We are independent & ad-supported. We may earn a commission for purchases made through our links.

Advertiser Disclosure

Our website is an independent, advertising-supported platform. We provide our content free of charge to our readers, and to keep it that way, we rely on revenue generated through advertisements and affiliate partnerships. This means that when you click on certain links on our site and make a purchase, we may earn a commission. Learn more.

How We Make Money

We sustain our operations through affiliate commissions and advertising. If you click on an affiliate link and make a purchase, we may receive a commission from the merchant at no additional cost to you. We also display advertisements on our website, which help generate revenue to support our work and keep our content free for readers. Our editorial team operates independently from our advertising and affiliate partnerships to ensure that our content remains unbiased and focused on providing you with the best information and recommendations based on thorough research and honest evaluations. To remain transparent, we’ve provided a list of our current affiliate partners here.

What are Protostomes?

Michael Anissimov
By
Updated May 21, 2024
Our promise to you
All The Science is dedicated to creating trustworthy, high-quality content that always prioritizes transparency, integrity, and inclusivity above all else. Our ensure that our content creation and review process includes rigorous fact-checking, evidence-based, and continual updates to ensure accuracy and reliability.

Our Promise to you

Founded in 2002, our company has been a trusted resource for readers seeking informative and engaging content. Our dedication to quality remains unwavering—and will never change. We follow a strict editorial policy, ensuring that our content is authored by highly qualified professionals and edited by subject matter experts. This guarantees that everything we publish is objective, accurate, and trustworthy.

Over the years, we've refined our approach to cover a wide range of topics, providing readers with reliable and practical advice to enhance their knowledge and skills. That's why millions of readers turn to us each year. Join us in celebrating the joy of learning, guided by standards you can trust.

Editorial Standards

At All The Science, we are committed to creating content that you can trust. Our editorial process is designed to ensure that every piece of content we publish is accurate, reliable, and informative.

Our team of experienced writers and editors follows a strict set of guidelines to ensure the highest quality content. We conduct thorough research, fact-check all information, and rely on credible sources to back up our claims. Our content is reviewed by subject matter experts to ensure accuracy and clarity.

We believe in transparency and maintain editorial independence from our advertisers. Our team does not receive direct compensation from advertisers, allowing us to create unbiased content that prioritizes your interests.

Protostomes (Greek: mouth first) are a huge clade of animals that includes the Ecdysozoans (arthropods, nematodes, and friends), Platyzoa (flatworms and allies), and Lophotrochozoa (molluscs, annelids, and allies). Their counterpart, the deuterostomes, includes the chordates, hemichordates, and echinoderms. There are many differences between the two groups, which make up all bilateral animals, but the most frequently mentioned is that the mouth is formed in the embryo first, rather than the anus. This is a pretty fundamental difference.

Another difference between protostomes and deuterostomes is that in protostomes the embryo displays spiral cleavage, where the position of each cell is deterministic. In deuterostomes, which display radial cleavage, there is less certainty in where differentiated cells ultimately end up. The protostomes' embryological development might therefore be considered relatively conservative and predictable. Unsurprisingly, protostomes encompass the world's simplest animals, including the earliest bilateral animals to appear in the fossil record, such as arthropods and molluscs.

When it comes to species count, protostomes win over deuterostomes. While there are many millions of protostome species, especially thanks to phylum Arthropoda, there are only about 70,000 species of deuterostomes, though these encompass all fairly complex animals. Protostomes encompass all simple animals, but their abundance is a testimony to their success. Ignoring fish and whales, protostomes make up the majority of biomass in the world's oceans, though they are less dominant on land, where the vertebrate body plan has proven highly successful. The suspension qualities of water make a spinal column much less necessary.

Protostomes and deuterostomes are thought to have split at least 558 million years ago, and possibly much earlier. This is inferred by the existence of Kimberella, a primitive mollusk, 555 million years ago, which is a known protostome. However, just because Kimberella existed at the time, why does that mean that deuterostomes didn't evolve later, branching off from protostomes? There is little solid consensus on the subject, but apparently some Late Ediacaran (older than 542 million years) fossils, Ernettia, have a likely deuterostome affinity, though consensus on the classification of Ediacaran fossils is frequently absent. In any case, it is certain that protostomes and deuterostomes had separated by the dawn of the Cambrian, 542 million years ago.

All The Science is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.
Michael Anissimov
By Michael Anissimov , Writer
Michael Anissimov is a dedicated All The Science contributor and brings his expertise in paleontology, physics, biology, astronomy, chemistry, and futurism to his articles. An avid blogger, Michael is deeply passionate about stem cell research, regenerative medicine, and life extension therapies. His professional experience includes work with the Methuselah Foundation, Singularity Institute for Artificial Intelligence, and Lifeboat Foundation, further showcasing his commitment to scientific advancement.

Discussion Comments

By Spiffany — On Jun 04, 2011

@elizabeth23 - You are partially correct. While "proto" is the Greek for "first", "stoma" (or its plural, "stomes") is the Greek word for "mouth". Thus, the Greek naming of these two is dictated by whether the mouth of the animal developed first (the protostomes) or second (the deuterostomes).

By elizabeth23 — On Jan 26, 2011

In Greek, "proto" means first, while "deutero" means second. The naming, at least, suggests that protostomes might be the older or more dominant when comparing protostomes vs deuterostomes, although maybe this naming also comes from the fact that there are more species of protostome.

Michael Anissimov

Michael Anissimov

Writer

Michael Anissimov is a dedicated All The Science contributor and brings his expertise in paleontology, physics, biology...
Learn more
All The Science, in your inbox

Our latest articles, guides, and more, delivered daily.

All The Science, in your inbox

Our latest articles, guides, and more, delivered daily.