Animal Diversity

Besides their names, what do horses, and seahorses, have in common? Although they look vastly different, they are both animals. Generally, we define animals as multicellular organisms that are motile, meaning capable of independent motion, consume organic matter for energy, and are capable of sexual reproduction. Species have evolved their different traits with specific functions to maximize their fitness in their habitats like lungs and gills for example. A good rule of thumb is that form follows function, meaning that the function of a feature can be predicted by its structure and appearance. For example, different finch species in the Galapagos Islands have evolved different beaks for specific diets like thin beaks to consume nectar and thick beaks to crack open large seeds. Thus, we can predict the diets of a species by observing structures involved with feeding.

Certain features mark branching points on the evolutionary tree and this is true all the way up throughout the whole tree of animal life. If we look at the basic structure of the evolutionary tree of animals, we can see that it is split into many different phyla with each phylum defined by major shared characteristics. Let's walk through these briefly.

The first major phylum is Porifera, simply meaning pore bearing. Stationary as adults, Porifera include filter feeders such as sponges. Lack of symmetry and distinct tissue layers separates Porifera from other groups and gives them enormous regeneration capacity where a whole organism can regrow from a small group of cells.

The next phylum, Cnidaria, include sea anemones and jellyfish and this group is characterized by their tentacles with specialized cells in the tips which they use to sting and subdue prey. Cnidarians are radially symmetric since they can be divided into equal parts along a central axis and possess two tissue layers, endoderm on the inside and ectoderm on the outside. This enabled Cnidarians to develop a simple gut with a single opening.

The Platyhelminthes, literally meaning flatworms, is our next group. Flatworms exhibit bilateral symmetry which means they have mirror image halves along a single axis. Bilateral symmetry enabled cephalization which is a concentration of sensory organs near the anterior end or head where these animals will continuously interact with their surroundings. Cephalization in turn enabled directional movement such as moving towards a food source. Platyhelminthes also developed a third tissue layer, mesoderm, which is sandwiched between ectoderm and endoderm.

As the animals grow more complex, it was the segmented phylum, Annelids like the earthworm, that developed a mesoderm-lined body cavity called the coelom. This new feature enabled further tissue specialization and paved the way for organ development.

Complexity of this specialization is increased in mollusks, a group which includes body plans as diverse as octopuses, snails, and clams. This group has many specialized organs covered and kept in place with a mantle.

Arthropods are arguably the most diverse phylum and include insects, crustaceans, and arachnids. They were the first group to develop jointed appendages and so were named accordingly. They also have sensory antennae and their bodies are divided into three specialized segments, head, thorax, and abdomen, all covered with an exoskeleton.

Finally, the Chordate phyla includes our old friend the seed eater and all other birds as well as tunicates or sea squirts, fish, amphibians, reptiles, and mammals. Chordates get their name from a novel physical trait they developed, the nerve cord, seen clearly in this typical Chordate embryo. This develops into the brain and spinal cord in most adults. They also have a cartilaginous structure, the notochord, which becomes the spine or backbone.

Here, we have characterized the main animal phyla in an extremely simplified manner. But as you can see, even within a single phylum, there are numerous differences in form and function to be found. This means that we can observe the diverse structures and physiologies of different members of a single phylum and predict their functions and life histories.

In this lab, you will observe crayfish and cricket structures and predict their functions.