Let's consider a number of animals that biology studies - type Annelids. Learn about their species, lifestyle and habitat, internal and external structure.
General characteristics
Annelled worms (also called simply annelids, or annelids) are one of the vast groups of animals, which includes, according to various sources, about 18 thousand species. They are non-skeletal vertebrates that are not only involved in the destruction of organic matter, but are also an important part of the nutrition of other animals.
The body of annelids is divided by internal partitions into segments, which correspond to the outer rings. It is this feature that gave the type its name. Among annelids, one can meet not only those who process the soil, but also mutualists (worms living in symbiosis with another organism), ectoparasites (living on the surface of the body), blood-sucking parasites, predators, scavengers, filter feeders.
Anned worm habitat
Where can you find these animals? The habitat of annelids is very extensive - these are seas, and land, and fresh water. Annelids are very diverse,living in the s alt waters of the ocean. Kolchetsov can be found in all latitudes and depths of the World Ocean, even at the bottom of the Mariana Trench. Their density is high - up to 100,000 specimens per square meter of the bottom surface. Marine annelids are a favorite food for fish and play an important role in the marine ecosystem.
In fresh water bodies there are mainly blood-sucking parasites - leeches, which, in particular, are used in medicine. In tropical countries, leeches can live on trees and in the soil.
Aquatic species not only crawl along the bottom or burrow into the mud, some of them can build a protective tube and live without leaving it.
The most famous are the annelids that live in the soil, they are called earthworms. The density of these animals in meadow and forest soils can reach up to 600 specimens per square meter. These worms are actively involved in soil formation.
Classes of annelids
Approximately 200 years ago, Georges Cuvier worked on the classification of the animal world and identified 6 types of its representatives. These included arthropods - creatures whose bodies are divided into segments: crayfish, spiders, insects, wood lice, earthworms and leeches.
You can name some features of annelids, thanks to which they were singled out as a separate type. This is the presence of celloma (secondary body cavity), metamerism (segmentation) of the body and the circulatory system. In addition, annelids are characterized by the presence of specific organs of movement - parapodia. Rings have a developed nervous system,which consists of the supraesophageal ganglion and the ventral nerve cord. The structure of the excretory system is metanephridial.
Type annelids are divided into 4 classes. Annelids classes:
- Polychaete annelids (also called polychaetes). Three subclasses can be distinguished within this class: free-moving, sessile-attached, and misostomids.
- Small bristle annelids (oligochaetes).
- Leech. There are 4 orders in this class: pharyngeal, jawed, proboscis and bristle-bearing leeches.
- Echiuride.
External structure of annelids
Kolchetsov can be called the most highly organized representatives of the group of worms. Their body sizes vary from a few fractions of a millimeter to two and a half meters! The body of the worm can be divided into three parts: the head, trunk and anal lobe. The features of annelids are that annelids do not have a clear division into departments, as in higher-organized animals.
There are various sense organs on the head of the worm. Many annelids have well developed eyesight. Some types of rings boast especially sharp eyesight and complex eye structure. However, the organs of vision can be located not only on the head, but also on the tail, body or tentacles.
Developed in worms and taste sensations. Worms sense smells due to the presence of olfactory cells and ciliary pits. The organs of hearing are arranged according to the type of locators. Some Echiruids are able to distinguish very quiet sounds thanks to the organs of hearing,similar in structure to the lateral line in fish.
Respiratory organs and circulatory system of annelids
Little-bristle worms breathe through the entire surface of their body. But polychaetes have respiratory organs - gills. They are bushy, leaf-like or pinnate outgrowths of the parapodia, penetrated by a large number of blood vessels.
The circulatory system of the annelids is closed. It consists of two large vessels - abdominal and dorsal, which are connected by annular vessels in each segment. The movement of blood is carried out due to contractions of certain parts of the spinal or annular vessels.
The circulatory system of the annelids is filled with the same red blood as in humans. This means that it contains iron. However, the element is not part of hemoglobin, but of another pigment - hemerythrin, which captures 5 times more oxygen. This feature allows the worms to live in conditions of oxygen deficiency.
Digestive and excretory systems
The digestive system of annelids can be divided into three sections. The anterior intestine (stomodeum) includes the oral opening and oral cavity, sharp jaws, pharynx, salivary glands, and narrow esophagus.
The oral cavity, which is also called the buccal region, is capable of turning inside out. Behind this section are the jaws, which are bent inward. This apparatus is used to capture prey.
Then comes the mesodeum, the midgut. The structure of this department is homogeneous inthe entire length of the body. The middle intestine narrows and expands, it is in it that food is digested. The hindgut is short, ending in the anus.
The excretory system is represented by metanephridia located in pairs in each segment. They remove waste products from the cavity fluid.
Nervous system and sense organs
All classes of annelids have a ganglion type nervous system. It consists of a parapharyngeal nerve ring, which is formed by the connected supraoesophageal and suboesophageal ganglia, and of pairs of a chain of abdominal ganglia located in each segment.
The sense organs of the rings are well developed. Worms have sharp eyesight, hearing, smell, touch. Some annelids not only capture light, but can also emit it themselves.
Reproduction
The characteristic of the annelids suggests that representatives of this type of animal can reproduce both sexually and asexually. Asexual reproduction can be carried out by dividing the body into parts. The worm splits into halves, each of them becomes a full-fledged individual.
At the same time, the tail of the animal is an independent unit and can grow a new head for itself. In some cases, a second head begins to form in the center of the worm's body before separation.
Budding is less common. Of particular interest are species in which the budding process can cover the entire body, when the posterior ends bud off from each segment. Duringreproduction, additional mouth openings may also form, which will later separate into independent individuals.
Worms can be dioecious, but some species (mainly leeches and earthworms) have developed hermaphroditism, when both individuals play the role of both female and male at the same time. Fertilization can occur both in the body and in the external environment.
For example, in sexually reproducing marine worms, fertilization is external. Animals of different sexes throw their germ cells into the water, where eggs and sperm merge. From fertilized eggs, larvae appear that do not look like adults. Freshwater and terrestrial annelids do not have a larval stage, they are born immediately similar in structure to adults.
Class Polychaetes
Polychaete worms have the largest number of species among the annulus. Mostly the class is represented by free-living marine animals. There are single freshwater and parasitic species.
Marine annelids belonging to this class are very diverse in form and behavior. Polychaetes are distinguished by a well-defined head region and the presence of parapodia, peculiar limbs. They are predominantly heterosexual, the development of the worm occurs with metamorphosis.
Nereids actively swim, can burrow into the mud. They have a serpentine body and many parapodia; animals make passages with the help of a retractable pharynx. Sandworms in appearance resemble earthworms and are deeplyburrow into the sand. An interesting characteristic of the annelids is that it moves in the sand in a hydraulic way, pushing the cavity fluid from one segment to another.
Sessile worms, serpulids, which live in spiral or twisted calcareous tubes, are also curious. Serpulids protrude from their dwelling only their heads with large fan-shaped gills.
Class Low Bristle
Little-bristle worms mainly live in soil and fresh waters, they are found singly in the seas. The structure of annelids of this class is distinguished by the absence of parapodia, homonomous segmentation of the body, and the presence of a glandular girdle in mature individuals.
Head region is not expressed, may be deprived of eyes and appendages. On the body are setae, rudiments of parapodia. This body structure is due to the fact that the animal leads a burrowing lifestyle.
Earthworms that live in the soil are very common and familiar to all. The body of the worm can be from a few centimeters to three meters (such giants live in Australia). Also in the soil are often found small, about a centimeter in size, whitish enchytreid worms.
In fresh water you can find worms living in whole colonies of vertical tubes. They are filter feeders, feeding on suspended organic matter.
Leech Class
All leeches are predators, mostly feeding on the blood of warm-blooded animals, worms, mollusks, fish. Habitat of annelids of the leech classvery varied. Most often, leeches are found in fresh water, wet grass. But there are also marine forms, and even terrestrial leeches live in Ceylon.
The digestive organs of leeches are of interest. Their mouth is equipped with three chitinous plates that cut through the skin, or proboscis. The mouth contains numerous salivary glands that can secrete poisonous secretions, and the pharynx acts as a sucking pump.
Class Echiuridae
One of the rare animal species studied by biology is the annelids Echiurids. The Echiurid class is small, it has only about 150 species. These are soft, sausage-like marine worms with a proboscis. The mouth is located at the base of a non-retractable proboscis, which the animal can discard and grow back.
The habitat of Echiurid annelid worms is deep seas, sandy burrows or rock crevices, empty shells and other shelters. Worms are filter feeders.
