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:
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.
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).
Parasites called tapeworms. They have a distinctive head (cephalic) region called the scolex and body segments known as proglottids.
These are a whole mounts (wm) of Clonorchis sinensis. Please note that there are two suckers: the oral_sucker at the anterior end of the body and the ventral_sucker, which is located just above the uterus. The testes are located at the posterior end of the organism, and the ovaries are just above the testes. The ovaries look like they are pressing against the base of the uterus.