Protostomes: Definition and Characteristics

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  • 0:01 What Are Protostomes?
  • 2:28 Examples of Protostomes
  • 4:14 Lesson Summary
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Lesson Transcript
Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

Though you may not be able to tell just by looking at them, protostomes are an evolutionarily related group based on shared characteristics of their body plans. In this lesson, you will identify these characteristics, as well as see examples of the variety of animals this group encompasses.

What Are Protostomes?

When you compare a crab and a lobster, you can understand how they may be related in the Animal Kingdom. But compare that same crab with a snail or an earthworm, and you might have a difficult time seeing how these animals are part of the same evolutionary branch.

The reason we put these animals together as a group is they share the same characteristics in their body plan; this is the combination and organization of an animal's body structures. When we look at how an animal is a sum of its parts, we can start to see relationships that might not otherwise be obvious.

We first determine if the animal has true tissues, or tissues and cells that are specialized. If so, we then determine the body symmetry of the animal. Is it radially symmetric, meaning that it is symmetrical from a center point on the body, or is it bilaterally symmetric, meaning that each side is a mirror image of the other?

If it is bilaterally symmetric, we have one more question to ask about the animal: How did it develop as an embryo? This is a critical point because if the anus developed first and the mouth second, as is the case with humans, then the animal is a deuterostome, which literally means second mouth. However, if the mouth developed first, then we have a protostome, which means first mouth.

Combined, these factors are the basis for all protostomes: They have true tissues, they are bilaterally symmetric, and the mouth developed first during embryonic development. Our worms, crab, and snail all fit these criteria, which is why we can say they are evolutionarily similar, even though they don't really look alike.

But protostomes are so much more than this. Many protostomes also have an anterior brain, a central nervous system, and a coelom, which is a fluid-filled body cavity between the digestive tract and outer body wall. The coelom provides space for internal organs to grow and develop independently of the other body tissues.

And as mentioned, the most defining factor of a protostome is how it proceeds through embryonic development. Protostomes not only develop the mouth before the anus, they also have spiral and determinate cleavage. Spiral cleavage refers to the rotational pattern of the dividing cells, and determinate simply means that each cell has a certain fate - they cannot become whole organisms if isolated from other cells.

Examples of Protostomes

The protostome group encompasses an incredible variety of animals. Some of the most common protostomes are flatworms (such as tapeworms), nematodes (such as heartworms), mollusks (such as snails and slugs), annelids (such as earthworms), and arthropods (spiders, insects, lobsters, ticks, barnacles, and more!).

Some of these groups are more diverse than others. For example, there are over 25,000 species of named nematodes, but most of them have the same body form.

But when we look at the mollusks, we can see that this group has many variations of their body plan. Squids, snails, mussels, octopi, clams, scallops, and nautiluses are all mollusks! The reason they are all in this same category is that they share many of the same body features.

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