The class of Polychaetes includes free-living annelids, in which numerous long bristles are collected in bundles and are located on the sides of each segment. There are about 7000 species of polychaete worms. Most of them live in the seas, where they crawl along the bottom, bury themselves in silt or swim in the water column.
Like all annelids, the body of the Polychaete consists of segments, the number of which varies from 5 to 800 in different species. In polychaete worms, the head section and the anal lobe are distinguished.
On the sides of each segment of the body, skin and muscle outgrowths are noticeable – the organs of movement, which are called parapodia. The worm rakes with parapodia from front to back, clinging to the unevenness of the substrate, and thus crawls forward.
Among the polychaete worms, there are sessile forms that build a protective tube and never leave it.
In sedentary forms of worms, a partial reduction (contraction) of the parapodia occurs: they often remain only in the front of the body.
The musculocutaneous sac consists of a thin cuticle, cutaneous epithelium and muscles. Under the cutaneous epithelium there are two layers of muscles: transverse (annular) and longitudinal. Under the muscle layer there is a single-layer internal epithelium, which lines the secondary body cavity from the inside and forms partitions between the segments.
The digestive system begins with the mouth, which is located on the ventral side of the head lobe, continues with the muscular pharynx (many predatory worms have chitinous teeth in it, which serve to capture prey). The esophagus and stomach follow the pharynx.
The intestine is divided into three sections: the anterior, middle, and hindgut. The anal opening is located on the anal lobe.
Free-living polychaete worms are mainly carnivores that feed on crustaceans, molluscs, coelenterates and worms. The sedentary ones feed on small organic particles and plankton suspended in water.
In polychaete worms, gas exchange (absorption of oxygen and the release of carbon dioxide) is carried out either by the entire surface of the body, or by sections of parapodia, inside which blood vessels enter. In some sedentary forms, the respiratory function is performed by the corolla of tentacles on the head lobe.
The circulatory system in annelids is closed: in any part of the worm’s body, blood flows only through the vessels.
There are two main vessels – dorsal and abdominal (one vessel passes over the intestine, the other – under it), which are connected by numerous semicircular vessels. There is no heart, and the movement of blood is provided by contractions of the walls of the dorsal vessel, in which the blood flows from back to front, in the abdominal – from front to back.
The excretory system is represented by paired tubes located in each segment of the body. Each tube begins with a wide funnel facing the body cavity. The edges of the funnel are covered with ciliated cilia. The opposite end of the tube opens outward on the lateral side of the body. With the help of the system of excretory tubes, waste products that accumulate in the coelomic fluid are removed to the outside.
The nervous system consists of paired supraopharyngeal nodes (ganglia), connected by cords in the periopharyngeal ring, paired abdominal nerve chain and nerves extending from them.
The sense organs are most developed in free-living polychaete worms. On the head of these worms there are a pair of palps, a pair of tentacles and antennae. These are the organs of touch and chemical sense. Many of the polychaete worms have eyes. There are organs of balance.
Reproduction and development
Most of the polychaete worms are dioecious. The sex glands are found in almost every segment. Mature germ cells (in females – eggs, in males – sperm) first enter the whole, and then through the tubules of the excretory system – into the water.
Fertilization is external. A larva develops from the egg, which swims with the help of cilia. Then it sinks to the bottom and turns into an adult worm.
Some species also reproduce asexually. In some species, the worm divides across, and each half restores the missing part. In others, the daughter individuals do not diverge, and as a result, a chain is formed, including up to 30 individuals, but then it breaks up.
The development of polychaete worms occurs with the alternation of life forms. Their larvae do not look like adults.
In some polychaete worms, care for the offspring is observed (for example, they guard the laid eggs).
The importance of polychaete worms in nature is quite large: they filter water, purifying it; are orderlies of reservoirs, destroying a mass of decomposing remains. Polychaetal annelids are eaten by crustaceans, fish, echinoderms, and coelenterates.