The gastrointestinal worms can be divided into two major groups: the Nemathelminthes or nematodes, which are also known as "roundworms," and the Platyhelminthes, also known as flatworms and which include the Trematoda (flukes) and Cestoda (tapeworms).
The Platyhelminthes are the more primitive of the two groups. Although they have organ systems, their organs are embedded in a "parenchyma," which is defined as cellular tissue between the body wall and organs. That is because the flatworms do not have a body cavity or "coelum". In other words, if you were to attempt to dissect out their organs, you could not.
In the nemathelminthes, on the other hand, the organs can be dissected out; they are found within a "pseudocoelum." Although a pseudocoelum is indeed a body cavity, it is different from true coelum in that it is not completely surrounded by mesodermal tissue.
