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 Mesozoa?

Michael Anissimov
Updated Jan 28, 2024
Our promise to you
AllTheScience 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 AllTheScience, 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.

Mesozoa are tiny, simple, worm or blob-like parasites that were once thought of as intermediates between protozoa (motile single-celled organisms) and metazoa (multicellular animals). However, now they are generally thought of as degenerate metazoa, that is, metazoans that descended from more complex animals but were secondarily simplified. Still, there is no strong consensus as of yet, and it may be that mesozoa are truly just basal metazoans.

While the term "Mesozoa" was once meant to refer to a natural group consisting only of exclusive descendants of a common ancestor, it is now known that mesozoa consist of three largely unrelated groups, all of which have been given their own phyla — the Placozoa (which consists of a single species, Trichoplax adhaerens), Orthonectida (tiny parasites of flatworms, mollusks, and echinoderms), and Rhombozoa (kidney parasites of cephalopods like squid). The Mesozoa are remarkable in their simplicity — though they have differentiated and organized tissues, this differentiation is quite simple — for instance, cell types may be layered.

Trichoplax adhaerens, the only species of phylum Placozoa, is a soft-bodied animal 0.5 mm across, reminiscent of a giant amoeba. Also called "tablet animals," Tricoplax is named for its ability to strongly adhere to surfaces, including such as microscope slides or glass pipettes. In 2006, Dellaporta et al reported the complete mitochondrial genome sequence of Tricoplax and demonstrated that it is the most basal known living metazoan phylum, branching off even earlier than cnidarians (jellyfish and corals). Cnidarians may have evolved from an organism as simple as Tricoplax.

Another of the mesozoa, Orthonectida, consist of a wall of ciliated cells surrounding a core of reproductive cells. The Orthonectida are mobile, swimming by means of their cilia, and have a larval stage as well as distinct males and females. The phylum only includes 20 species, Rhopalura ophiocomae being the best-known. The Orthonectida were once thought to be closely related to Rhombozoa, another mesozoan group, though now they are known to be unrelated.

The Rhombozoa (also known as dicyemids), another parasite mesozoan phylum, range in size from 0.1 - 9.0 mm and inhabit squid kidneys. Like another microscopic animal, rotifers, dicyemidsare eutelic, meaning adult individuals all have a constant number of cells. Instead of adding new cells to grow, the size of each individual cell simply increases. Dicyemids are found most frequently in temperate benthic environments, and more rarely in tropical areas.

AllTheScience 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

Michael is a longtime AllTheScience contributor who specializes in topics relating to paleontology, physics, biology, astronomy, chemistry, and futurism. In addition to being an avid blogger, Michael is particularly passionate about stem cell research, regenerative medicine, and life extension therapies. He has also worked for the Methuselah Foundation, the Singularity Institute for Artificial Intelligence, and the Lifeboat Foundation.

Discussion Comments

Michael Anissimov

Michael Anissimov

Michael is a longtime AllTheScience contributor who specializes in topics relating to paleontology, physics, biology...

Read more
AllTheScience, in your inbox

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

AllTheScience, in your inbox

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