Animals come in a wide variety of shapes, sizes, and structure. We are animals, but so are insects, fish, earthworms, tapeworms, jellyfish, and sponges. Roughly speaking, there are over 35 phyla of animals, and it is a continuing challenge for zoologists to understand both the evolutionary relationships among animals and the evolutionary processes that produced this great diversity of form and function in animals.
There are a few underlying themes in animal evolution, though, which you should keep in mind during the next series of labs. Three related themes are the evolution of
1. Body size, because larger animals can move faster, and can consume larger prey (and avoid being consumed by small predators),
2. Locomotory ability, because the ability to find food and escape predators is an important part of life for most animals, and
3. The ability to capture and digest food, because sophisticated means of capture allow animals to consume more food.
Animals are multicellular, ingestive heterotrophs. That is, they are made of many cells, and they don’t make their own food, but instead they eat organic material such as plants, protistans, other animals, or dead stuff like corpses and fecal matter. The simplest animals, sponges, cannot move in search of food, and can only eat food items that are smaller than one cell, because they don’t have a gut to digest food. All of their digestion occurs inside of cells (intracellular digestion), like protozoans. In other animals, though, we see the ability to move about in search of food (and to avoid becoming food!), and the ability to eat larger pieces of food, which are digested within the cavity of the alimentary canal using extracellular digestion. The ability to move, the ability to capture larger food items, and the ability to avoid being eaten, are all aided by larger body size. Increased locomotory ability, increased body size, and the ability to capture and digest large food items all require an extensive “infrastructure.” Sense organs, a nervous system to process sensory information and coordinate movement, respiratory systems for gas exchange, circulatory systems to move things around the body, and excretory systems to remove metabolic wastes are some examples of the infrastructure required to keep a large, mobile animal functioning properly. In this lab we will introduce several features of body plans that are related to the evolution of locomotory ability, increased body size, and the ability to capture and digest food in animals. In subsequent labs we will address these features of animals in greater detail.
Another theme in the evolution of animals is the importance of embryology in the evolution of body plans. All of the organs and systems of animals develop from a single cell, the zygote. Any subsequent changes in organs and systems depend on changes in patterns of development. In this lab will explore some of the basic features of animal embryology, and introduce some ways that changes in body plans depend on simple differences in patterns of early development.