An Introduction to Zoology: Chapter 11

Acoelomates, Pseudocoelomates, and Coelomates:

Because it is triploblastic, a bilaterian has the capacity to form a coelom. You may recall that a true coelom is a fluid-filled (occasionally, an air-filled) body cavity that is entirely lined by mesodermal tissue. Obviously, only an organism that actually has mesoderm can have a true coelom, so radiate animals cannot be coelomates. You may also recall that many internal organs are derived from mesodermal tissue, so triploblastic animals generally have much more complex bodies than do diploblastic animals.

We often divide the Bilateria into the acoelomates, the pseudocoelomates, and the coelomates, according to what kind of coelom (if any) is present. These terms are simply descriptions of how animals’ bodies are built; they do not necessarily indicate evolutionary relationships.

An acoelomate is any triploblastic animal that has no internal, fluid-filled body cavity. (“A” means “without,” so an “acoelomate” is “without a coelom.”) In an acoelomate, the space between the ectoderm and the endoderm is occupied by organs derived from mesodermal tissue, plus a loose collection of mesodermally-derived cells called parenchyma.


You may recall that, since water is incompressible, a coelom, if present, can form a hydrostatic skeleton that helps its possessor resist external pressure. A hydrostatic skeleton can also be used to help redirect force produced by muscle contractions. Because they lack internal skeletons of any sort, few acoelomates are capable of burrowing, since they cannot resist external pressure. Similarly, few acoelomates can generate sufficient muscle force to be capable of swimming.

Because they cannot resist external pressure, most acoelomates are pressed flat by a combination of the downward pull of gravity and the pressure exerted by the surrounding atmosphere or water. That’s why these animals are frequently referred to as “flatworms.”

A cross-section through the body of a typical acoelomate (in this case, a member
of the phylum Platyhelminthes). Note how there is no internal body cavity of any sort.
You can also see that the animal’s body is flattened by gravity and external pressure.