Polychaete worms are today considered the largest group of marine organisms. Most often, members of the class live at the bottom of a sea reservoir and are much less likely to lead a planktonic way of life.
Polychaete Worms: Body Structure
The body of a representative of this class consists of the head section, a long trunk and a specific anal lobe. In most cases, the body of such an animal is clearly divided into several segments, to each of which a parapodia is attached.
Parapodia are nothing more than primitive limbs with small antennae and setae. Interestingly, the parapodia of some members of the group transformed into gills.
Like other representatives of the Annedil type (leeches, small-bristle worms), in such an animal the body consists of a skin-muscle bag. On top of the worm's body is covered with a thin protective cuticle, under which a single-layer epithelium is located. Under the skin is the musculature, which consists of longitudinal and ring 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 oral cavity. Then the food particle enters the muscular pharynx. By the way, it is in the throat that contains powerful jaws of chitin. Some species are even able to twist it out.
After grinding, food enters the esophagus, where the main glands that produce saliva open. Only a few representatives have a small stomach. The middle intestine of the animal serves to completely digest and absorb the necessary nutrients. The posterior intestine is responsible for the formation of feces and opens with the anus on the dorsal part of the anal lobe.
Polychaete worms have a closed circulatory system, which consists of the dorsal and abdominal arteries. By the way, the spinal vessel is large and has contractile functions, therefore it works like a heart. In addition, the large arteries are connected by the so-called annular vessels, which carry blood to the limbs and gills.
The respiratory system of 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 front, head part of the body.
The extraction system consists of small methanephridia, which remove unnecessary metabolic products from coelomic fluid into the external environment. Each segment has its own pair of excretory organs, which open outwards through small holes - nephropores.
As for the nervous system, it consists of a typical periopharyngeal ring, from which the abdominal nerve 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
To begin with, it is worth noting 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.
However, animals have a well-developed reproductive system. Reproduction of worms is exclusively dioecious. Gonads form on the wall of the secondary cavity of the body. Germ cells can be released through tissue rupture - in this case, an adult dies. Some representatives have specific holes through which gametes stand out. Fertilization occurs in the aquatic environment. From the zygote, a larva develops that looks little like an adult. Accordingly, the development of a young worm occurs with metamorphoses.