sponges). The characteristic presence of a gastrointestinal
tract
explains why animals are sometimes referred to as traveling stomachs.
The single feature that taxonomists rely upon to distinguish animals from
all other forms of life is the blastula. The blastula is an early embryo consisting
of a sphere of cells enclosing a central cavity known as the blastocoel. Like the
gastrointestinal tract, the blastula is present in all eumetazoan animals.
Three classes within Class Eukaryota (organisms with nucleated cells)
are multicellular, with specialized cells and organs: classes Plantae, Fungi,
and Animalia. These three classes account for virtually every organism that
we can see with the naked eye. Consequently, prior to the invention of the
microscope (about 1590 A.D.) and the advent of scientific observations of
the microscopic world (about 1676 A.D.), these three classes accounted for
the totality of the observed living world.
Fungi, plants, and animals can be distinguished by the method by which
their included organisms develop. Fungi develop from spores. Plants, like
animals, develop from embryos, but they do not have a blastula phase (i.e.,
the plant embryo does not develop a cavity). Animals develop from an
embryonic blastula.
If you are a skeptic (and you should be), you may be wondering why the
blastula serves as the defining feature that separates animals from other com-
plex, multicellular organisms. For example, plants are complex organisms,
with specialized tissues and organs. Plants, like animals, develop from
embryos. Though it is easy to observe morphologic differences between plants
and animals, it is difficult to imagine how the presence or absence of a blastula
would account for the fundamental difference between plants and animals.
The importance of the blastula to Class Animalia may turn out to be
something of a red herring. To form a blastula from a solid embryo, animals
must have a fundamental property that is lacking in Class Fungi, Class
Plantae, and in all eukaryote classes other than Class Animalia. This special
