INVERTEBRATE ZOOLOGY IN THREE VOLUMES - BOOK 1 - H.I. Shcherbak - 1995

SUBKINGDOM MULTICELLULAR ANIMALS (METAZOA)

SECTION TRUE MULTICELLULAR ANIMALS (EUMETAZOA)

PHYLUM ROTIFERA (ROTIFERS)

CLASS ROTATORIA (ROTATORS)

Most rotifers are free-living, motile animals; some species lead a sessile lifestyle, while a few are parasites of certain invertebrates, particularly Protozoans, such as heliozoans. Many rotifer species form colonies. These colonies are formed by several hundred or sometimes thousands of young free-swimming individuals that aggregate and secrete large amounts of a gelatinous substance. The colony either attaches to a substrate (Lacinularia flosculosa) or swims freely (Sinantherina socialis).

The body of rotifers exhibits A wide variety of shapes—most commonly, it is elongated and circular or flattened in cross-section (Fig. 175). The body is more or less clearly divided into three regions: the HEAD, trunk, and tail (or FOOT). This division is due to the presence of external integumentary constrictions and is unrelated to true body segmentation. The presence of thinned integument in certain areas of the body provides flexibility and The ability to retract the head, and sometimes the tail region, into the trunk using specialized Muscles. Rotifers with a trunk covered by a cuirass (lorica) defend themselves against predators in this manner. The head region bears the corona (rotatory apparatus), which is a collection of cilia closely arranged along the edges of disk-shaped cephalic outgrowths that resemble the blinking spokes of a wheel when beating. During locomotion in Water, the corona Functions as a locomotory organ. When the rotifer is attached to a substrate, the beating of the cilia creates a vortex that draws food particles toward the Mouth opening. The Structure of the corona may be simple—appearing as a ciliated area around the mouth—or complex, yet its functions remain the same. The head of rotifers often bears various outgrowths and appendages, and in sessile forms, the margin of the head transforms into a more or less broad funnel.

Class="center">

Fig. 175. Diagram of rotifer structure — dorsal view (a, b), lateral view (c, d): 1 — corona; 2 — retrocerebral organ; 3 — toe; 4 — adhesive gland; 5 — muscles; 6 — nerve cords; 7 — salivary gland; 8 — Pharynx with mastax; 9 — Esophagus; 10 — digestive gland; 11 — Stomach; 12 — protonephridia; 13 — hindgut; 14 — Urinary Bladder; 15 — cloaca; 16 — cerebral ganglion; 17 — eyespot; 18 — tentacle; 19 — body cavity; 20 — oviduct; 21 — vitellarium ( yolk gland); 22 — Ovary; 23 — mouth

The trunk region may bear fixed or movable outgrowths in the form of spines, keels, oars, or fins. This region houses most of the Internal Organs. Posterior to the anus lies a muscular body outgrowth of varying structure—the tail region, or foot. It terminates in one or two movable outgrowths, the toes, at the tips of which open the ducts of specialized glands that secrete a sticky substance, or a plate with openings for these ducts. The STRUCTURE OF THE foot largely determines the rotifer's lifestyle: by means of the foot, they regulate the direction of movement while swimming or attach themselves to the substrate.

The hypodermal layer in rotifers is very thin (0.5 µm), thickening only near the corona and the foot. The hypodermis is a syncytium lacking Cell boundaries, with a small number of nuclei. Electron Microscopy studies indicate that within the hypodermal Cytoplasm, directly beneath The Plasma Membrane, There is a dense fibrous layer 0.2–0.4 µm thick. In loricate rotifers, it reaches a greater thickness. In certain species, the hypodermis secretes a dense outer envelope, the cuticle (for example, in Asplanchna). The integument of rotifers contains scleroprotein molecules that provide mechanical strength (Chitin is absent from the integument).

Thickened integument or a massive lorica is predominantly found in rotifers inhabiting aquatic vegetation in the littoral zones of water bodies, where the probability of damaging unprotected integument is higher.

In many species of sessile rotifers, protective cases, tubes, or similar structures form around the body (Fig. 176). In some species, these are completely transparent and composed of a gelatinous substance secreted by the hypodermis; in others, foreign particles adhere to the outer sticky surface, making the case clearly visible. The young females of Floscularia ringens construct their tube-cases in an exceptionally interesting manner (see Fig. 176). The walls of their tubes are built from uniformly sized pellets composed of the rotifer's excrement and foreign particles. Such pellets are formed in the "pellet-forming organ," which is a ciliated depression on the head located beneath the lobe; during Ciliary movement, unformed particles constantly rotate, become compacted, and take on the shape of a pellet. The rotifer then bends its head region and glues the pellet onto the wall of the case among similar pellets attached previously.

Fig. 176. Rotifers:

a — Philodina brevipes; b — Macrochetus collinsi; c — Rotaria neptunia; d — Trichocerca cylindrica; e — Floscularia ringens; f — Ptygura thienemanni

The ducts of adhesive glands open at the bottom of the depression. Driven by the water currents generated...

Rotifers possess well-developed muscles that form circular and longitudinal bands, designed for retracting the head and foot, enveloping the intestine, and operating the mastax and urinary bladder.

Rotifers have a primary body cavity (schizocoel), which consists of fluid-filled spaces between the internal organs. Externally, it is bounded by the integument and lacks a true cellular lining. The internal organs are situated quite freely within the cavity, surrounded by fluid containing floating Cells. The primary body cavity performs a skeletal (supportive) function and also plays a crucial role in metabolic processes—through it, nutrients are transported from the intestine to the musculature, nerves, and Reproductive System, metabolic wastes and gases are transported, and End products of METABOLISM are temporarily accumulated. Thus, the primary body cavity acts as the internal environment of the Organism. The schizocoel facilitates Metabolic exchange between the intestine and other Tissues; The transport of nutrients and metabolic wastes occurs faster than through the parenchyma of Flatworms. Therefore, in animals possessing a body cavity, the intestine does not branch and appears as a straight tube.

The Digestive System begins with the mouth opening, located on the ventral side near the anterior end of the body, in the center of the corona. This leads to the Oral Cavity, which transitions into the pharynx. Its expanded muscular portion, or mastax, contains a chewing apparatus formed by cuticular thickenings of the walls and consists of an anvil (incus) and two hammers (mallei). The shape of the individual PARTS OF THE chewing apparatus is extremely diverse.

During the movement of the mallei and incus, food is ground up. The walls of the pharynx bear two to eight Salivary Glands, the ducts of which open into the pharynx. The pharynx leads into a narrow esophagus, followed by a sac-like stomach that occupies most of the trunk cavity. The ducts of paired digestive glands, located on the sides of the anterior portion of The Stomach, empty into it. The stomach leads to a narrow hindgut, the terminal section of which is called the cloaca and opens via the anus. In some rotifers, the posterior region of the digestive system is absent; the gut terminates at the stomach, and undigested food remnants are expelled through the mouth. Rotifers feed on Algae, Bacteria, and detritus, and there are predatory forms that hunt protozoans, other rotifers, etc. In slow-swimming animals, food is captured by the corona during locomotion, whereas in fast-swimming ones, only after attachment. Predators actively capture food using the chewing apparatus, during which the pharynx can be everted outward to seize prey.

The excretory system begins with terminal cells called cyrtocytes (flame cells), located in the head region (3–50 on each side). These give rise to capillaries and collecting tubules that empty into a common urinary bladder or a pair of ducts opening into the cloaca. The excretory system also performs an osmoregulatory function. In addition to protonephridia, amoeboid Cells of the coelomic fluid (excretophores) participate in excretion, as well as the retrocerebral organ—a gland located in the posterior dorsal part of the head that opens externally via a pore. Its functions are not yet fully elucidated, but it is known to secrete mucus containing dissimilation products.

The Nervous system of rotifers consists of a main (cerebral) ganglion—the dorsal Brain—mastax and foot ganglia, small ganglia scattered throughout the body, and nerves that connect the ganglia, innervate Muscle bands, Sense Organs, etc. Consequently, the nervous system of rotifers does not form a regular orthognon (ladder-like) nervous system.

Sense organs are represented by tentacles, which function as organs of Touch, equilibrium, and chemoreception, and photoreceptor eyespots, which may number two or just one. The eyes, as in turbellarians, are inverted.

Rotifers are dioecious animals exhibiting Sexual Dimorphism. Females are found much more frequently than males; therefore, the morphological descriptions provided above pertain to females. Males are, firstly, considerably smaller than females and, secondly, exhibit a simplified structure—their intestine is vestigial, and the excretory system is underdeveloped. The lifespan of males is short, ending after Fertilization of the female. The Male Reproductive System consists of a single Testis from which a sperm duct leads, emptying into the cloaca. Its terminal section is enclosed within a muscular copulatory organ. The FEMALE REPRODUCTIVE SYSTEM is somewhat more complex and is represented by an ovary, a vitellarium (yolk gland), and an oviduct that opens into the cloaca. The ovary together with the vitellarium forms a single organ enclosed in a membrane, the continuation of which is the oviduct.

In some species, copulation occurs via the cloaca, while in others it takes place through any part of the body. In the latter case, the male attaches to the female using a sucker located just above the copulatory organ. The mechanism by which spermatozoa penetrate the body cavity remains unclear. From the body cavity, the spermatozoa actively migrate into the ovary, where egg fertilization takes place. Most

rotifer species lay eggs, and viviparity is observed in only a few. Cleavage is spiral, bearing similarities to that of turbellarians, though it exhibits several specific features. The blastopore, as in turbellarians, develops into the mouth opening. The anus (cloacal aperture) forms at later Selection/3.html">Stages of development.

Embryonic development lasts three to four days. A characteristic feature of rotifers is that their organs are formed from a very small number of cells. Once embryonic development is complete, Cell Division ceases, and the Total Cell Count remains constant throughout life. This accounts for the constancy of the cellular composition in rotifers and their lack of regenerative capacity.

Development in rotifers is direct, without metamorphosis. Their life cycle follows a heterogonic pattern, meaning there is an alternation of bisexual and parthenogenetic generations (Fig. 177). Resting eggs give rise to amictic females, which produce amictic eggs parthenogenetically (without fertilization) with a diploid chromosome set. Reduction division does not occur during their formation. These eggs develop into identical amictic females, and this process repeats for several generations. Subsequently, under the Influence of Environmental factors (changes in Temperature, food quantity and quality, light, Chemical composition of the medium, etc.), mictic females appear. They lay mictic haploid eggs, a process accompanied by reduction division. These eggs develop, without fertilization, into males, all cells of which possess a haploid chromosome set. The males fertilize the mictic females, which then lay thick-shelled diploid eggs from which the amictic female generation eventually develops. These eggs are covered with sturdy membranes and can remain dormant from two weeks to a year, resuming development when favorable conditions return. The number of life cycles per year varies among species; there are mono-, di-, and polycyclic species. Moreover, the same species in different water bodies may exhibit a varying number of annual cycles depending on environmental conditions.

Fig. 177. Life Cycle of rotifers: amictic female (A), mictic female (M), male (n): 1 — parthenogenetic reproduction, 2 — bisexual reproduction

Thus, the rotifer life cycle proceeds with intermediate reduction—alternating between Two Types of adult generations: those with a diploid chromosome set (amictic and mictic females) and those with a haploid set (males). This is one of the unique cases of such a form of heterogony among Multicellular animals, with the other being The life cycle of single-celled foraminiferans.

In addition to developmental cyclicity, rotifers also exhibit cyclomorphosis—the periodic variation in the external Morphology of certain generations, which is clearly visible, for example, in seasonal differences in the size and shape of external spike-like body projections (Fig. 178).

Fig. 178. Annual cycle of Kellicottia/Anuraea cochlearis:

a — winter egg; b–e — generations of parthenogenetic females with summer eggs; f — male

Most rotifer species are cosmopolitan, and some are also undemanding regarding their habitat conditions, living in warm and cold, saline and fresh waters, as well as on land. Animals that have successfully colonized all types of habitats are referred to as ubiquists.

Terrestrial and certain aquatic rotifers do not lose viability after prolonged complete desiccation; upon rehydration, they resume their metabolic activity. Dried rotifers can withstand heating up to 100 °C for 5 minutes and freezing down to –270 °C for 4 hours.

Rotifers play a significant role in aquatic ecosystems by purifying water through filtering massive quantities of bacteria, algae, and detritus. They serve as an essential food source for other organisms, particularly for newly hatched fish larvae.