Annelids are a fairly large group of invertebrate organisms. In addition, they are considered the most organized representatives of the worms. They live mainly in fresh and salty water bodies, as well as in the soil. Some species of tropical leeches have adapted to the terrestrial mode of existence.
Ringworm Type: General Description
The sizes of representatives of this group range from a few millimeters to six meters. A rather characteristic feature of such an organism is the presence of segmentation - their body consists of many rings, which explains the name of the type. External ringing corresponds to internal segmentation. That is why with wounding or damage to the body, the annular worm loses only a few segments, which are soon regenerated.
From the outside, the body is covered with cuticles, which are not discarded. Chitinous bristles grow from it - another characteristic feature of this group of animals. Some representatives on the segments may have parapodia - rather primitive limbs, which in some cases are equipped with sensitive setae or gills.
Annelid worms: structural features of internal organs
Representatives of this type are characterized by the presence of a secondary body cavity - coelom. This cavity is filled with a specific fluid, due to which normal indicators of the internal environment of the body are maintained .
There is a skin-muscle bag, which consists of balls of the epithelium, as well as muscles, grouped into circular and longitudinal groups.
The digestive system is through, begins with the mouth and ends with the anus. Annelids have three sections of the intestine - anterior, middle and posterior. In some species, primitive salivary glands are formed.
The breath of the body is through the skin. The exception is only some marine species of animals in which gills are located on parapodia. As for the circulatory system, it is usually closed. It consists of the abdominal and dorsal aorta, which are interconnected by annular vessels. These organisms do not have a heart - the movement of blood is provided by the contraction of the spinal aorta. Roundworm blood can contain a wide variety of respiratory pigments.
The nervous system is still quite simple. At the front end of the body is the large nerve ganglion, which acts as the brain. A nerve chain departs from it, which in each segment of the body forms a small ganglion - an accumulation of neurons. The sensory organs are represented by the eyes, organs of chemical sensitivity, as well as mechanoreceptors, which are distributed throughout the body of the worm.
Annelids: features of reproduction and development
Organisms of this group can be both heterosexual and hermaphrodites (they are much less common). For example, an earthworm has a hermaphroditic reproductive system, but two individuals are needed for fertilization. It can occur both in the external environment and by introducing sperm into the female’s internal channels.
An interesting fact is that annelids with pronounced segmentation are prone to rapid and intensive regeneration. Due to this, asexual reproduction is characteristic of some species . The development of organisms is direct, without metamorphoses.
It is worth noting that the role of annelids is quite significant. For example, the well-known earthworm is responsible for aeration of the soil. This group includes leeches, which are often used in modern medicine. Hirudin, produced by a leech, is of particular value, as it dilutes blood and is used in the fight against thrombosis and other dangerous diseases.