|Notes: Acoelomates and Pseudocoelomates|
These are flatworms, flukes and tapeworms. They have a gastrovascular cavity (no body cavity), true muscle tissue, and moderate levels of cephalization.
Turbellaria. This class of acoelomates includes free-living marine organisms. The most well-known example of this class is Planaria. Planaria is one of the few aquatic turbellarians. In it, nitrogeneous waste diffuses through the ectoderm. Planaria lack gas exchange and circulation specialization. It has a fine-branching gastrovascular cavity, a simple excretory system that functions only when osmotic forces are balanced and a flat shape - so all the cells are close to water. Osmoregulation is regulated by ciliated cells called flame cells that move water in and out of the flatworm. Flatworms move via cilia on the ventral side where mucus is often secreted when not "swimming" (for a planarian to swim it undulates). Eyespots detect light and lateral flaps locate pheromones.
Trematoda and Monogenea. These are flukes, parasites with suckers and a tough epidermis. Some may require an intermediate host (e.g., snails/human). A fluke lifecycle can be sexual or asexual. The bulk of a fluke is primarily reproductive organs.
Cestoda. Tapeworms are parasitic flatworms. The head is called the scolex. The posterior is a string of proglottids; each proglottid is a reproductive unit. It releases eggs from the posterior into the host's feces. The tapeworm has no digestive system.
These are ribbon worms or proboscis worms. They are structurally acoelomates but contain a small fluid-filled sac that hydraulically moves a proboscis. It may/may not be a true coelom. They are unique because they have a simple blood vascular system with some having RBCs with hemoglobin, and a complete digestive tract (separate mouth and anus).
Pseudocoelomates do not have inner mesoderm-derived tissue.
These tiny animals live in an aquatic environment. Their pseudocoelom acts as a hydrostatic skeleton and medium for internal transport of compounds and molecules. For reproduction some rotifers undergo parthogenesis: females develop from unfertilized eggs. Others create two eggs; one produces a female, the other yields a degenerate male that only survives long enough to undergo meiosis (create sperm). What makes the male degenerate, you ask? He cannot feed himself.
Nematodes are aquatic pseudocoelomate worms - "pests" to farmers. The pseudocoelom serves as a blood vascular system. It has a complete digestive tract. Nematodes have a tough cuticle and longitudinal muscles.