In are called acoelomates. Germ Layers Before we

In this lesson, we will go over what an acoelomate is and how its body is different from other animal body types.

We will also look at some examples of acoelomates.

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What Is an Acoelomate?

Do you ever think about what’s inside your body? You most likely have, and you can probably even name some organs that are crucial to your survival. But how do those organs form during development? A key part of this process involves the coelom, or body cavity. Complex animals like us have a body cavity that houses our internal organs and are called true coelomates. However, some primitive animals survive without a body cavity and are called acoelomates.

Germ Layers

Before we can talk about acoelomates, we first need to understand germ layers. Germ layers are groups of cells that exist in the embryo.

There are three specific germ layers, each of which forms different parts of the animal.The ectoderm is the outer covering and will eventually form the skin, nails, hair and nervous system. The mesoderm is the middle layer and eventually forms muscle and some organs.

The endoderm is the inner germ layer and will form the digestive tract. Think of germ layers like a sandwich. The ectoderm and endoderm are like the bread, and the mesoderm is what goes in between.

The Body Cavity

The three germ layers may organize themselves in slightly different ways depending on the type of animal. Think of our bodies: we have our skin, then some muscle, and then inside is a space where many of our organs are held. That space is the body cavity. The mesoderm, which lines both the ectoderm and the endoderm, creates this space.

When the space is hollow, this is a true body cavity, or coelom. Think of a sandwich again: if we line the inside of our bread with mayo and mustard but put nothing else in between, that’s what the body cavity of a true coelomate is like.In acoelomates, the mesoderm fills up all the space that occurs between the ecotoderm and endoderm.

So instead of being hollow, this space is filled with mesoderm, and there’s no body cavity. This is more like how we would normally think of our sandwich, where in between the two bread layers, we fill our sandwich with meat, cheese, and/or vegetables.


All acoelomates have bilateral symmetry, meaning that just like your body, the two halves of their bodies mirror each other. This adaptation allowed for forward facing movement. Acoelomates also evolved complex internal organ systems, such as those for waste excretion and complex nervous systems, including the development of a primitive brain.

Planaria are an especially interesting example of acoelomates. These flat worms live as scavengers in freshwater ponds. Amazingly, these acoelomates have the ability to completely regenerate a new body when cut in half. In fact, you can cut a planarian dozens of times to make multiple new worms. No other animal with three germ layers can do this.

Scientists interested in regenerative medicine, where new limbs and organs can be grown or regrown for patients, study the processes controlling growth in these special worms.Another example you may be familiar with is the tapeworm. Tapeworms are large parasitic worms that grow in the intestines of humans and animals. These acoelomate worms can grow to up 50 feet long inside a host! Tape worms are acquired mainly through eating contaminated, under-cooked meat.

Lesson Summary

True coelomates are animals that have a coelom, or hollow body cavity.

This cavity is formed based on how their three germ layers organize themselves. The ectoderm is the outermost layer. The mesoderm is the middle layer and lines the hollow body cavity in coelomates. And, the endoderm is the innermost layer.In acoelomates, there is no body cavity because the mesoderm fills the entire space that occurs between the ectoderm and endoderm. Acoelomates have bilateral symmetry and more complex organs than other animals without the three germ layers.

Specific examples of acoelomates include flatworms and the parasitic tapeworm. Planaria are one type of flatworm that live in freshwater ponds and have the incredible ability to regenerate entirely new bodies when cut into pieces.


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