What is the natural habitat of annelids
The Annelid worms (Annelida) or also Arthropods form a trunk within the trunk group of Lophotrochozoa (Lophotrochozoa), which belong to the primordial mouths (Protostomia), due to their independent construction plan. Annular worms are divided into two classes: polychaeta and belt worms (Clitellata), which are divided into oligochaeta and leeches (hirudinea). There are around 18,000 different species in total. The largest known species are the giant earthworm, native to Australia Megascolides australis, which can reach a length of up to 3 meters, as well as the sea worm Eunice aphroditois with a similar length and up to 1000 individual segments. The smallest species live in the groundwater and in the sand gap fauna and reach lengths of 300 micrometers (genus Diurodrilus), the dwarf males of the species Dinophilus gyrociliatus are even only 50 micrometers long.
The most important evolutionary changes compared to the blueprints of more simply built worm-shaped animals are the coelom and segmentation of the annelids. The Coelom allows space for complex organs and allows novel, differentiated forms of movement. Skin muscle tube and viscera muscles can perform independent movements. The segmentation makes it possible to specialize individual segment sections for certain tasks (heteronomous segmentation). In total, the annelid worm can be divided into three regions:
- Head (prostomium and peristomium)
- Trunk (similar segments)
- Rear end (pygidium)
This division can no longer be clearly recognizable in some groups.
Segments and parapodies
As the German name "Ringelwurm" already describes, it consists of several rings, so-called segments. On the individual segments, especially well-developed in the case of the polychaetes, there are pair-shaped lateral stubby feet (parapodia) which are criss-crossed with bristles, the so-called chaetae. The parapodia are protuberances on the body wall into which the body cavity expands and muscles are drawn into. The segments are formed one after the other at the sprouting zone in front of the rear end of the body (teloblastia). In some species the number of segments is constant in all individuals, in other species the number fluctuates by a few segments. Most annelids live in water, some species (especially larger ones) also have gills, these are usually located on the parapodia.
Body cavity and locomotion
The secondary body cavity (coelom) of the segmented body, which appears for the first time in evolution in the annelid worms, is completely filled with fluid and is traversed by a straight, tubular intestine. The hydrostatic skeleton enables an effective locomotion system in interaction with the circular muscles, the longitudinal muscles and the bristles. "Errante", ie annelid worms that do not live in tubes or structures, receive their typical locomotion pattern through this system. Depending on the habitat and sometimes also after different phases of life, annelids can crawl, swim, meander or undulate (rhythmic movement to ensure that water flows through living tubes and thus for ventilation).
Blood circulation and nervous system
The blood vessel system is closed and consists of a back and an abdominal vessel. The abdominal vessel carries the blood from the front to the back, the back system lies above the intestine and carries the blood from the back to the front. The dorsal vascular system is usually contractile and sometimes double. The longitudinal vessels are often connected by side vessels. There is no endothelium, the blood flows in the spaces between adjacent tissues. Hemoglobins, chlorocruorin (green) and haemythrin (violet) were detected as respiratory pigments. The gas exchange and thus also breathing takes place through the skin.
Cephalization occurs especially in originally built annelids. The neural centers are originally concentrated in the head area (prostomium) together with the most important sensory cells as the upper pharyngeal ganglion. The prostomium is equipped with light and chemoreceptor organs and movable appendages. There are also receptors on these appendages (palps and antennae). To control the individual segments, however, there are still accumulations of nerve cells, so-called ganglia, in pairs in the individual segments. Here, too, there may be a concentration of ganglia. The rope ladder nervous system typical of the annelids is found as the abdominal marrow on the ventral side of these animals. The nerve rope ladder, which is often shown schematically in textbooks, cannot be easily recognized in the prepared animal. With the naked eye you can only see a collection of nerve fibers in large annelids, which at first glance does not look like a rope ladder. This is because in these taxa the nerve cords are fused into a single cord, or the nerve cell bodies are distributed over the connective. The peristomium joins the prostomium with the mouth opening and other appendages. These attachments may be missing as a secondary feature. The rear end is also originally provided with appendages on which there are receptors. Eyes come in many forms - from the two-celled ocelles to the complex lens eye (tomopteris). Most often they are on the prostomium, but they can also occur in other places (e.g. the segments). Nuchal organs occur within the annelids only in the polychaetes. There are paired, strongly ciliated structures at the rear end of the prostomium, which mainly contain chemoreceptors.
Epidermis and skin muscle tube
The soft epidermis of the annelids has many glands. These glands produce, for example, mucus, secretions for building tubes or even luminous secretions. The epidermis is surrounded on the outside by a cuticle which usually contains a lattice of collagen fibers. The bristles characteristic of most annelids, which are mobile, are also deposits of the epidermis. This means that these bristles are arranged segmentally in deep epidermal pockets. The number and shape of the bristles is very constant for each type and are an important distinguishing feature. Two layers of muscle run under the epidermis: a circular muscle layer and a longitudinal muscle layer. Diagonal fibers can lie between these layers. In addition to these muscles, there are various other muscles such as those that move the bristles, parapodial muscles or dorsoventral muscles. A transfer of forces takes place through the water-filled coelom, which together with the collagen fibers of the cuticle form the hydrostatic skeleton. The muscle cells are mononuclear and belong to the type with diagonally striated muscles.
Intestinal tract and excretory organs
The intestinal tract of the annelids is primarily a tube covered with a single layer of epithelium, which is straight and divided into three sections. It extends from the ventral opening of the mouth at the peristomium to the anus at the pygidium. Based on this, there are many variations depending on the diet and lifestyle.
Excretion takes place through the nephridia. Originally, these are available in pairs in each segment. The majority of annelids have metanephridia, but protonephridia also occur.
The poly-bristles are mostly marine animals. The sedentary species (Sedentaria) feed on plankton, the free-living species (Errantia) mostly feed as predators or as grazers. Some poly-bristles are basically free to move, but are counted among the Sedentaria because they live in tubes and are adapted to a local way of life, such as the pier worm or lugworm (Arenicola marina). You and also the earthworms belonging to the few bristles such as Lumbricus terrestris feed on bacteria and organic matter by eating the surrounding substrate and digesting the nutrient-rich parts. Earthworms and lugworms are therefore significantly involved in the upheaval and loosening of the inhabited soils. The leeches, which predominantly occur in fresh water, also belong to the annelids. They are either blood-sucking ectoparasites (e.g. adult animals of the medical leech, fish leech and snail leech) or are predatory (e.g. horse leech, roller gel).
The reproduction of annelids is extremely varied. There is hermaphroditic and sexual reproduction, live-bearing species, larval development, sprouting, generation change. Depending on the way of life, suitable forms of reproduction have developed.
The possibility of repairing amputations with new sprouting zones is well developed among the annelids and is also used by them for reproduction. In some species, a single segment is enough to fill in the remaining missing ones. Regenerating earthworms go into rigidity.
The sexual reproduction of the annelids originally takes place freely in the water. This means that the gametes are released into the water and fuse there. The direct transmission of the sperm to the sexual partner has developed independently of this several times, and some species have external sex organs.
Originally, larval development takes place via a trochophora larva. However, a number of polychaetes and all clitellates evolve directly.
The syringe worms, the hedgehog worms and the beard worms were until recently considered to be independent tribes, or their systematic classification was long controversial and was only recently assigned to the annelid worms. Like all annelids, beard worms have ciliated trochophoric larvae. In addition, the segmentation of the end piece and the oxygen transport molecule hemoglobin point to a classification in the annelid worms.
- Wilfried Westheide, Reinhard Rieger (eds.): Special zoology, Volume 1. Spektrum Akademischer Verlag 2003, ISBN 3-8274-1482-2
- ↑ From the point of view of classical Latin, the name should be "Anellida", derived from (lat.) anellus "Small ring" (cf. anus "Anus" [sphincter muscle]); Annelida however is based on annulus "Ring" (post-classical Latin).
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