Echinoderms are peculiar animals. They cannot be compared in structure to other types. The appearance of these animals resembles a flower, a star, a cucumber, a ball, etc.
Study history
Even the ancient Greeks gave them the name "echinoderms". Representatives of this species have long been interested in humans. The history of their study is associated, in particular, with the names of Pliny and Aristotle; and in the 18th and at the beginning of the 19th century many famous scientists (Lamarck, Linnaeus, Klein, Cuvier) studied them. Nevertheless, most zoologists at that time correlated them with either intestinal or worms. I.I. Mechnikov, a Russian scientist, found out that they are related to intestinal glands. Mechnikov showed that these organisms are closely related to representatives of the chordates.
Variety of echinoderms
In our time, it has been established that echinoderms - animals that belong to the group of the most highly organized invertebrates - are secondary. They appeared on our planet more than 520 million years ago. The remains of echinoderms are found in sediments belonging to the Early Cambrian. This type includes about 5 thousand species.
Echinoderms - marine animals, benthic, the bulk of which are free-living organisms. Less common are those attached to the bottom with the help of a special stalk. The organs of most organisms are located along 5 rays, but their number in some animals is different. It is known that the ancestors of echinoderms had two-sided symmetry, which of the modern species have free-floating larvae.
Internal structure
Representatives of echinoderms in the subcutaneous connective layer develop a skeleton consisting of calcareous plates and needles, spikes, etc., on the surface of the body. As in chordates, in these organisms the secondary body cavity is formed by separating mesodermal sacs from the intestine. Gastropore during their development overgrows or is converted into the anus. In this case, the larval mouth is formed anew.
Echinoderms have a circulatory system. Nevertheless, their respiratory organs are developed quite poorly or even completely absent. It is necessary to briefly describe other signs of echinoderms. These animals lack special excretory organs. The nervous system of the organisms that interest us is quite primitive. It is located partially in the skin epithelium or in the epithelium of the parts of the body that are pushed inside.
External structure
The characteristics of echinoderms should be supplemented by the features of the external structure of these organisms. The outer epithelium in the main part of the echinoderms (with the exception of holothurians) has cilia, which create a flow of water. They are responsible for the supply of food, gas exchange and cleansing the body of dirt. In the echinoderms, there are various glands (causing luminescence and poisonous) and pigments, which give an amazing color to these animals.
The skeletal elements of starfish are calcareous plates that are arranged in longitudinal rows, usually with spines protruding outward. The body of the sea urchins is protected by a calcareous shell. It consists of a series of plates tightly interconnected, with long needles sitting on them. Holothurians have calcareous bodies that are scattered across their skin. The skeleton of all these organisms is internal by origin.
Musculature and Ambulacral System
The muscles of these animals are represented by muscle bands and individual muscles. It is developed quite well, as much as one or another animal is mobile. In most echinoderms, the ambulacral system is used for touch, movement, and in some sea urchins and sea lilies it is intended for breathing. These organisms are dioecious; they develop with metamorphosis of the larva.
Echinoderm classification
There are 5 classes of echinoderms: Ophiuras, Starfish, Sea Urchin, Sea Lilies and Holothurians. The type is divided into 2 subtypes: freely moving echinoderms are represented by ophiuras, holothurians, sea urchins and starfish, and attached by sea lilies, as well as some extinct classes. About six thousand modern species are known, as well as twice as many extinct species. All echinoderms are marine animals that live only in salt water.
Sea stars
The most famous representative of the type we are interested in is a starfish (a photo of one of them is presented above). These animals belong to the class Asteroidea. It is no coincidence that starfish received this name. In their form, many of them represent a five-pointed star or a pentagon. However, there are also species whose number of rays reaches fifty.
Look at what an interesting body a starfish has, the photo of which is presented above! If you turn it over, you can see that from the bottom of the rays there are rows of small tubular legs with a suction cup at the end. The animal, sorting them, crawls along the seabed, and also climbs to vertical surfaces.
All echinoderms have the ability to quickly regenerate. In a starfish, every ray that has separated from the body is viable. It regenerates immediately, and a new organism emerges from it. Most starfish feed on the remains of organic matter. They find them in the ground. Their diet also includes corpses of fish and algae. However, some starfish are predators that attack their prey (fixed invertebrates). After the prey is found, these animals dump their stomach out. Thus, the digestion of some predatory starfish is carried out outside. The rays of these animals have very powerful muscles. It allows them to easily open the clams. Starfish, if necessary, can also crush its shell.
The most famous among predatory animals is Acanthasterplanci - the crown of thorns. It is the worst enemy of marine coral reefs. About 1,500 species are in this class (echinoderm type).
Starfish can reproduce both sexually and asexually (regeneration). The bulk of these animals are dioecious organisms. Fertilization in them is carried out in water. The body develops with metamorphosis. Some starfish live up to 30 years.
Wormtail (Ofiura)
These animals are very reminiscent of stars: they have thin and long rays. Ofiur (a type of echinoderm) has no hepatic appendages, anus, and hindgut. In their lifestyle, they also look like starfish. These animals are dioecious, however, capable of both regeneration and asexual reproduction. Some species are luminous forms.
The body of the snake-tails (ofiur) is represented by a flat disk, the diameter of which is up to 10 cm. 5 or 10 thin long segmented rays extend from it. Animals use these curving rays to move around, crawling along the seabed. These organisms are driven by tremors. They pull forward two pairs of their "hands", after which they sharply bend them back. Wormtail feed on detritus or small animals. Ofiurs live on the bottom of the sea, sponges, corals, sea urchins. There are about 2 thousand species. These animals are known from the Ordovician.
Sea lilies
Echinoderms are very diverse. Examples of sea lilies, which are also of this type, are presented above. These organisms are exclusively benthic. They lead a sedentary lifestyle. It should be emphasized that sea lilies are not plants, namely animals, despite their name. The body of these organisms consists of a calyx, stem and hands (brachioli). Using their hands, they filter food particles out of the water. Most modern species are free-floating and stemless.
Stemless lilies can slowly crawl. They are even able to swim in water. Their diet consists of small animals, plankton, the remains of algae. The total number of species is estimated at 6 thousand, of which less than 700 are currently represented. These animals are known from the Cambrian.
Beautifully colored species of sea lilies live mainly in the seas and oceans of the subtropics. They are attached to various underwater objects. It is believed that these are endangered animals, but in the Mesozoic and Paleozoic era, their role in the waters of the seas and oceans was very great.
Sea cucumbers (holothuria)
These organisms are called by different names: sea cucumbers, sea egg capsules or holothurians. They represent a class of invertebrate type echinoderms. There are species that a person eats. The common name for edible holothurians is trepang. Trepang is mined on a large scale in the Far East. There are also poisonous holothurias. Various drugs are obtained from them (for example, holoturin).
Currently, about 1150 species of sea cucumbers are represented. Their representatives are divided into 6 squads. The Silurian period is the time to which the oldest holothurian fossils belong.
These organisms differ from the rest of the echinoderms in an oblong, spherical or worm-shaped form, as well as in the reduction of the skin skeleton and in that they do not have protruding spines. The mouth of these animals is surrounded by a corolla consisting of tentacles. With their help, holothurians capture food. These animals are bottom, although very rarely found and living in silt (pelagic). They lead a sedentary lifestyle. Holothurians feed on small plankton or silt.
Sea urchins
These animals live at the bottom or in the bottom. The body of most of them is almost spherical, it can also be ovoid. Its diameter is from 2-3 to 30 cm. Outside, the body is covered with rows of spikes, calcareous plates or needles. As a rule, the plates are interconnected motionlessly, forming a shell (dense shell). This shell does not allow the animal to change shape. Today, there are about 940 species of sea urchins. The largest number of species was represented in the Paleozoic. Currently, there are 6 classes, while extinct - 15.
As for nutrition, some sea urchins use dead tissue (detritus) for food, while others scrape algae from stones. In the latter case, the animal’s mouth is equipped with a special chewing apparatus called the Aristotelian lantern. In appearance, it resembles a drill. Some species of echinoderms (sea urchins) with its help not only get food, but also modify rocks, drilling holes in them.
The value of sea urchins
These animals are a valuable species of sea biological resources. Of commercial interest is mainly sea urchin caviar. In Japan and other countries of the Asia-Pacific region, it is a delicacy. The caviar of these animals contains many biologically active substances. Scientists believe that the elements that are available in it can be used for cancer as a therapeutic and prophylactic agent. In addition, they normalize blood pressure, increase potency, and remove radionuclides from the human body. It is proved that eating caviar increases resistance to various infections, helps with gastrointestinal diseases, reduces the effects of radiation therapy, improves the functions of the genital and thyroid glands, and the cardiovascular system.
Given the above, it is not surprising that the sea urchin is a marine echinoderm that becomes a welcome dish. For example, the inhabitants of Japan eat about 500 tons of caviar of this animal each year both in-kind and as additives to dishes. By the way, the consumption of this food product is associated with such a long life in this country, where people live on average 89 years.
In this article, only the main echinoderms were presented. We hope you remember their names. Agree, these representatives of the marine fauna are very beautiful and interesting.