Polychaetes today considered the largest group of marine organisms.The most common representatives of the class live at the bottom of the pond and the sea is much less lead a planktonic way of life.
Polychaetes: body structure
The body representative of the class consists of a department head, long body and specific anal lobe.In most cases, the body of the animal is clearly divided into several segments, each of which is attached parapodium.
Parapodia are none other than the primitive limb with a small mustache and bristles.It is interesting that some members of the group parapodia transformed into gills.
Like other representatives of the type annedil (leech, Oligochaeta) from an animal body is made of skin-muscular sac.On top of the worm's body is covered with a thin protective cuticle, which is located under a single-layer epithelium.Below the skin is musculature, which consists of longitudinal and circular muscle responsible for the movement and reduce the animal's body.
Polychaetes: internal structure
representatives of this class have a fairly well-developed digestive system, which consists of three parts.The front part consists of a mouth opening which opens into the oral cavity.Then, a piece of food falls into the muscular pharynx.By the way, in the pharynx contains powerful jaws of chitin.Some species are even able to twist it out.
After grinding food into the esophagus, which opens the main glands that produce saliva.Only a few representatives have a small stomach.The mid-gut of the animal is used to complete digesting and absorption of essential nutrients.Rear intestine is responsible for the formation of faeces, and the anus opens on the dorsal part of the anal lobe.
Polychaetes have a closed circulatory system, which consists of the dorsal and ventral arteries.By the way, the dorsal vessel has a large and contractile function, so it works like a heart.In addition, the large arteries connected to the so-called circular vessels that carry blood to the limbs and gills.
respiratory system the representatives of this class is missing.The bodies of gas exchange are the skin and gills, which are placed either on parapodia or in the front, the head section of the body.
allocation system consists of small metanephridia, which remove waste products of metabolism from the coelomic fluid into the environment.Each segment has its own pair of excretory organs that open out small holes - nefroporami.
As for the nervous system, it consists of a typical peripharyngeal ring, which departs from the ventral nerve cord.Interestingly, almost all members of this class have a highly developed sense of touch and smell organs.Some species have eyes.
Polychaetes: reproductive system and reproduction
For a start it is worth noting that almost all the species of this group are capable of asexual reproduction, which in most cases is represented by the fragmentation of the body, sometimes budding.
However, the animals have a well-developed reproductive system.Reproduction exclusively dioecious worms.Gonads are formed on the wall of the secondary cavity body.The release of sex cells can be carried out through a gap of tissue - in this case the adult dies.Some members have specific openings through which gametes are allocated.Fertilization occurs in an aqueous medium.From zygote develops a larva, which apparently bears little resemblance to adult.Accordingly, the development of the young worm occurs with metamorphosis.
