BIO 120
Field Natural History
Spring
1
LECTURE 16
Introduction to the Animal Kingdom
(Evolution of functional morphology)
I.
What is an Animal?
1. We often think of mammals or any vertebrate (= backbone), but these represent only a
small fraction of the animal kingdom.
2. Although nearly 1.5 million species of animals have been described, 95% of them are
invertebrates - animals without backbones.
A. The characteristics shared by all animals.
1. Animals are multicellular.
a. Specialized groups of cells with a similar function -tissues. E.g muscles.
b. Different tissues combine to form organs. e.g. the heart
c. Various organs that carry out major body functions are called organ systems.
2. Animals are heterotrophic.
3. Animals are motile – Animals are capable of locomotion at some stage of their life.
4. Animals engage in sexual reproduction.
a. Animals consist of diploid cells that contain two sets of chromosomes.
b. There is no counterpart in animals to the alternation of generations found in plants.
II.
Elements of a Body Plan
1. Three of the most important properties that distinguish the body plans of animals are
a. Symmetry, b. Body cavities and c. Segmentation.
A. Body Symmetry: The arrangement of the Body Parts.
1. An object has symmetry if it can be bisected into two mirrored halves.
2. Animals exhibiting radial symmetry usually have a cylindrical body composed of
similar parts that are arranged in a circular fashion around a central axis.
a. Animals are either sessile (sedentary) e.g. sea anemone, or slow moving e.g. jellyfish.
b. Adaptive since sensory information and food matter from all directions.
3. Bilateral symmetry is exhibited by animals that are more motile.
a. When a plane can bisect an animal into mirrored, left and right halves.
b. Found among animals that actively search for food and shelter, e.g insects and animals.
c. Concentration of sense organs and nervous tissue at the leading end of the body to form
a “head”. This evolutionary phenomenon is known as cephalization.
d. Allows animal to gather information about environment into which they are heading.
B. Internal Cavities – the spaces within.
1. Nearly every animal on earth has at least one internal chamber in which food matter is
collected. This chamber is called the digestive tract.
a. A blind sac, e.g. sea anemone, hydra, jellyfishes, coral. They are radially symmetric.
b. In other animals, it may consist of a complete internal tube, which we call the gut.
2. Why is a digestive tract important? Diffusion limits the size of the organism.
a. Demand for energy and oxygen greater in a motile organism. Need constant supply of
energy and oxygen, and removal of waste products like carbon dioxide and lactic acid
BIO 120
Field Natural History
Spring
2
3. In addition to digestive tract, most animals have at least one other major body cavity that
develops between the digestive tract and the body wall.
a. Like body symmetry, the difference in the nature and the location of the body cavity
distinguish major groups of bilaterally symmetric organisms.
b. The cavity is called a coelom (Greek koiloma – cavity.)
c. There are 3 major types of body cavities.
4. No coelom (acoelomate). – no body cavity.
a. this is characteristic of a group of organisms called flatworms (phylum
Platyhelminthes) flatworms , flukes (parasites) and tapeworms.
b. They have a gut cavity, but no body cavity.
5. Flatworms are thin because of their acoelomate body design.
a. They lack any circulatory system, so dissolved substances such as oxygen, carbon
dioxide, and nutrients must pass through the solid body by diffusion.
6. False coelom or psuedocoelomates. False cavity.
a. This is the typical body plan of the roundworms. Phylum Nematoda. Nematodes
b. The body plan of roundworms is that of a tube within a tube.
c. The evolutionary similar to the coelomates.
7. The coelomates – those animals that possess a body cavity.
a. The main distinction between the psuedocoelomates and the coelomates is the origin
of the tissues in the body cavity.
b. Coelomates include the phylum Chordata (backbones), the phylum Arthropoda (no
backbone, but an exoskeleton) and the phylum Mollusca (shellfish, snails,
octopuses).
8. The three advantages to having a body cavity.
a. Circulation – Fluids that move within the body cavity can serve the function of a
circulatory system. This makes larger bodies possible or faster ones.
b. Movement – Fluid in the body cavity makes the animal’s body rigid. This is called a
hydrostatic skeleton. It is essentially a sealed, fluid filled tube.
• This increases the effectiveness of an animal’s muscular contractions.
• In addition to the muscles working against the water or a substrate, the muscles
have something to work against in the animal’s body.
c. Organ function – the body muscles can move and distort the outer body wall, but the
internal organs are protected from being squished by the coelomic space.
• Digestion was no longer a function of the movement of the animal.
• Digestion could proceed at a constant or faster rate, supplying more energy to the
animal, enabling it to go faster, or become larger.
• The coelomic space also allowed the development of other organ systems, like a
separate circulatory system.
In summary, the body cavity allowed for larger, faster and more complex animals to
evolve.
C. Segmentation: Dividing the body into repeating segments.
1. Earthworm body is constructed of repeating subunits or segments.
a. Each segment is separated by a partition called a septum.
b. Each segment has its own set of muscles which operate independently, its own
excretory organs, nerves, etc.
BIO 120
Field Natural History
Spring
2. Segmentation was the most novel developments in evolution and the first animals to
evolve with this body plan were the annelid worms, phylum Annelida.
a. Important because growth of an organisms was simplified by repeating units.
b. Allowed for specialization of different segments for different functions.
c. The most important is the specialization of different segments for locomotion.
3. Segmentation underlies the body plan of all coelomates.
a. Not just annelids, but arthropods (e.g. lobster) and chordates – the vertebrates.
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