List the characteristics that define animals and distinguish them from other organisms.
Animals are multicellular, heterotrophic eukaryotes. In contrast to the autotrophic nutrition of plants
and algae, animals must take into their bodies preformed organic molecules; they cannot construct them
from inorganic chemicals. Most animals do this by ingestion--eating other organisms or organic material
that is decomposing.
Animal cells lack the cell walls that provide strong support in the bodies of plants and fungi. The
multicellular bodies of animals are held together by structural proteins, the most abundant being collagen. In
addition to collagen, which is found mainly in extracellular matrices, animal tissues have unique types of
intercellular junctions--tight junctions, desmosomes, and gap junctions--that are composed of other
Also unique among animals are two types of tissues responsible for impulse conduction and movement:
nervous tissue and muscle tissue. A few key features of life history also distinguish animals. Most animals
reproduce sexually, with the diploid stage usually dominating the life cycle. In most species, a small
flagellated sperm fertilizes a larger, nonmotile egg to form a diploid zygote. The zygote then undergoes
cleavage, a succession of mitotic cell divisions. During the development of most animals, cleavage leads to
the formation of a multicellular stage called a blastula, which in many animals takes the form of a hollow
ball. Following the blastula stage is the process of gastrulation, during which layers of embryonic tissues
that will develop into adult body parts are produced. The resulting developmental stage is called a gastrula.
Some animals develop directly through transient stages of maturation into adults, but the life cycles of many
animals include larval stages. The larva is a sexually immature form. It is morphologically distinct from the
adult stage, usually eats different food, and may even have a different habitat than the adult, as in the case of
a frog tadpole. Animal larvae eventually undergo metamorphosis, a resurgence of development that
transforms the animal into an adult.
The transformation of a zygote to an animal of specific form depends on the controlled expression in
the developing embryo of special regulatory genes called Hox genes. All eukaryotes have genes that
regulate the expression of other genes. And many of these regulatory genes contain common "modules" of
DNA sequences called homeoboxes. But among eukaryotes outside the animal kingdom, homeoboxes are
not found in homeotic genes, those regulatory genes that function in the development of body form. So far,
genes that are both homeobox-containing in structure and homeotic in function--Hox genes--have been
discovered only in animals. And all animals, from the simplest sponges to the most complex insects and
vertebrates, have Hox genes containing homeoboxes of DNA sequences that are clearly related. In general,