Station 1: Introduction to the Basal Animals

Before proceeding with this lab you should review the textbook and lecture material on this subject.  In particular you should be able to define the following terms:

• Bilateral symmetry

• Blastula

• Body Plan

• Cnidaria

• Diploblastic

• Gastrula

• Porifera

• Radial Symmetry

• Triploblastic

You should also be able to describe the following concepts in your own words:

• The advantages of cellular specialization to form tissues and organs

• The advantages of bilateral symmetry

Imagine the Earth about 700 million years ago. The oceans are full of microscopic life—single cells, drifting colonies, strange little protists waving their flagella to pull in food. Among them are choanoflagellates, tiny organisms with a collar of delicate filaments around a beating flagellum. They’re not animals, not yet, but they hold the secret of what’s to come.

Now picture this: one colony of choanoflagellates begins to change. Some cells stop pulling in food and instead give their energy to supporting the colony’s shape. Others focus on reproduction. Still others remain the feeders. For the first time, a colony is no longer just a cluster of identical cells—it’s a team. That moment marks the birth of the Metazoa.

What Makes an Animal?

Animals are still defined today by the echoes of those first experiments in multicellularity. They are multicellular, heterotrophic, eukaryotic organisms, yes, but that only scratches the surface. Their real uniqueness comes from a set of shared derived traits that set them apart from everything else alive.

Unlike fungi, animals don’t just absorb nutrients. They ingest food and break it down in a controlled space outside their cells. Their cells have no cell walls, relying instead on collagen and specialized junctions to hold together. They developed nerves and muscles, the machinery for sensing and moving. Their life cycles are dominated by the diploid stage, with a characteristic embryonic journey through cleavage, blastula, and gastrula. Many even pass through a wandering, self-feeding larval stage before transforming into adults.

And beneath all of this lies a genetic toolkit—conserved rRNA genes and the famous Hox genes—that ties every animal, from sponge to human, into the same great story.

The Big Picture of Animal Diversity

From those humble beginnings, animals radiated outward into a staggering variety. Today, biologists recognize about 35 phyla and over a million species—and molecular evidence suggests there are many more. Yet here’s a twist: the creatures most familiar to us, the vertebrates, make up less than 5% of named species. The rest are invertebrates, an entire world of animals most people never notice.

Early zoologists tried to make sense of this diversity by asking a few basic questions:

• Do they have tissues?

• What kind of symmetry do they show?

• How do their embryos develop?

• Do they have a body cavity?

For decades, those questions built the skeleton of animal classification. But like all good scientific stories, the plot shifted. With the rise of molecular biology, we could suddenly read the genetic script itself. Sequences of DNA and proteins told us which animals were truly related, which ones only looked alike, and which old ideas we had to discard.

Some discoveries reinforced what we already suspected: clades based on tissues and symmetry held up. Others overturned old assumptions. For example, we once thought that the type of coelom—a body cavity—revealed deep evolutionary relationships. Molecular evidence now shows the truth is messier: body cavities evolved, disappeared, and reappeared across different lineages. The tree of life isn’t a straight line; it’s a tangled, branching history of trial and error.

The Animal Tree: Built on Innovation

If you step back, the animal family tree can be read as a sequence of critical innovations. Each new trait was like a doorway opening into new possibilities:

• Multicellularity gave us the first animals.

• True tissues allowed organs and coordinated functions.

• Bilateral symmetry created animals that could move with purpose and direction.

The most dramatic chapter came with the Cambrian Explosion (~565–525 million years ago), when bilateral animals burst onto the scene. In a geological blink of an eye, the ancestors of nearly every modern phylum appeared, experimenting with new body plans that still define the animal kingdom today.

How to Approach This in Lab

So what do you do with all this? The temptation in a lab manual is to memorize lists of traits. But animals aren’t just lists—they’re solutions to problems. Every group of animals has faced the same challenges:

• How do you eat?

• How do you breathe?

• How do you move?

• How do you circulate nutrients?

• How do you keep water and salts balanced?

• How do you sense the world around you?

• How do you reproduce?

Each clade answers these questions in its own way. As you move through this lab, think like a storyteller. For every animal you study, ask yourself: what problems did it inherit from its ancestors, and what new solutions did it invent?

If you can see animals this way—as chapters in a story of innovation—then the diversity you’ll encounter in lab won’t feel overwhelming. It will feel like what it really is: a fascinating story 700 million years in the making.

Now let’s step past the sponges into the rest of the animal kingdom: the Eumetazoa. This name literally means “true animals,” and the big innovation here is true tissues. With tissues, cells can organize into organs and carry out complex functions. This one step opened the door to almost all the animal diversity you’re familiar with.

But the Eumetazoa don’t all look alike. Early on, they split into two very different paths: the Radiata and the Bilateria.

As you begin examining basal animals, it is important to recognize that not all animals are built the same way. Some represent very simple levels of organization, while others show increasing complexity in how their bodies are constructed. One of the most powerful ways biologists understand these differences is by looking at how animals develop from an embryo.

Early in development, cells begin to organize into layers known as germ layers. These layers will ultimately give rise to all tissues in the adult organism.

From No Tissues to Two Layers 

Sponges represent the most basic condition. Their cells are specialized, but they are not organized into true tissues. As a result, their bodies lack the structural organization seen in other animals.

In contrast, cnidarians and ctenophores show a significant step forward. Their cells are organized into two distinct layers, making them diploblastic. These layers allow for more coordinated structure and function, but still impose limits. Without additional layers, these animals cannot develop many of the internal structures seen in more complex organisms.

At this stage in evolution, animals have tissues, but their overall organization remains relatively simple.

How Diploblastic Animals Form Two Germ Layers

All animals, starting with the cnidarians, begin as a single cell called a zygote. This cell undergoes a series of rapid divisions called cleavage, producing a solid ball of cells. As division continues, a hollow cavity forms, creating a stage known as the blastula.

At this point, the embryo consists of a single layer of cells surrounding a fluid-filled cavity. The key transformation occurs next.

During gastrulation, part of the blastula folds inward, a process called invagination. This inward movement reorganizes the cells into two distinct layers.

The outer layer becomes the ectoderm, which will form structures such as the outer body covering and nervous tissue. The inner layer becomes the endoderm, which lines the digestive cavity.

The opening created during this folding process is called the blastopore, and it marks the beginning of a digestive system.

At the end of gastrulation, the embryo now has two clearly defined germ layers. This is the diploblastic condition, seen in organisms such as cnidarians and ctenophores.

Connecting This to What You Will Observe

As you examine cnidarians and ctenophores in this lab, you are looking at animals whose body plan is established at this stage of development.

They have:

• an outer layer (ectoderm)

• an inner layer (endoderm)

• and no additional layer between them

This developmental pathway limits the types of tissues and structures they can form, which helps explain the simplicity of their body organization.

A Major Evolutionary Transition: The Third Layer 

A major transition occurred when a third germ layer evolved. This layer, called the mesoderm, forms between the outer and inner layers.

This change was not minor. The mesoderm made possible:

• the development of muscles

• the formation of internal organs

• more complex body organization

Animals with three germ layers are called triploblastic, and this group includes most of the animals you are familiar with.

From an evolutionary perspective, this represents a shift from relatively simple body plans to organisms capable of greater size, movement, and specialization.

The Addition of a Third Layer: Mesoderm 

In diploblastic animals, gastrulation produces two layers: the ectoderm and the endoderm. In more complex animals, this same process continues further, resulting in the formation of a third layer called the mesoderm.

During gastrulation, as cells move inward and reorganize, some cells begin to occupy the space between the ectoderm and endoderm. These cells differentiate into the mesoderm, forming a new layer between the outer and inner tissues.

This change fundamentally alters what the organism is capable of becoming.

The mesoderm gives rise to:

• muscles

• connective tissues

• many internal organs

With this additional layer, animals are no longer limited to relatively simple body plans. Instead, they can develop structures that allow for greater movement, internal transport, and specialization.

Animals that develop this third layer are called triploblastic, and they include most major animal groups.

Connecting Back to Basal Animals

The organisms you are about to study—sponges, cnidarians, and ctenophores—do not form a mesoderm during development. Their body plans reflect this limitation.

As you move to later animal groups, you will begin to see how the addition of this third layer allows for increasingly complex organization. The appearance of the mesoderm represents one of the most important transitions in animal evolution, marking the shift from simple tissue-level organization to the development of true organ systems.

Connecting Development to Evolution

The animals you are about to study represent early stages in this progression:

• Sponges → no true tissues

• Cnidarians and ctenophores → two germ layers (diploblastic)

• Later animals → three germ layers (triploblastic)

Understanding how these body plans arise during development allows you to see animal diversity not as a list of unrelated groups, but as a series of evolutionary innovations built on earlier forms.

As you move through this lab, consider not only what structures each organism has, but what it lacks, and how those limitations reflect its place in the evolutionary history of animals.