Ancient fossils: bryozoans, crinoids and others. Living earth - know your home Larvae of the fungiform bryozoan and the birth of a colony: video
13.0.54 Bryozoans.- Bryozoa
If in the middle of summer you take a leaf of an egg capsule or water lily floating in a river or pond, you will often find some kind of embroidery on the side facing the water that looks like it was made from horn (Fig. 13.1). These embroideries are nothing more than the home of the bryozoan - Plumatella repens. To verify this, you just have to throw this leaf into an aquarium or even just into a glass of water and look at it from below. In less than a few minutes, small, white, fluffy stars will begin to appear from these flyers, and soon all the branches of the flyers will be dotted with many such tufts. These fluffy stars are the bryozoan itself. Rock the glass, touch the leaf, and the bryozoans, feeling the push, will instantly all disappear. And everything will calm down, the danger will pass, and they will all look out again.
These curious animals constitute one of the last stages of the animal kingdom, and for a long time they were classified, due to some similarity of the colonies they form, as coral polyps, but now zoologists have recognized that they have a much higher organization, and therefore consider them as separate, independent class.
The most characteristic feature of bryozoans is their coloniality and attached lifestyle. They secrete a substance similar to parchment from themselves, and make from it something like interconnected tubes. This substance is so dense that it persists even after their death, so that in this respect, bryozoans really form a dense skeleton, similar to the calcareous skeleton of corals.
As for the builders of these tubes themselves, thanks to the numerous tentacles surrounding their mouths, of which only one peeks out of the tube, they look like some kind of small flowers. But if you take them out from there and examine them, at least with a large magnifying glass, then their body has the appearance shown in Fig. 13.2, where under the letter d the esophagus is located under e- stomach, and under f- hindgut. Looking further, we see that their mouth, which does not have any chewing devices, goes into the stomach, which is a large bag, that their nervous system is made up of only one nerve node located on the dorsal wall of the esophagus, and that the sense organs, respiratory organs ( excluding tentacles), as well as a heart and blood vessels, they are completely absent.
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Reproduction of bryozoans occurs in several ways. Firstly, by sexual reproduction, with live bait and eggs developing in the same animal. A larva with cilia emerges from the fertilized egg, which leaves the mother animal and, rotating around its longitudinal axis, merrily rushes around the aquarium until it finds a suitable place: a pebble, a twig or a floating leaf of a nymph; then its eyelashes fall off, it turns into a sessile animal and gradually grows into a whole colony. Such larvae can be found in especially large numbers in the water at the beginning of July, and if you plant an adult colony of bryozoans in an aquarium at this time, in other words, throw a nymphal leaf with a colony of bryozoans into it, then hundreds of oval free-swimming larvae will immediately appear in this aquarium, which at first glance can be mistaken for ciliates.
The second method of reproduction is budding, through which the colony itself is mainly formed. Finally, the third is with the help of overwintering buds, the so-called statoblasts, an enlarged image of which we see in Fig. 13.2, g.
These statoblasts have the appearance of lentils and are so different in different species that they even serve as the main characteristic for identifying these species. They are perfectly protected from cold, drought and tolerate winter well, and in the spring, under the influence of the life-giving rays of the spring sun, their constituent halves open like clockwork, and an embryo emerges, which, without turning into a larva, directly attaches to some underwater object and begins to grow to the colony.
Such statoblasts are found in abundance in autumn on the surface of the water or on the shore in the sand. Having collected them, you can save them all winter in a jar of water, which should only be kept closer to the window, and in the spring a colony will develop from each of them.
In addition to this creeping bryozoan (Plumatella repens), in rivers, on floating, broken reed stems and twigs, another species is found in the form of a sponge-like growth - the spongy bryozoan (Pl. fungosa) (Fig. 13.1, A) and, finally, the bryozoan Pl. is even more common. fruticosa, branching upward like corals.
The last species of bryozoans lived in my aquarium for entire years, then disappearing, then appearing again, and at times covered entire plants and even the grotto with its beautiful colonies. At the slightest fluctuation in the water, at the slightest impact on the aquarium, all the animals instantly disappeared, but the water calmed down, and the white fluffy crests appeared again. The main condition for the preservation of these bryozoans is a possibly rare and careful change of water. It’s better not to change it at all even in the aquarium where they live, since even a small addition of fresh water usually ruins them. The aquarium where they are placed must, of course, be planted with aquatic plants and be in a bright place. My water temperature was constantly +15° R., but at one time in one of the aquariums of the Society of Aquarium Lovers a colony appeared that lived in water with a temperature of +10° and even +9° R.
You can get these curious animals everywhere, but most often they are introduced by themselves along with aquatic plants taken from rivers.
Type of bryozoan (Bryozoa)
Bryozoans- a kind of group. They are mostly immobile, sessile animals. However, among them there are also individual mobile forms. For example, the freshwater Cristatella mucedo, whose worm-like colonies have a wide, muscular sole; with its help, they slowly crawl along underwater objects, for example, along the stems of aquatic plants (Fig. 307, 14).
The movement speed of Cristatella is about 1-15 mm per day. Young colonies of Lophopus, Lophopodella and Pectinatella can also move.
The vast majority of bryozoans are colonial animals (Fig. 308), outwardly similar even to plant organisms (hence the name - bryozoans, i.e. similar to moss).
The position of bryozoans in the animal world was unclear for a long time. Old authors (Linnaeus and others), based on purely external similarities, classified them as coelenterates; those with a soft skeleton (Fig. 307) are classified as hydroids, and those with a calcareous skeleton (Fig. 307) are classified as corals.
A more detailed study of bryozoans showed that they are built much more complex than polyps (Fig. 308). Then they began - without sufficient grounds - to be combined together with some other groups into one type under various names: worm-shaped, mollusc-shaped, tentacled. This connection is artificial, and at present both zoologists and paleontologists have abandoned it. Bryozoans are now considered as a special, ancient type of the animal world, in evolutionary development standing between protostomes and deuterostomes. The presence of a secondary cavity (Fig. 308) and some other characteristics allows them to be placed on a level with annelids, higher worms.
Due to their sedentary lifestyle, they actually developed some characteristics similar to polyps, which led to their earlier misunderstanding.
Bryozoans live in both seas and fresh waters, although they are much more numerous in the seas.
Colonies of marine bryozoans are very diverse in shape (Fig. 307): on the one hand, there are soft ones with a horn-like skeleton - bushy (Fig. 307), leaf-shaped (Fig. 307) or fleshy without any specific shape (Fig. 307); on the other hand, there are species with a hard (calcareous) skeleton, sometimes complex in shape (Fig. 307).
In some cases, colonies in the form of thin crusts cover underwater objects, such as stones (Fig. 307), mollusk shells, etc. All these colonies are very variable and depend on the conditions in which they develop and live: on depth, substrate, water movement, etc. environmental factors (ecological variability).
The colonies of a number of marine bryozoans are remarkable in that the individuals composing them are not the same, but differ in both structure and function, that is, in such colonies there is specialization and a “division of labor” between individuals. Such colonies are called polymorphic or multiform, in contrast to monomorphic single-form ones, in which all individuals are the same.
Freshwater bryozoans have only monomorphic colonies.
In polymorphic colonies, the following groups of individuals are distinguished.
First group- ordinary individuals, i.e., having a lophophore, a crown of tentacles, a normally arranged intestine and forming reproductive products. There are always more such individuals in or all colonies than others - they capture food particles, digest, assimilate them and feed the entire colony. In this regard, they are called feeding or ordinary individuals. Often special individuals, the so-called oecia, i.e., individuals serving as brood chambers in which eggs develop.
Rice. 310. Structural features of marine bryozoans: A - avicularia of a marine bryozoan with an open “lower jaw”. The muscles that close the “jaw” are shown in black. B - vibracularium of a marine bryozoan. A tourniquet (2) with muscles moving it is visible. On the left - feeling peg (1)
Second group sharply modified individuals that have the function of protecting colonies from various “uninvited guests” - small worms, crustaceans and other small predators. Among these protective individuals differ avicularia, having the appearance of “bird heads” (Fig. 310, A). They do not have tentacles and therefore cannot feed themselves, but receive food from ordinary individuals. They strongly develop a special appendage in the form of a “lower jaw”, which can slam shut when special muscles contract. With the help of such a device, avicularia capture “uninvited guests” and, thus, free the colony from them. The second form of protective individuals (less common) are vibracular(Fig. 310, B). They develop a special long movable appendage, which can, with the help of special muscles, vibrate and drive away “uninvited guests”, preventing them from even climbing onto the colony.
The avicularia and vibracularia have special sensory formations that are connected to the nervous system and signal the presence of an enemy. Some forms have special individuals with the help of which colonies are attached to the substrate.
In addition to these protective individuals that “actively” protect the colony, many forms have passive defense formations - various outgrowths of the outer wall - thorns, prickles, etc. In some forms they cover the entire colony, making it prickly and thereby scaring off enemies. In others, spikes develop around the openings of the cells, preventing enemies from getting into them.
An example of the first case can be the species Uschakovia gorbunovi described by our famous bryozoan researcher G. A. Kluge from the Siberian seas. Its colonies are covered with long spiny-spike-like outgrowths (Fig. 311), making the colony of this species inaccessible to enemies. This species is found at depths of up to 700 m at temperatures from -0.9° to -1.4°C and is considered as high-arctic. This species is also interesting in another respect: in the lower part of its colonies there are individuals (Fig. 311) that lack a tentacle apparatus and do not form reproductive products; their cells are almost always filled with a white granular mass. These peculiar cells are modifications of the colony's individuals, "warehouses" of reserve nutrients to support the growth of these complex colonies.
An example of the second case, when the spines are located mainly around the openings of the cells, can be: firstly, Flustrella hispida is a very common species in the tidal zone of the Murmansk coast of the White Sea and other boreal and Arctic seas, where they form rather thick brown colonies, mainly on fucoids; secondly, the Black Sea bryozoan Discora, which forms reddish thin calcareous crusts on algae, mollusk shells and other substrate at a depth of 20-80 m in the Sevastopol Bay, along the southern coast of Crimea and in the northwestern part of the Black Sea.
Colonies of freshwater bryozoans are less diverse. They are either in the form of branched thin lying tubes located on underwater objects and underwater vegetation, for example, on the lower surface of the leaves of water lilies and egg capsules (Fig. 307), or form massive colonies on underwater objects - stones, sunken logs, plants (Fig. 307), and sometimes on animals - mollusks (anadonta, viviparus), occasionally even on crayfish.
Bryozoan colonies consist of a large number of individual very small individuals. For example, a piece of a colony of Flustra foliacea (Fig. 307) weighing 1 g contains 1330 individuals. Each individual is placed in a separate cell, which has a rather large cavity (Fig. 308). The front part of the individual, the so-called lophophore, with the body cavity entering it, can protrude from the cell.
On the lophophore there is a mouth opening surrounded by a crown of tentacles, which are arranged in a circle or in a horseshoe shape. The mouth can be open (naked bryozoans) or covered with a special outgrowth, the so-called eppstome (angiostomated bryozoans, Fig. 308). The mouth opening leads into the pharynx, which passes into the esophagus, which goes down, that is, into the depth of the cell, and passes into a rather voluminous stomach; from the latter the hindgut extends upward and opens with the anus on the lophophore, outside the crown of the tentacles (Fig. 308).
Eating bryozoans using tentacles covered with cilia. The latter are in continuous movement, creating a current of water moving towards the mouth and then exiting between the tentacles. At the same time, various microorganisms and organic suspension (detritus) are filtered out of the water. All this is driven by the cilia to the mouth, swallowed, enters the stomach and is digested there. Thus, by the nature of their feeding, bryozoans are typical filter feeders and in this regard they bring some benefit by cleaning water bodies, but at the same time they cause more significant harm by overgrowing and clogging various hydraulic and water supply structures. This is especially true for massive freshwater forms of bryozoans (Fig. 307).
Respiratory apparatus, circulatory and excretory systems are absent in bryozoans. The exchange of gases occurs through the tentacles. The cavity fluid serves as blood, and the excretory functions are performed by special cells located in it.
Nervous system due to a sedentary lifestyle, it is greatly simplified and in each individual consists of only one nerve ganglion located between the oral and anal openings (see Fig. 308, 309). Nerves extend from it to the tentacles and to all organs of the individual. There is no single nervous system for the entire colony. The sensory organs are the tentacles.
Reproduce bryozoans both sexually and asexually. The budding rate of some bryozoans is very high, especially in warm seas. There is evidence that off the Hawaiian Islands, bryozoans form colonies up to 2 m in height in just a few hours. In the port of Odessa, a sunken ship was covered with a continuous crust of bryozoans in 3-4 months.
Reproductive products are formed in the body cavity of cells from coelomic epithelial cells, and most bryozoans are hermaphrodites. Some marine bryozoans have specially modified individuals that serve as brood chambers, the so-called oecia, in which the development of the egg to the larva occurs. If there are no oeciums, then the larva develops in the body cavity of the mother. The formed larva comes out and leads a planktonic lifestyle for some time; Thus, the development of bryozoans occurs with metamorphosis.
The larvae of bryozoans are very different from the larvae of other groups (annelids, brachiopods, etc.) both in structure and in their transformation into the primary individual. This once again proves that bryozoans are a unique and independent group (type) of the animal world.
The larvae of bryozoans are very diverse (Fig. 313), all of them have a more or less developed ciliary apparatus, with the help of which they can actively swim in plankton. Then they land on the substrate and turn into a primary individual, which forms colonies by budding. The most famous larva is tsifonautes, triangular in shape, has a thin transparent bivalve shell (Fig. 313, 1).
The larvae of freshwater (angiostomata) bryozoans have a much simpler structure than the larvae of marine (naked) bryozoans. They are microscopic oval bodies without any outgrowths. Larvae are formed in each cell from the mesodermal epithelium. In almost every cell, growing, they occupy its entire cavity, displacing the feeding individual. Thus, in angiostomes, all individuals are a kind of oecium, but, unlike those in barestomes, they are a temporary formation. When the larva hatches, everything returns to normal and the feeding individual is restored.
Typically, the larvae live very briefly, only a few hours, and due to their small size and simple structure, they are often not noticed by researchers. A characteristic feature of the larvae of angiostomes is that they contain not one primary individual, but two. When the larvae land on the substrate (near Moscow this usually happens at the end of summer), both individuals begin to develop, one to the right, the other to the left, and form characteristic two-lobed colonies. These colonies are so unique in shape that they were previously distinguished as special species. In some years, such forms literally shower the lower surface of the leaves of water lilies and egg capsules.
The colonies then grow and form flotoblasts, and in the spring they form typical colonies. This is how perennial, sometimes very large colonies are formed, reaching, for example, in Plumatella fungosa (Fig. 307) sizes more than 1 m in height and 25-30 cm in diameter.
Asexual reproduction occurs by budding: each individual budding (Fig. 309) new ones; thus the number of individuals in the colony increases and it grows; colonies of some bryozoans reach a height of several tens of centimeters.
A special case of asexual reproduction occurs in freshwater bryozoans. It lies in the fact that inside the individual (Fig. 309) special internal buds are formed, the so-called statoblasts, covered with a dense shell. Statoblasts arise when conditions unfavorable for the life of colonies are created in a reservoir (in our country - in the fall, in tropical countries - with the onset of a dry period).
Statoblasts, thanks to their durable shell, survive unfavorable conditions, and in the spring a young individual emerges from them, from which a new colony is formed by budding. There are different types of statoblasts. Pipetoblasts, having a simple oval or bean-shaped shape, are covered with a dense shell and lie freely in the tubes of the colonies. When colonies break up, pipetoblasts fall out and form new colonies in the same place. In this way, the species is preserved under unfavorable conditions.
Next form - flotoblasts, or floating statoblasts. They form a special chitinous cellular ring around the capsule, in the cells of which air bubbles accumulate. With the help of this “hydrostatic” apparatus, flotoblasts can float up and passively float for some time in the water column and are carried by currents to new places, where new colonies are formed from them. Thus, flotoblasts not only ensure the preservation of the species, but also its dispersal.
The most complex statoblasts are spinoblasts, in which chitinous hooks are formed on the capsule or on the floating ring, with the help of which they can cling to moving underwater objects or to other animals, for example, to the feathers and legs of waterfowl, and with their help they can be carried over long distances. For example, in the summer of 1962, numerous spinoblasts of the Indo-African bryozoan Lophopodella carteri (Fig. 312) were unexpectedly discovered in the reservoirs of the Volga delta front, where they were apparently brought by migratory birds. Since in the reservoirs of the Volga delta, especially in summer, temperature conditions are close to those in subtropical reservoirs, it is possible that new colonies will develop from these introduced spinoblasts and this species of bryozoans will take root in the reservoirs of the Volga delta.
As already mentioned, most bryozoans live in the seas; there are few of them in fresh waters - mostly angiostomata (Fig. 308). Marine bryozoans are very widely distributed throughout all seas and oceans, from the tidal zone (Flustrella hispida) to great depths, almost 8 thousand m, for example Bugula sp. The largest number of species are found at depths of no more than 200-300 m. Bryozoans live in water at very different temperatures (from -2° to 29 C). Littoral species also tolerate significantly lower temperatures, although falling into a state of suspended animation.
In total, there are currently about 4 thousand modern species of bryozoans and almost 15 thousand fossils.
Of our seas, bryozoans are especially numerous in the Far Eastern and northern ones; for example, in the White Sea there are 132 species of them. Moreover, there it is especially necessary to note the species Flustra foliacea (Fig. 307), which in some areas of the sea, for example in Velikaya Salma, forms, according to observations of the White Sea Biological Station of Moscow State University, powerful thickets. This Arctic-boreal species is widespread in the seas surrounding Western Europe, but is not found on the Murmansk coast, although it has been recorded in the Czech Bay of the Barents Sea and in the Kara Sea.
In our southern seas, due to their low salinity, the bryozoan fauna is poorer: about 30 species are known in the Black Sea, 7 species in the Azov Sea, and only 6 in the Caspian Sea, and one of them, namely Membranipora crustulenta (Fig. 307), penetrated there in recent years along with other Azov-Black Sea organisms, apparently through the Volga-Don Canal.
A special fauna of bryozoans is found in brackish water bodies, for example in the Caspian and Aral seas. Their distribution is associated with the geological past of some water bodies, for example, the Tethys Ocean and its derivatives. Some brackish-water coelenterates also have a similar distribution.
In the freshwater and brackish water bodies of our country, 20 species of bryozoans are known: four from the class of barestomes, the rest are angiostomes, mainly representatives of the genus Plumatella. They are found in a wide variety of reservoirs: in large lakes - Baikal, Onega, Sevan; in small ponds, in large rivers and small streams *.
* (In Lake Sevan, on coastal stones, a special form of the widespread Plumatella fungosa is very common, forming rather dense crusts consisting of thick, compacted tubes with a thick brown cuticle. Its flotoblasts are the same in structure and shape as those of the typical form.)
Freshwater bryozoans are very widespread, although they are more abundant and diverse in the water bodies of tropical and subtropical countries (India, Indonesia); are almost never found in water bodies of the North; only a few cases of their occurrence are known in water bodies north of the Arctic Circle, for example in water bodies of Greenland, Iceland, on the Kola Peninsula, about 66° N. w. Statoblasts of bryozoans were found on Spitsbergen and Novaya Zemlya. The southernmost location of freshwater bryozoans is the reservoirs of Tierra del Fuego.
Some species, for example Plumatella emarginata, Cristatella mucedo, have a very wide distribution, while other species, on the contrary, have very narrow habitats associated with water bodies of certain areas. For example, Stephanella hina is found only in water bodies of Japan, Lophopusella - only in water bodies of South Africa.
The geographic distribution of bryozoans sometimes expands greatly. The presence of statoblasts, especially spinoblasts, allows them to overcome large spaces with the help of ships, floating wood, fish and other animals. For example, Lophopodella carteri was considered until recently to be an Indo-African species, then it was discovered in Indonesia, Australia, China, Japan, and in recent years its spinoblasts were brought to North America and the Volga delta.
In the geographic distribution of freshwater bryozoans, it is necessary to note an interesting feature - the parallel development of identical forms in water bodies with approximately the same abiotic and biotic conditions of existence. For example, in the Altai Lake Teletskoye and the Balkan Lake Ohrid, the same subspecies of the common Fredericella sultana subsp. is found. lepnevae.
Particularly interesting are the bryozoans of the reservoirs of the southern part of our Far East, where, along with the usual widespread forms, there are forms of a southern character, for example Australella indica, with large massive, gelatinous, translucent colonies, in which chains of individual individuals are visible (Fig. 307).
The fauna of Lake Baikal is also interesting, where a species from the genus Hislopia (from naked bryozoans), also common in subtropical countries. In Baikal, this species is considered a relic of past, warmer geological periods. This species has particularly well-expressed ecological variability (Fig. 315).
Bryozoans, both freshwater and marine, together with other sessile organisms, are part of the fouling of ships, port and hydraulic structures, and water pipelines, thereby causing quite significant harm. Especially in this regard, freshwater massifs of the form Plumatella fungosa should be noted. They not only disrupt the normal operation of the mentioned structures, but after the death of the bryozoans, the particles clog the water supply network. The fight against them is carried out by destroying colonies, clearing them out; but it must be borne in mind that this cleaning must be very thorough so that no statoblasts remain (especially those attached to the substrate), since otherwise colonies will develop from them again. Basically, to protect against fouling by bryozoans, as well as other organisms, preventive measures are used - coating the surface with anti-fouling paints, ultrasonic protection, etc.
Quite a strong overgrowth of bryozoans occurs in the Caspian Sea. Representatives of the genera Victorella and Bowerbankia, together with some other forms, form dense tufts covering port structures in the polluted waters of the Baku and Krasnovodsk ports, and Membranipora forms a dense calcareous crust over hydraulic structures and ships in the clean waters of the Middle and Southern Caspian Sea.
The nutritional value of bryozoans is very small. Osborne(1921) provides some evidence that bryozoans can serve as food for some birds and fish. This especially applies, of course, to forms that do not have a skeleton. Some mild forms like Alcyonidium are used by residents of the northeastern regions (Chukotka) to feed their pets (dogs).
Geological history of bryozoans . The most ancient remains of fossil bryozoans belonging to the class of baremouths are known from the beginning of the Ordovician. Representatives of four orders, including two that are currently extinct, were found in the sediments of this system. To date, reliable remains of Cambrian bryozoans are unknown. However, it can be assumed that they existed already from the beginning of this period. Their development in different directions during the Cambrian led to the formation of orders.
In modern seas and fresh waters, representatives of two subclasses - naked and angiostomes. The first includes the orders Ctenostomata, Cheilostomata, Cyclostomata. The rest, namely Gryptostomata, Tipostomata, became extinct, as already indicated, in the Mesozoic. In some places, fossil bryozoans formed quite powerful reefs, similar to modern corals. For example, the sediments of the Kerch Peninsula mostly consist of bryozoan reefs.
General characteristics. Bryozoans, or bryozoans (gr. bryon - moss, zoon - animal), are aquatic, mainly marine, colonial secondary animals leading an attached lifestyle. Colonies of various shapes and sizes consist of small zooids, monomorphic in freshwater bryozoans, polymorphic in marine ones; Among the latter, autozooids and heterozooids are distinguished. In an autozooid, the rear part of the body is transformed into a cystid, protected by a thick cuticle, and the front is modified into a delicate polylipid, bearing a corolla of tentacles retracted into the cystid with the help of a retractor muscle. In many forms, the cuticle is calcified and the colony has a strong calcareous skeleton. The mouth is located in the center of the lophophore. The digestive tract is curved in a loop, the anus is located on the dorsal side, outside the corolla of the tentacles. The body cavity is divided by the diaphragm into two sections - trunk and lophophoral. The suprapharyngeal ganglion innervates the internal organs and tentacles of the lophophore. The coelomic cavity is bipartite or tripartite. The circulatory, respiratory and often excretory organs are absent. Bryozoans are hermaphrodites; Embryonic development occurs in ovicella, or gonozooids. After attachment, a free-swimming larva undergoes necrobiotic metamorphosis and gives rise to a primary zooid, which by budding forms the zooids of a new colony. Bryozoans appeared in the Ordovician and exist to this day.
The structure of a soft body. All bryozoans are colonial organisms. Colonies can be similar to moss (hence the name of the type - bryophytes) or algae; sometimes they have the form of crusts that grow on underwater rocks, algae or shells of mollusks, brachiopods, skeletons of arthropods, corals; some have the appearance of a mesh consisting of rods and loops of different sizes and shapes. Colonies often take on the appearance of spherical or nodular bodies. The shape and size of the colony depend on the living conditions: salinity and temperature of the water, the nature of the substrate, the presence of currents or disturbances. One and the same species can have a different colony shape: when living in the surf zone - massive, in calm water conditions - branched, bushy. The colony is composed of individual zooids (Fig. 221), microscopic in size (usually up to 1 mm). In freshwater bryozoans, the colony consists of monomorphic zooids, in marine ones - from polymorphic individuals. Among the latter, two groups of individuals are distinguished: autozooids, or feeding zooids, and heterozooids - modified individuals that have lost the function of nutrition and sexual reproduction and perform the functions of protection, attachment, etc. in colonies.
The autozooid consists of a cystid, the posterior part of the zooid, which has a strong, often calcified cuticle, and a polylipid, the anterior protruding part of the zooid. The mouth is located at the anterior end of the polypid and is surrounded by a corolla of tentacles located on the lophophore in a ring or horseshoe shape. The tentacles are hollow, covered with cilia, creating inlet and outlet currents of water; With the first, food comes in, with the second, metabolic products are removed. The main food of bryozoans are various small organisms, algae, protozoa and organic detritus. The mouth leads to a loop-shaped digestive tract consisting of the pharynx, esophagus, midgut, or stomach, and hindgut, which opens on the dorsal side outside the ring of tentacles with the anus. The digestive tract is attached to the body wall with the help of a retractor muscle, designed to draw the polylipid into the cystid in case of danger. Protrusion of the body and straightening of the tentacles occurs hydrostatically in different ways in different groups. In addition, the midgut is attached to the wall of the cystid by a special cord - a cord. Freshwater bryozoans have a special outgrowth above the mouth - the epistome (hence the name of the class of freshwater bryozoans - angiostomata), which is absent in marine representatives. The wall of the zooid body in the former consists of epithelium and a layer of circular and longitudinal muscles and coelomic epithelium; it looks like the wall of the skin-muscular sac of worms. In marine forms, it is devoid of muscles and consists of epithelium and cuticle of organic origin, very often impregnated with calcium carbonate, as a result of which the wall of the cystid acquires a strong mineral calcareous skeleton.
The nervous system consists of the suprapharyngeal ganglion, the peripharyngeal ring and nerves - sensory and motor, going to the tentacles, digestive tract and other organs. The cavity lying between the wall of the zooid and the digestive tract is filled with coelomic fluid, which performs the function of the circulatory system. In freshwater bryozoans, the coelom is divided into three parts, in marine ones - into two. The coelomic cavity continues into the epistome, tentacles. In a number of marine bryozoans, the body cavities of neighboring zooids communicate with each other. There is no circulatory system; gas exchange, movement of the cavity fluid, removal of metabolic products, apparently, occur in the process of retraction and protrusion of the small body of the polylipid. The reproductive system consists of testes and ovaries; the former are located in the wall of the anterior section, the latter arise on the cord and in the wall of the posterior part of the body.
Heterozoids designed to perform specialized functions include gonozooids, avicularia, vibracularia, kenozoids and nanozooids. Gonozooids are peculiar zooids arising from autozooids (order Cyclostomata), in which fertilized eggs develop (Fig. 221, 4). A gonozooid occurs when a polylipid degenerates and is often pitcher-shaped. Inside the gonozooid, the embryo develops, which in turn gives rise to numerous secondary embryos (polyembryony), which feed on the tissue located here.
To protect the colony from uninvited settlers, many modern bryozoans have developed avicularia and vibracularia. The first resemble a bird's head (Latin avis - bird), their polypid is reduced and the operculum covering the mouth of the cystid is turned into a grasping organ (Fig. 221, 2). The vibracularia carry a long movable cord, which is also a modified cap (Fig. 221, 6). Kenozooids form root threads and creeping stolons - outgrowths of the colony that perform attachment and support functions. Nannozooids are thin tubes (three times thinner than the cystids of autozooids); the polypid sitting in them has a rudimentary intestine, one tentacle, but normally developed muscles; their role and functions are not clear enough. Special structures in autozooid cheilostomata are ooecium, or ovicella. They are located above the mouth of each zooid. Embryogenesis occurs in them and the larva develops; it is still unclear whether it represents a simplified individual or a differentiation product of an autozooid cystid (see Fig. 229).
Skeletal structure. According to the chemical composition of colonies, or zoaria, they can be organic or calcareous. In some, the walls of zooids, absorbing a large amount of water (up to 95%), form a protective membrane that is not capable of being preserved in a fossil state; in others, the cystid cuticle is built from a protein substance containing mucopolysaccharide and a stereoisomer of true chitin (B-polymer, N-acetylglucosamine). The middle layer of the cuticle has a fibrous structure and represents a matrix on which crystals of calcium carbonate are formed in the form of calcite or aragonite, forming a mineral skeleton. Some bryozoans contain strontium as an impurity (<0-6% SrCO 3) или магний (>7-4% MgCO 3). In cheilostomata, the skeleton is built of calcite.
The size of a colony of modern bryozoans sometimes reaches 1-2 m, among fossils up to 60 cm are known, but usually their size is insignificant. The zooecia, or cells that make up a colony, have the character of long cylindrical tubes or prismatic short chambers; sometimes they take on the appearance of a pear with a wide base and an elongated tubular vestibule. In cross-section, the cells can have the shape of a parallelipiped with a rectangular, oval, or polygonal base. Each cell opens with an aperture that has a different shape: round, oval, star-shaped, polygonal. The aperture is located at the top of the cell, at its anterior end, or on the upper part of the front wall. A peristome (a raised ridge) or a lunarium (a semilunar projection near the mouth) may be developed around the mouth. The mouth is equipped with a cap. In cyclostomata there is no operculum and the mouth is closed by a thin membrane with a terminal pore in the center.
In many tubular cells, the internal cavity is divided by partitions, or diaphragms, which reflected certain stages of the life of the zooid (Fig. 222). Diaphragms are either solid or with a hole in the center. Special semi-septa are also known - hemisepts, separating the vestibule from the base of the cell (in cryptostomats), and cystifragms - special bubble-like structures developed in the cells; their formation is possibly associated with polylipid degeneration. The cell wall is usually permeated with pores, through which communication occurs between the polylipids of the zoaria. In cryptostomats, the thickened parts of the skeleton were penetrated by capillary tubes that served for gas exchange.
In addition to cells of normal size, fossil bryozoans have cells of smaller diameter, which are the remains of the skeleton of heterozooids; Among them are mesopores and acanthopores. Mesopores have a round or angular cross-section and are equipped with diaphragms. Acanthopores are tubular formations of small diameter with a narrow cavity and thick walls; They rise above the surface of the colony in the form of spines. A number of marine bryozoans have spines near the mouth and along the keel of reticulate zoaria, which in modern cheilostomata represent the skeletons of reduced zooids. Various calcareous supporting and mechanical elements of colonies (kenozoids), located on the underside, also develop due to modification of zooids. The plasticity of the organization of bryozoan zooids is thus very great.
Reproduction and development. Almost all bryozoans are hermaphrodites; The testes are located in the upper part of the cystid, and the ovaries are located in the lower part. The eggs are fertilized by the male reproductive cells of other individuals, which enter the body cavity with a current of water. The first stages of embryonic development take place in gonozooids, in ovicella or in the body cavities of the maternal organism. The gastrula is formed by invagination; mesoderm cells develop teloblastically after the closure of the blastopore. The larva is free-swimming. After floating in the water column for some time, it sinks to the bottom and attaches itself to solid objects. The intestine and other larval organs disintegrate, and after necrobiotic metamorphosis, the first cell is formed - the ancestrum with a flattened initial part - the protoecium (see Fig. 221, 3). Individual development in freshwater and marine bryozoans proceeds differently and will be considered when characterizing the classes.
Fundamentals of taxonomy and classification. Based on the characteristics of astogenesis (ontogenesis of the colony) and the structure of the colonies, the phylum of bryozoans is divided into three classes: Phylactolaemata, Stenolaemata and Gymnolaemata.
For a long time now I have had several pebbles of limestone-shell rock with fossilized imprints of ancient organisms. They were picked up at different times and in different places, I can’t remember now. Some were probably found in a limestone quarry, some were brought to me from the Atarskaya Luki, some, perhaps, brought from the Crimea.
I’ve had them for a long time, I just haven’t gotten around to photographing and describing them. Today the planned walk in the forest was cancelled, I had some free time and I took a few pictures. This is what one of the pebbles looks like. It is small in size, a little more than 3 cm.
What it consists of used to be the remains of living organisms of warm shallow seas that fell onto the muddy bottom. Here you can see pieces of shells of ancient mollusks, imprints of bryozoans and pieces of the stem of crinoids (sea lilies). Let's figure out which one is which.
Bryozoans, especially the order Gymnolaemata is easily recognized by its reticulate structure. These are colonies of marine invertebrate organisms, known since the Ordovician period, and still existing in seas of varying salinity. As the name suggests, the colonies of some bryozoans resemble a continuous cover of moss. Some bryozoans form colonies in the form of crusts and clumps on hard surfaces (rocks, shells, etc.), others have a fan-shaped or bush-like appearance. Modern bryozoans, for example, look like this:
They make up the bulk of recognizable fragments on the stone. But don’t forget, bryozoans are not plants, although they look like them, they are full-fledged animals that feed on various microorganisms and diatoms.
Let's look at another stone:
Here, in the same way, the bulk of the fossils are reticulated fragments of bryozoans.
At the bottom in the middle you can see a round piece with notches and a hole in the center (the same “gear” can be found on the right side in the first photo). This is one of the stalk segments sea lily(or crinoids, lat. Crinoidea). These are bottom-dwelling animals with a sedentary lifestyle, belonging to the phylum echinoderms. They are even more similar in appearance to plants - their body consists of a stem, a calyx and brachioles - arms.
Most species of modern crinoids have lost this stalk. During the life of the animal, the stalk consisted of round segments connected by muscles; in the fossil state they often fall apart. Fossilized segments of crinoids are called trochites. Due to their similarity to gears, theories about alien contact millions of years ago constantly arise, and attempts are made to present trochites as ancient parts of alien mechanisms. And they have been known since ancient times; the first written mentions date back to the 17th century. The British called the star-shaped polygonal segments of crinoids “stone stars” and made various assumptions about their connection with celestial bodies. On the Northumberland coast these fossils are called "St Cuthbert's rosary". Whole sea lily prints look like this:
Crinoids (photo by user galamish from Yandex.photos)
Of course, the stone contains a large number of fragments and impressions of shells of various mollusks:
Moreover, they have a completely recognizable shape, characteristic of modern seashells. For example, the shell at the top center of the bottom photo, next to the trochite, is quite similar to a modern scallop.
It’s hard for me to say what kind of long fossil is in the photo below. Maybe a piece of stem, maybe something else.
And just a couple more pictures, try to identify something in them yourself:
Also known and common fossils that you can find, for example, on the banks of rivers are belemnites(popularly called the “devil’s finger”), which are the remains of the fossilized internal shell of ancient mollusks that resemble squid in appearance. Well-preserved mother-of-pearl shells or simply imprints of cephalopod shells are also widely known. ammonites. Their spiral-twisted ribbed shells can range from 1-2 centimeters to 2 meters in diameter.
