Consider a number of animals that biology studies - the type of annelid worms. We learn about their species, lifestyle and habitat, internal and external structure.
general characteristics
Ringworms (also called simply ringworms, or annelids) are one of the vast groups of animals, which, according to various sources, includes about 18 thousand species. They are skeletal-free vertebrates, which not only participate in the destruction of organic substances, but are also an important component 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 name to the type. Among annelids can be found only on soil-processing, but also mutualists (worms living in symbiosis with another organism), ectoparasites (living on the surface of the body), blood-sucking parasites, predators, scavengers, and filtrators.
Ringworm habitat
Where can I meet these animals? The habitat of annelid worms is very extensive - this is the sea, and land, and fresh water bodies. Annelids living in the salty waters of the ocean are very diverse. Kolchetsov can be found in all latitudes and depths of the oceans, even at the bottom of the Mariana Trench. Their density is high - up to 100,000 copies per square meter of bottom surface. Marine annelids are a favorite fish food and play an important role in the marine ecosystem.
In fresh water bodies, mainly blood-sucking parasites are found - leeches, which, in particular, are used in medicine. In tropical countries, leeches can inhabit trees and soil.
Water species not only crawl along the bottom or bury themselves in silt, some of them can build a protective tube and live without leaving it.
The most famous 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.
Annelid classes
About 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 annelid worms, due to which they were isolated in a separate type. This is the presence of celloma (secondary body cavity), metamerism (segmentation) of the body and circulatory system. In addition, the annelids are characterized by the presence of specific organs of movement - parapodia. The ring system has a developed nervous system, which consists of the supralopharyngeal nerve node and the abdominal nerve chain. The structure of the excretory system is metanephridial.
Type annelids is divided into 4 classes. Classes of annelids:
- Polychaete annelids (also called polychaetes). In this class, three subclasses can be distinguished: free-movable, sedentary-attached, and misostomids.
- Small bristle annelides (oligochaetes).
- Leeches In this class, 4 orders are distinguished: pharyngeal, maxillary, proboscis and bristle-legged leeches.
- Echiurid.
The external structure of the annelid worm
Kolchetsov can be called the most highly organized group of worms. Their body sizes range from a few fractions of a millimeter to two and a half meters! The body of the worm can be divided into three parts: head, trunk and anal lobe. The peculiarities of annelids are that annellids do not have a clear division into departments, as in animals with higher organization.
On the head of the worm are various sensory organs. Many annelids have well-developed vision. Some types of rings can boast particularly sharp vision 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.
Worms also have a taste sensation. The worms smell due to the presence of olfactory cells and ciliary fossa. Hearing organs are arranged like locators. Some echiruids are able to distinguish very quiet sounds due to hearing organs, similar in structure to the lateral line in fish.
Respiratory organs and circulatory system of annelid
Small-worm worms breathe the entire surface of their body. But in many-setled respiratory organs - gills. They are bushy, leaf-shaped or cirrus outgrowths of parapodia penetrated by a large number of blood vessels.
The circulatory system of the annelid worm is closed. It consists of two large vessels - the abdominal and dorsal, which are connected by annular vessels in each segment. Blood movement is due to contractions of certain sections of the spinal or ring vessels.
The circulatory system of the annelid worm is filled with the same red blood as a human. This means that iron is present in it. However, the element is not part of hemoglobin, but another pigment - hemerythrin, which captures 5 times more oxygen. This feature allows 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 mouth opening and oral cavity, sharp jaws, pharynx, salivary glands and narrow esophagus.
The oral cavity, which is also called the buccal section, is able to turn inside out. Behind this department are the jaws, which are bent inward. This unit is used to capture prey.
Then follows the mesodeum, the middle intestine. The structure of this department is uniform along the entire length of the body. The middle intestine narrows and expands; it is in it that food is digested. The hind gut is short and ends in the anus.
The excretory system is represented by metanephridia, located in pairs in each segment. They remove waste products from the abdominal fluid.
Nervous system and sensory organs
All classes of annelids have a ganglionic nervous system. It consists of a peri-pharyngeal nerve ring, which is formed by the connected supra-pharyngeal and sub-pharyngeal ganglia, and of pairs of chains of abdominal ganglia located in each segment.
The sensory organs in the annuli are well developed. Worms have keen eyesight, hearing, smell, touch. Some annelids not only capture light, but can themselves emit it.
Breeding
The characteristic of the annelid worm suggests that representatives of this type of animal can reproduce both sexually and asexually. Asexual reproduction can be done by dividing the body into parts. The worm splits into half, 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 process of budding can cover the entire body when the posterior ends bud from each segment. In the process of reproduction, additional oral openings can form, which later will separate into independent individuals.
Worms can be dioecious, but some species (mainly leeches and earthworms) developed hermaphroditism, when both individuals play the role of both females and males. Fertilization can occur both in the body and in the external environment.
For example, in marine worms that reproduce sexually, external fertilization. Different-sex animals throw their sex cells into the water, where eggs and spermatozoa merge. From fertilized eggs, larvae appear that are not similar to adults. Freshwater and terrestrial annelids do not have a larval stage, they are immediately born similar in structure to adult individuals.
Grade Polychaete
Polychaete worms have the largest number of species among 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. Polychaetae are distinguished by a well-defined head region and the presence of parapodia, peculiar limbs. They are mainly heterosexual, the development of the worm occurs with metamorphosis.
Nereids actively swim, can bury themselves in silt. They have a serpentine body and many parapodia, animals make moves with the help of a retractable pharynx. Sandworms in appearance resemble earthworms and dig deep into the sand. An interesting characteristic of the annular sandworm is that it moves in the sand hydraulically, pushing the cavity fluid from one segment to another.
Sedentary worms, serpulids, which live in spiral or twisted calcareous tubes, are also curious. Serpulids protrude from their homes only a head with large fan-shaped gills.
Class Small bristle
Small-worm worms predominantly live in soil and fresh waters, and are found singly in the seas. The structure of the annelid worm of this class is distinguished by the absence of parapodia, homogeneous segmentation of the body, and the presence of the glandular girdle in mature individuals.
The head is not expressed, it may be devoid of eyes and appendages. Setae, rudiments of parapodia are located on the body. This body structure is due to the fact that the animal leads a digging lifestyle.
The earthworms that live in the soil are very common and familiar to everyone with little bristles. The body of the worm can be from a few centimeters to three meters (such giants live in Australia). Also in the soil there are often small, about a centimeter in size, whitish enchitreida worms.
In fresh water bodies, worms living in entire colonies of vertical tubes can be found. They are filtrators that feed on suspended organic matter.
Leech Class
All leeches are predators, mostly feeding on the blood of warm-blooded animals, worms, mollusks, and fish. The habitat of ringworms of the leech class is very diverse. Most often, leeches are found in fresh water, moist grass. But there are marine forms, and even terrestrial leeches live on Ceylon.
Of interest are the digestive organs of leeches. Their mouth is equipped with three chitinous plates that cut through the skin, or proboscis. In the oral cavity are numerous salivary glands that can secrete a toxic secret, and the pharynx acts as a pump during sucking.
Ehiurida Class
One of the rare species of animals that biology studies is ringed worms of echiurida. The ehiurid class is small, it has only about 150 species. These are soft, sausage-like sea worms with a proboscis. The mouth is located at the base of the non-retractable proboscis, which the animal can discard and grow again.
The habitat of ringed worms of the echiurid class is great depths of the sea, sandy holes or clefts of rocks, empty shells and other shelters. Worms belong to the filter.