General characteristics and structure of annelids

Ringworms (Ringworms) type – includes animals whose bodies are made up of repeating segments, or rings.

The annelids are the most progressive group of worms.

These are the first animals to have a circulatory system, a segmented body, paired organs of movement – a prototype of future limbs. For the first time in evolution, annelids have a secondary body cavity (whole).

Secondary body cavity, or whole

The body of annelids consists of three layers of cells: ectoderm, endoderm, and mesoderm (that is, they are three-layer animals).

The secondary body cavity (whole) develops from the cells of the mesoderm (the space between the body wall and internal organs).

In contrast to the primary body cavity, the secondary cavity is lined from the inside with its own internal epithelium. The whole is filled with liquid, which creates the constancy of the internal environment of the body. Thanks to the pressure of the liquid, the secondary cavity maintains a certain shape of the worm’s body, serves as a support during movement (i.e., serves as a hydroskeleton) and ensures the functioning of organ systems (participates in metabolism: transfers nutrients, accumulates and removes harmful substances, and also removes reproductive products ).

Body segments

Ringed worms are mostly free-living animals and have bilateral symmetry. On their body, the head section, trunk and tail section can be distinguished.

The body of annelids is divided into successive sections – segments, or rings (hence the name – annelids). There may be several or hundreds of such segments in different species. Each segment is an independent compartment: it has its own external outgrowths, nodes of the nervous system, excretory organs and gonads.


The movement is provided by bundles of annular and longitudinal muscles, as well as special paired outgrowths of the body located on the sides of each segment – parapodia (similar to legs), which not all annelids have.

Digestive system

The digestive system includes the mouth, pharynx, esophagus, middle and hind intestines, and anus.

Respiratory system

Breathing is carried out through a wet surface of the body or through the gills.

Excretory system

The excretory system is found in every segment of the body of the worms.

Nervous system and senses

In comparison with roundworms, annelids have a more perfect nervous system and sense organs.
The nervous system is characterized by an accumulation of nerve cells above the pharynx – the periopharyngeal ring and the abdominal nerve cord with nerve branches in each segment.

Ringed worms have sense organs: sight, touch, taste, smell, hearing, balance.

Circulatory system

The circulatory system of annelids is closed, that is, the blood does not pour freely into the body cavity, but moves only through the vessels.


Ringed worms are dioecious and hermaphrodites.
Reproduction is possible asexually and sexually.
Sexual reproduction takes place with the participation of two individuals, even in hermaphrodites.
During asexual reproduction, the body of the worm breaks up into several parts, and then each of them completes the missing head and tail sections.


Ringworms evolved from primitive (lower) worms with an undivided body, similar to flat ciliary worms. In the process of evolution, they developed a secondary body cavity (whole), a circulatory system, and the body was divided into rings (segments). From the primitive polychaetal worms the small-bristled worms have evolved.

The type of annelids is divided into several classes, among which the most significant are three: Polychaetae, Malochaeta and Leeches.

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