Figure 11.1: Animals are heterotrophs, meaning they must eat to get molecules necessary
for their growth and energy. ( 9 )
Classification of Animals
Recall that each kingdom of life, including the animal kingdom, is divided into smaller
groups called phyla based on their shared characteristics. For example the phylum Mollusca
largely consists of animals with shells like snails and clams. Although modern classification
is also based on looking at molecular data, such as DNA sequencing, animals have long been
classified in their current phyla largely by their physical characteristics.
One example of a physical characteristic used to classify animals is body symmetry. In
radially symmetrical organisms, such as sea stars, the body is organized like a circle
(Figure11.2). Therefore, any cut through the center of the animal results in two identical
halves. Other animals, such as humans and worms, arebilaterally symmetrical,meaning
their left and right sides are mirror images.
Animals are also often classified by their body structure. For example, segmentation, the
repetition of body parts, defines one phylum of worms (Figure11.3). Animals that have
a true body cavity, defined as a fluid-filled space, and internal organs are also classified in
separate phyla from those animals that do not have a true body cavity. Finally, the structure
of the digestive system of animals can also be used as a characteristic for classification.
Animals withincomplete digestive tractshave only one opening in their digestive tracts,
while animals withcomplete digestive tracthave two openings, the mouth and anus.