The acoelomate animals include three phyla: the flatworms (Platyhelminthes), ribbon worms and jaw worms. These are primitive organisms but have bilateral_symmetry. They have three germ_layers and are at the organ-system level of organization. They are protostomes and have spiral_cleavage. They exhibit cephalization. Their excretory mechanism includes the flame_cell. They have simple sense organs such as eyespots. Their nervous system consists of sensory ganglia and a longitudinal nerve. They have mucus-producing structures known as rhabdites in their epidermis.

The classes of the Phylum Platyhelminthes include:

Class_Turbellaria: These are the "free-living flatworms", a/k/a planarians. These organisms are hermaphrodites. They move by gliding with help of cilia. They feed by projecting a pharynx from their mouth and they are carnivores. Gas exchange and some waste removal is accomplished by diffusion. Both asexual and sexual reproductive mechanisms are utilized. These organisms can also reproduce by regeneration.

Class_Monogenea: Parasites known as flukes, a/k/a the trematodes. They have a protective body covering called the tegument. Two types of flukes affect humans: blood_flukes (Schistosoma) and liver_flukes (Clonorchis).

Class_Cestoda: Parasites called tapeworms. They have a distinctive head (cephalic) region called the scolex and body segments known as proglottids.

This is a whole mount of a Planarian that provides an overview of its major external and internal features. The cephalic (head) region is clearly visible, as are the eyespots and auricles associated with the cephalic region. The pharynx is centrally located in the body and the mouth is at the lower end of the pharynx, not in the head region. Multiple dark-staining loops of intestine surround the pharynx on all sides.