Flatworms are traditionally considered dangerous helminths that cause significant harm to many animals and humans. However, these organisms possess many progressive structural features, in comparison with the previous systematic units, and occupy their niche in the system of the organic world. From our article you will learn what bilateral worms are for flatworms, why some of them do not have a digestive system, and why they are an important link in the food chain.
Structural Features of Flatworms
A characteristic feature of flatworms is their body shape. It has the appearance of a sheet or ribbon. In this regard, these animals do not have a cavity, and the gaps between the organs are filled with a special type of connective tissue - the parenchyma. The concept of "bilateral symmetry" in flatworms occurs for the first time, because its intestinal cavity is characterized by its other form - radiation.
A progressive feature is the presence of formed organ systems. So, the musculoskeletal system consists of a combination of integumentary epithelium and a layer of muscles located under it. The digestive system is closed. Excretory is represented by a system of tubules, which are released outward by the pores. Due to the presence of the nervous system, flatworms have sense organs: vision, touch, balance and chemical perception. Most of these animals are hermaphrodites. This means that both male and female germ cells are formed in their body. But the respiratory organs flatworms are devoid. They carry out gas exchange throughout the surface of the body. Classes are distinguished inside the type: ciliary, tapeworms and flukes.
What is bilateral symmetry in flatworms?
In the process of evolution, this trait first appears in worms of this type. And what does the concept of symmetry mean? This is the location of individual parts of the body about an axis or center. The first multicellular animals, which include sponges, generally have no symmetry. Growing along the substrate, they can form any shape.
Remember what a jellyfish looks like. In the middle of her body you can put an imaginary point from which to draw lines in all directions. This is radial, or radial, symmetry. It is characteristic of animals that live in the thickness of the seas and oceans.
What is bilateral symmetry in flatworms? Pay attention to the photo. An imaginary line can be drawn along the entire body of the worm, which seems to divide it into two equal parts. This is bilateral, or bilateral, symmetry.
Features of bilateral symmetry
What is the reason for the emergence of a new type of symmetry? First of all, with the change in the lifestyle of organisms and the complexity of their structure. Bilateral symmetry is characteristic of animals that actively move in space. This allows them to move in one line and turn. In such organisms, the front specialized department of the body is clearly distinguished. It hosts receptors for attachment and capture of prey. Flatworms have formed organ systems. This is another progressive feature of their structure. The organs of flatworms, like other animals with bilateral symmetry, are located on the sides of an imaginary line.
Ciliary worms
All worms of this class are free-living organisms, the sizes of which do not exceed 6 cm. The ciliary worms owe their name to the ciliary epithelium covering their body. They live in fresh, salty water bodies, forest litter. What is bilateral symmetry in flatworms, it is best to consider it on their example, since their body has the form of a fairly wide sheet, flattened in the dorso-abdominal direction. Ciliary worms are predators. They crawl with their whole body on the victim, holding it. Then a small proboscis appears from the mouth opening. With its help, the worm sucks the contents of the victim. These organisms feed on smaller animals: crustaceans, hydras, and insect larvae. Representatives of this class are dairy and freshwater planarians.
Flukes
But flukes are typical parasites. Their most common species are feline and hepatic. They got this name by the appearance of the final owner. Their body can reach 8 cm. With the help of the oral and abdominal suckers, they attach to the internal organs of the host and feed on its tissues. Their development cycle includes a number of stages, during which they change several owners. So, a cat fluke develops in the body of a freshwater mollusk and fish. Only after this the worm forms a sexually mature individual that parasitizes in the liver of dogs, wolves or cats.
Tapeworms
The most gigantic representatives of this type of animal are tapeworms. Their body consists of a small head, on which are the attachment organs, neck and joints. The number of the latter can reach several thousand pieces or tens of meters. Bovine and pork tapeworm, wide tape, echinococcus parasitize in the lumen of the small intestine of many animals and humans. Therefore, tapeworms do not have digestive organs. They absorb already split organic substances.
So, representatives of the flat type have a number of progressive structural features. These include the appearance of bilateral body symmetry, which is associated with the formation of these organ systems.