Annelids, also called ringworms or annelids, include a huge number of animal species. Their body consists of numerous repeating rings (segments), which is why they got this name. The general characteristic of annelids unites about 18 thousand of their various types. They live on land in the soil and on the surface in tropical rainforests, in the sea water of the oceans and fresh water of rivers.
Classification
Annelids are a type of invertebrate animal. Their group is called the primary. Biologists of 5 classes of annelids are distinguished:
- girdle, or leeches;
- low-bristle (the most famous representative of this class is the earthworm);
- polychaetae (grit and nereid);
- misostomids;
- dinofilides.
There are also ectoparasites and mutualists. Among annelids, you can see predators and blood-sucking individuals, scavengers and representatives of filtrators. Especially many representatives of this large type of animal are distributed in the depths of the ocean. They are found at a depth of 11 km and at all latitudes of the planet. At the bottom of the ocean, their density is 1 square. meter of the seabed up to 600,000 individuals.
The important role of such worms
Considering the general characteristics of annelids, you understand their important biological role in the processing and aeration of soils. Earthworms produce loosening of the soil, which is favorable for the entire surrounding vegetation of the planet. To understand how many there are on earth, imagine that in 1 square. from a meter of soil aeration is carried out from 50 to 500 annelids. This increases the yield of agricultural land.
Ringworms are one of the main links in the food chain of the ecosystem both on land and in the oceans. They feed on fish and turtles, birds and other animals. Even people use them as a top dressing when breeding commercial fish species in both fresh and marine bodies of water. Fishermen plant worms as bait on a hook when fishing on a fishing rod.
Everyone knows about the importance of medical leeches, which suck blood from sore spots, saving a person from hematomas. People understood their therapeutic value a long time ago. Apply leeches with hypertension, increased blood clotting. Leeches have the ability to produce hirudin. This is a substance that reduces blood coagulation and dilates the blood vessels of the human blood system.
Origin
Studying the general characteristics of annelids, scientists have established that they have been known since the Cambrian period. Considering their structure, biologists came to the conclusion that they came from an older type of lower flatworms. The similarity is evident in certain structural features of the body.
Scientists believe that the main group of polychaete worms appeared first. In the process of evolution, when this type of animal came to life on the surface and in fresh water, small-bristled, later called leeches, appeared.
Describing the general characteristics of annelids, we note that this is the most progressive type of worms. It was they who first born the circulatory system and the ring-shaped body. Paired organs of movement appeared on each segment, which later became a prototype of the limbs.
Archaeologists have found extinct annelids with several rows of calcareous plates on their backs. Scientists believe that there is a definite connection between them and mollusks and brachiopods.
general characteristics
In grade 7, the type of annelid is studied in more detail. All representatives of this type have a fairly characteristic structure. What on the front, on the back, the body looks the same and symmetrical. It is conventionally divided into three main departments: the head lobe, numerous segments of the central part of the trunk and the back, or anal lobe. The central segmented part, depending on the size of the worm, can include from a dozen to several hundred rings.
The general characteristic of annelids includes information that their sizes vary from 0.25 mm to a length of 5 meters. The movement of worms is carried out in two ways, depending on its type. The first way - by reducing the muscles of the body, the second - with the help of parapodia. These are the bristles found in polychaete worms. They have lateral bilobate outgrowths on the walls of segments. In small-bristle worms, organs such as parapodia are absent altogether or have separately growing small tufts.
The structure of the head lobe
Annelids have sensory organs in front. These are the eyes, the olfactory cells, which are also present on the tentacles. Ciliary fossae are organs that distinguish between the effects of various odors and chemical irritants. There are hearing organs that have a structure resembling locators. And, of course, the main organ is the mouth.
Segmented part
This part represents the same general characteristic of annelid type. The central region of the body consists of rings, each of which is a completely independent part of the body. Such an area is called a coelom. It is divided by partitions into segments. They are noticeable when considering the appearance. The outer rings of the worm correspond to the internal partitions. By this sign, the worms got their main name - ringed, or ringed.
This division of the body for the life of the worm is very important. If one or more rings is damaged, the rest remain intact, and the animal regenerates in a short period of time. The internal organs are also located in accordance with the segmentation of the rings.
Secondary body cavity, or whole
In the structure of annelids, the general characteristic is as follows: the skin-muscle bag has a coelomic fluid inside. It consists of a cuticle, skin epithelium and ring and longitudinal muscles. In the fluid that is contained in the body cavity, the constancy of the internal environment is maintained. All the basic functions of the body are carried out there: transport, excretory, musculoskeletal and sexual. This fluid is involved in the accumulation of nutrients, removes all waste, harmful substances and reproductive products.
Ringworm type also has common characteristics in the field of the structure of body cells. The upper (outer) layer is called the ectoderm, then the mesoderm is located with a secondary cavity covered with its cells. This is the space from the walls of the body to the internal organs of the worm. The liquid contained in the secondary cavity of the body, thanks to the pressure, maintains the worm's constant shape and plays the role of a hydroskeleton. The last inner shell is called the endoderm. Since the body of annelids consists of three shells, they are also called three-layer animals.
Worm Food System
The general characteristic of annelids in grade 7 briefly describes the structure of the digestive system of the body of these animals. In front of the mouth is the opening. It is located in the first segment from the side of the peritoneum. The entire digestive tract has a through structure system. This is actually the mouth, then there is a periopharyngeal ring that separates the pharynx of the worm. The long esophagus ends with goiter and stomach.
The intestine has a common characteristic for the class of annelids. It consists of three departments with different purposes. These are the anterior, middle and hind intestines. The middle compartment consists of an endoderm, and the rest are ectodermal.
Circulatory system
The general characteristic of annelids is briefly described in a textbook of grade 7. And the structure of the circulatory system can be seen in the schematic image above. Vessels are marked in red. The figure clearly shows that the circulatory system of annelids is closed. It consists of two long longitudinal vessels. It is dorsal and abdominal. They are interconnected by the annular vessels existing in each segment, which resemble veins and arteries. The circulatory system is closed, the blood does not leave the vessels and does not spill out into the body cavity.
The color of blood in different types of worms can be different: red, transparent and even green. It depends on the properties of the chemical structure of the respiratory pigment. It is close to hemoglobin and has a different oxygen content. Depends on the habitat of the annelid worm.
The movement of blood through the vessels is due to contractions of some sections of the dorsal and, less commonly, circular vessels. After all, hearts have no worms . The rings contain special contractile elements in these vessels.
Excretory and respiratory systems
These systems of the type of annelid worms (a general description is briefly described in the textbook of grade 7) are associated with the skin. Breathing is carried out through the skin or gills, which in marine polychaete worms are located on parapodia. Gills are branched thin-walled outgrowths on the dorsal lobes. They can be of different shapes: leaf-shaped, cirrus or bushy. The inner part of the gills is penetrated by thin blood vessels. If the worms are small-bristled, then breathing occurs through the moist skin of the body.
The excretory system consists of metanephridia, protonephridia, and myxonephridia located in pairs in each segment of the worm. Myxonephridia is the prototype of the kidneys. Metanephridia have the shape of a funnel located in the whole, from which a thin and short channel displays the excretion products to the outside in each segment.
Nervous system
If we compare the general characteristics of round and annelid worms, the latter have a more advanced nervous system and sensory organs. They have an accumulation of nerve cells above the near-pharyngeal ring of the anterior lobe of the body. The nervous system consists of ganglia. These are the supralopharyngeal and subpharyngeal formations connected by nerve trunks into the periopharyngeal ring. In each segment, you can see a pair of such ganglia of the abdominal chain of the nervous system.
The figure above can be considered. They are marked in yellow. Large ganglia in the throat play the role of the brain, from which impulses diverge along the abdominal chain. The sensory organs of the worm are also referred to the nervous system. He has a lot of them. These are the eyes, and the organs of touch on the skin, and chemical feelings. Sensitive cells are located throughout the body.
Breeding
Describing the general characteristics of the type of annelid worms (Grade 7), one cannot fail to mention the reproduction of these animals. Mostly they are heterosexual, but some developed hermaphroditism. The latter include leeches and earthworms, well known to all. In this case, conception occurs in the body itself, without fertilization from the outside.
In many polychaetae, the development occurs from the larva, while in the remaining subspecies it is direct. The sex glands are located under the coelomic epithelium in each or almost every segment. When a rupture occurs in these cells, the germ cells enter the coelomic fluid and are excreted through the organs of the excretory system. For many, fertilization occurs on the outer surface, and in underground soil worms, inside.
But there is another type of breeding. In conditions favorable for life, when there is a lot of food, individual parts of the body begin to grow in individuals. For example, several mouths may appear. Subsequently, the rest grows. The worm breaks up into several separate parts. This is a asexual type of reproduction, when a certain part of the body appears, and the rest regenerate later. As an example, we can cite the ability to this type of reproduction of auloforus.
In the article, you learned in detail all the main characteristics of annelids, which are studied in the 7th grade of the school. We hope that such a detailed description of these animals will help to acquire knowledge more easily.