Polychaete worms: a brief description of the class

Polychaete worms are today considered to be the largest group of marine organisms. Most often representatives of the class live on the bottom of the sea basin and are much less likely to lead the planktonic mode of life.

Polychaete worms: the structure of the body

The body of the representative of this class consists of the head part, the long trunk and the specific anal lobe. In most cases, the body of such an animal is clearly divided into several segments, to each of which parapodium is attached.

Parapodia are nothing more than primitive limbs with small antennae and setae. It is interesting that the parapodia of some representatives of the group transformed into gills.

Like the other representatives of the annedil type (leeches, small worms), in such an animal the body consists of a skin-muscular sac. Above, the body of the worm is covered with a thin protective cuticle, under which is located a single-layered epithelium. Under the skin is the musculature, which consists of the longitudinal and annular muscles, which are responsible for the movement and contraction of the animal's body.

Polychaete worms: internal structure

Representatives of this class have a fairly developed digestive system, which consists of three parts. The front part consists of a mouth opening that opens into the mouth. Then the food particle gets into a muscular throat. By the way, it is in the throat that the powerful jaws of chitin are contained. Some species can even turn it out.

After grinding, the food enters the esophagus, where the main glands that produce saliva open. Only some representatives have a small stomach. The middle gut of the animal serves to fully digest and suck the necessary nutrients. The posterior part of the intestine is responsible for the formation of feces and opens with an anal opening on the dorsal part of the anal lobe.

Polychaete worms have a closed circulatory system, which consists of the dorsal and ventral arteries. By the way, the dorsal vessel is large and has contractile functions, therefore it works as a heart. In addition, large arteries are connected by so-called annular vessels that carry blood to the extremities and gills.

The respiratory system in representatives of this class is absent. The organs of gas exchange are the skin and gills, which are located either on the parapodia, or in the anterior, head part of the body.

The excretory system consists of small metanephridia, which remove unnecessary metabolic products from the coelomic liquid into the external environment. Each segment has its own pair of excretory organs, which open outward through small holes - nephropores.

As for the nervous system, it consists of a typical oclo-pharyngeal ring, from which the abdominal neural chain departs. It is interesting that almost all representatives of this class have highly developed organs of touch and smell. Some species have eyes.

Polychaete worms: reproductive system and reproduction

For starters, it should be noted that almost all species of this group are capable of asexual reproduction, which in most cases is represented by fragmentation of the body, less often by budding.

Nevertheless, animals have a well-developed reproductive system. Reproduction of worms is exclusively bipedal. Gonads form on the wall of the secondary body cavity. The release of germ cells can be carried out through a rupture of tissues - in this case, the adult dies. Some representatives have specific holes through which gametes are allocated. Fertilization occurs in the aquatic environment. A larva develops from the zygote, which does not seem to resemble an adult in appearance. Accordingly, the development of the young worm occurs with metamorphosis.