The cnidariocytes are located in the long tendrils Flatworms belong to the phylum Platyhelminthes. They are examples of acoelomates. This Dugesia has been stained to show the gastrovascular cavity . Roundworms like this vinegar eel, or parasitic roundworms like Ascaris, are pseudosoelomates. They do have an internal body cavity that surrounds their digestive tract, but the digestive tract is not lined with muscle tissue derived from the mesoderm layer of the embryo.
Annelids are the simplest group of true coelomates.
They have a digestive tract that is lined with mesoderm-derived muscle, and which is suspended in a body cavity. Annelids have a body that is made up of repeating segments. Earthworms, leeches, and certain marine worms are examples of annelids. Examples of annelids. Mollusks Mollusks are coelomates that often have shells, but always have a mantle.