Ringed worms - a large group of invertebrate organisms.In addition, they are the most organized representatives of worms.They live mostly in fresh and salt water bodies and in the soil.Some tropical species leeches have adapted to terrestrial mode of existence.
type Annelida: general characteristics
The size of this group range from a few millimeters up to six meters.Quite characteristic feature is the presence of this organism segmentation - their body is composed of a plurality of rings, which explains the name of the type.External annulation corresponds to the internal segmentation.That is why the injury or damage annelid body loses a few segments, which soon recovered.
Outside body is covered with cuticle, which can not be reset.From it grow chitin bristles - another characteristic feature of this group of animals.Some representatives on the segments can be parapodia - rather primitive limbs, which in some cases are equipped with sensitive bristles or gills.
Annelida: structural features of the internal organs
for representatives of this type is characterized by the presence of a secondary body cavity - coelom.This cavity is filled with a specific fluid, through which maintained normal levels of internal environment.
have a skin-muscular sac, which consists of balls of the epithelium, as well as muscle, grouped into the ring and longitudinal group.
digestive system sequentially starts the mouth and anal opening ends.Annelids have three intestine - front, middle and rear.In some species formed primitive salivary gland.
breath of the body through the skin.The only exceptions are some marine species, which are located on parapodia gills.As for the circulatory system, it is usually closed.Consisting of the ventral and dorsal aorta, which are interconnected annular vessels.Heart of these organisms is not - provide a reduction in the movement of blood dorsal aorta.Blood roundworms can contain a variety of respiratory pigments.
nervous system is still quite simple.In the front end of the body is a large nerve ganglion, which performs the functions of the brain.From it leaves nerve cord, which in each segment of the body forms a small ganglion - cluster of neurons.Senses presents the eyes, bodies chemical sensitivity and mechanoreceptors, which are distributed throughout the body of the worm.
Annelida: especially reproduction and development
Organisms of this group can be both opposite-sex and hermaphroditic (much rarer).For example, the earthworm has androgynous reproductive system, but it is necessary for the fertilization of two individuals.It can occur in the environment, and through the introduction of sperm in the female inner channels.
interesting fact that annelids with a pronounced segmentation have a tendency to rapid and intensive regeneration.Because of this characteristic for certain types of asexual reproduction.The development of organisms directly, without metamorphosis.
It is worth noting that the role of annelids quite weighty.For example, a well-known earthworm is responsible for soil aeration.This group includes and leeches, which are often used in modern medicine.Special value has hirudin produced by the leech, as it thins the blood and is used in anti-thrombosis and other dangerous diseases.
