Flatworms, also known as Platyhelminthes, belong in the Kingdom Animalia. They are unsegmented flat worms with a head and a tail end. Flatworms are considered the most primitive bilaterally symmetrical animals, meaning that their body exists in mirror images about a long anterior-posterior axis with definite upper and lower surfaces and anterior and posterior ends. The bilateral shape is an important feature because it permits cephalization, the concentration of sensory structures and nervous function in the head end. Flatworms have one single opening, the mouth, so both food and wastes must enter and leave in the same direction.
They have a hydrostatic skeleton used for support. The lack of a cuticle forces the flatworms to use their intracellular fiber and basal lamina to support their body wall, which actually makes their bodies more flexible. Aquatic flatworms usually use cilia to for movement, whereas most land-based flatworms use muscular contractions for movement.
Most reproduction in flatworms is sexual, sometimes consisting of self-fertilization or cross-fertilization, wherein both worms can produce offspring.
Furthermore, flatworms are triploblastic, which means that their body structure is based on three fundamental cell layers, the endoderm, mesoderm, and ectoderm. Their respiratory system and blood vessel system are missing and therefore diffusion is used for transport of oxygen inside the body. The flatworms are flat because for maintaining metabolism, no cell can be too far from the outside. The length of flatworms varies between 0.4mm for some free-living species (an organism that does not live off another organism) and several meters for parasitic forms (an organism that needs to live on or in another organism in order to survive) (Diphyllobothrium latum: 25m in length) Flatworms are divided into three groups. Most of them are parasitic. Of the 20,000 species known, 1,400 are parasites belonging to either the class Cestoda (tapeworms) or the class Trematoda (flukes). The rest are free-living and belong to the class Turbellaria (planarians).
Tapeworms are intestinal parasites in vertebrates. They use hooks and suckers to attach themselves to intestine of a host organism. A protective cuticle keeps them from being digested by their hosts' digestive juices. They absorb digested food from the gut of their host. Flukes have a more complex life cycles than tapeworms because they pass through a number of juvenile stages in more than one intermediate hosts before reaching adulthood. This Class is broken into two major groups based on how many intermediate hosts are found in the life cycle. If they only have one intermediate host, they belong to the Order Monogenia. If they have two intermediate hosts, they belong to the Order Digenia. Planarians are mostly free-living and are common in fresh water and inmost terrestrial habitats. The majorities of turbellarian flatworms are marine and can be found in the oceans and in shallow water. The Class gets its name because as these animals swim near the surface of the water they make little ripples or turbulence on the surface of the water.
How do flatworms make a difference in our life? Flatworms are usually friendly or harmful. The good sides of flatworms are that they scavenge on dead and dying invertebrates instead of hunting for moving prey and they are used by humans as research organisms in studies of regeneration and intelligence. The bad side is that flukes and tapeworms are pathogen or infectious agents that cause illness or disease in other living organisms. Some famous platyhelminth human parasites in the Tremetodes: Chinese liver fluke, sheep liver fluke, Schistosomes: and in the Cestodes: Beef tapeworm, Pork tapeworm.
As you see, flatworm is a unique phylum. Flatworms are a very complex animal. Some of the flatworms are pests and we know little about how to control them, while others are simply interesting and rather benign. They are not that bad and they actually possess a certain degree of intelligence.