General characteristics of flatworms (internal and external structure)

More than 25 thousand species belong to the type of Flatworms.
Flatworms have bilateral (bilateral) symmetry.
The body of Flatworms is formed from three layers of cells: ectoderm, endoderm, and mesoderm (that is, they are three-layer animals).
For the first time, they have a third layer of cells – the mesoderm.
The mesoderm is the middle layer of cells that lies between the ectoderm and endoderm.

Internal structure of flatworms
The body itself is a skin-muscle sac, which consists of the integumentary epithelium and three layers of muscles – annular, oblique and longitudinal. The musculocutaneous sac allows the animal to maintain a constant body shape.
Inside the musculocutaneous sac, there are internal organs, the space between which is filled with parenchyma.
Parenchyma is a group of cells with processes that fills the space between organs.

The body of free-living worms is covered with cilia from above, which ensure the movement of the worm (parasitic worms have no cilia).

In flatworms, the digestive, excretory, nervous and reproductive systems of organs are developed.

Digestive system
The digestive system is primitive, starting with the mouth located in the middle of the body on the ventral side. Next are the pharynx and intestines, which blindly ends (there is no anus). Undigested food debris is excreted through the mouth.

Excretory system
The excretory system is represented by protonephridia, consisting of tubules that begin with stellate cells with bundles of cilia. Bundles of cilia, oscillating, create a flow of fluid and direct it to the excretory pore.
Protonephridia is a system of tubules that permeate the entire body of the animal and open from the outside with pores.

Nervous system
The nervous system consists of nerve nodes (ganglia) at the front of the body and nerve trunks along the body. The nerve trunks are interconnected by transverse bridges. This type of nervous system is called the ladder.
Some flatworms (free-living) have primitive senses – eyes and statocysts (balance organs).

Reproductive system
The body of a flatworm contains both male and female reproductive organs. Such animals are called hermaphrodites.

There are male sex glands – testes and female – ovaries.
The parenchyma contains numerous vesicles – testes. From them there are tubular vas deferens. Male reproductive cells are transported via the vas deferens to the copulatory organ. These are all parts of the male reproductive system.

The female reproductive system consists of paired ovaries, from which tubes – oviducts – extend to the copulatory bag.
Hermaphrodites are organisms that simultaneously have both male and female genitals.

Internal fertilization is a type of fertilization in which the fusion of an egg and a sperm occurs inside the female body.

Cross fertilization is fertilization in which an egg is fertilized by the sperm of another individual.

Self-fertilization is fertilization in which an egg is fertilized by a sperm from the same individual.

Respiratory and circulatory systems
The respiratory and circulatory systems are absent. The absorption of oxygen and the release of carbon dioxide is carried out by the entire surface of the body. In order to transport nutrients to the tissues, numerous outgrowths of the intestine are used.

The meaning of flatworms
Flatworms live in all environments, including parasitizing other organisms.
Helminths (parasitic worms) cause dangerous diseases in animals and humans.
These parasites lead to severe pain, exhaustion, and anemia.
For example, the causative agent of the disease of opisthorchiasis is the feline fluke, which parasitizes in the liver and bile ducts, gall bladder. This disease is contracted by eating frozen carp fish.

A person becomes infected with pork tapeworm by eating contaminated pork, and bovine tapeworm by eating contaminated beef. Therefore, you should not eat meat that has not passed sanitary control.

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