Type Flatworms consists of non-parasitic and parasitic forms. At the same time, free-living organisms are combined in one class, and those living at the expense of other organisms - in six. Representatives of the class Ciliary (planaria, turbellaria) live in water bodies, most often they are predators.
Parasitic flatworms live in the body of animals and humans. Data
organisms are well adapted to such living conditions, because they have suction cups that attach to the walls of the internal organs of the host, and a joint with an ever-increasing number of segments. They are characterized by the absence of the digestive system (except for the class of flukes), nutrients are absorbed through special outgrowths of the body; anaerobic respiration (they breathe in an almost oxygen-free environment), as well as rapid reproduction (they are hermaphrodites).
All these features allow these organisms to settle in the host body for a long time and exist at its expense. Such helminths include: hepatic trematode, chickweed, cat fluke, pork tapeworm, echinococcus, etc. A person can become infected if he eats raw or poorly processed cattle meat, pork, and fish.
Type Flatworms unites organisms that have similar features in both external and internal structures. They are barren animals, have an elongated, flattened body from top to bottom, that is, it is flat or almost flat. It is also precisely for them that bilateral symmetry first appears for the first time and in the process of ontogenesis three germ layers are laid - ecto-, meso- and endoderm - from which later internal organs are formed. The type of Flatworms is also characterized by the presence of a skin-muscular sac, which is a combination of the epithelium and the muscle fibers located underneath. This allows them to "worm-like" to move around.
The digestive system in free-living forms has a primitive structure and consists of the anterior intestine or pharynx, middle intestine, which ends blindly. In helminths, this organ system is reduced.
The nervous system of flatworms is represented by a paired brain ganglion and nerve trunks that depart from it and are connected by ring jumpers. Two longitudinal abdominal trunks become highly developed.
There are no circulatory and respiratory systems. Representatives of the class Ciliary epithelium breathe with which their body is covered outside.
The excretory organs are protonephridia. They consist of a system of tubules that end in a stellate cell with cilia. The excretion of metabolic products into the environment occurs through special excretory holes.
The reproductive system is hermaphroditic and most often represents the system of ducts, which are needed for the excretion of reproductive products, and the copulatory organ for internal fertilization.
Thus, the type Flatworms are basically parasitic forms (helminths) that have been able to adapt and adapt to their lifestyle.