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

SUBKINGDOM MULTICELLULAR ANIMALS (METAZOA)

SECTION TRUE METAZOANS (EUMETAZOA)

PHYLUM PSEUDOCOELOMATES (NEMATHELMINTHES)

CLASS NEMATODES (NEMATODA)

Nematodes are one of the most numerous and widespread groups of animals. They inhabit seas, freshwater bodies, and soil, and include PLANT AND ANIMAL parasites. About 20,000 species of nematodes have been described; however, according to various estimates by scientists, the total number of species ranges from 80,000–100,000 to 1 million. The majority of nematodes are free-living animals inhabiting narrow capillary spaces between soil particles, sand, and silt at the bottom of Water bodies or on land, while only about 7,000 species (approximately a third of the known ones) are plant and animal parasites. More than 300 species of nematodes have been found in the freshwaters of Ukraine, and about 200 in the Black and Azov seas. In vertebrate animals, 561 species of parasitic nematodes have been recorded.

The body shape of nematodes is predominantly fusiform, circular in cross-section; in parasitic species, the body is more elongated and thread-like. Females of certain plant-parasitic species (such as Meloidogyne) are pear-shaped, which is attributed to the excessive development of their Reproductive System (Fig. 188).

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Fig. 188. General appearance of nematodes: a — free-living marine nematode Steineria mirabilis; b — Meloidogyne sp.; c — Aphelenchoides composticola, a plant parasite

Free-living nematodes are generally microscopic in size (0.3–1 mm), phytonematodes reach 8–10 mm, while the largest vertebrate parasites, such as the human roundworm, reach a length of 30–40 cm, and Placentonema gigantissima, a parasite inhabiting the Placenta of the sperm whale, reaches 6–8 m.

The Mouth opening is located at the anterior end of the body, and the anus is situated on the ventral side near the posterior end (Fig. 189).

Nematodes are characterized by poorly defined bilateral Symmetry, whereas radial symmetry is more pronounced in their external appearance. The DORSAL SIDE OF the body differs little from the ventral side. Their body is fusiform and circular in cross-section, which is the Origin of the name nematodes (Roundworms). At the anterior end, free-living species bear sensory setae, the arrangement of which follows a triradial symmetry; the arrangement of the three or six Lips around the mouth is also triradial. Bilateral symmetry is more pronounced in the internal Structure.

Fig. 189. Diagram of nematode structure — female (a), male (b):

1 — coelom; 2 — Esophagus; 3 — esophageal bulb; 4 — cervical gland; 5 — excretory pore; 6 — midgut; 7 — Ovary; 8 — oviduct; 9 — Uterus; 10 — egg; 11 — female genital opening; 12 — hindgut; 13 — anus; 14 — Testis; 15 — vas deferens; 16 — ejaculatory duct; 17 — spicules; 18 — bursa

The Skin-muscular sac of nematodes consists of the cuticle, hypodermis, and a layer of longitudinal Muscles.

The nematode cuticle is a multi-layered formation. It consists of four layers (epi-, exxo-, meso-, and endocuticle), each composed of several sublayers. Electron microscopic studies show that the cuticle alternates layers of parallel rods oriented perpendicularly to its surface, oblique, annular, and longitudinal microfibrils, and an amorphous dense substance. This structure ensures the strength and elasticity of the cuticle. The outer layers are permeated by a system of microscopic canaliculi. At the anterior end of many nematodes, the cuticle is thickened due to the proliferation of the loose mesocuticular layer (Fig. 190).

Fig. 190. Diagram of the nematode cuticle structure:

1 — epicuticle; 2 — exocuticle; 3 — mesocuticle layers; 4 — endocuticle

The chemical composition of the cuticle is quite complex, containing various Proteins (albumins, Collagen, scleroprotein), Glycoproteins, and Lipids. The presence of collagen imparts mechanical strength to the cuticle.

The cuticle not only covers the outer surface of the nematode body but also lines all ectodermal sections of the Digestive System—the Oral Cavity, Pharynx, and hindgut. In these regions, its structure is much simpler than that of the external cuticle.

The growth of nematodes is accompanied by molting. The old cuticle detaches from the hypodermis and is shed along with all its derivatives (setae, papillae, etc.) and the internal cuticle of the digestive tract, while the hypodermis produces a new cuticle.

One of the most important Functions of the cuticle is protective. The Specific features of the nematode habitat—spaces between soil particles, narrow intercellular spaces in plant and animal Tissues—require the body to be protected from mechanical damage. The thickening of the cuticle at the anterior end, which experiences the greatest mechanical stress, acts as a Shock absorber. The cuticle also serves as a chemical barrier, protecting body tissues from the effects of various harmful substances. This is particularly important for intestinal parasites of animals. However, the presence of Pores in the epicuticle indicates the selective permeability of the cuticle to certain substances, especially in parasitic forms.

The cuticle also functions as an element of The Musculoskeletal System. Nematode movements result from the antagonistic interaction between the longitudinal musculature and the cuticle, which is maintained in a tensioned state due to the high pressure of the coelomic fluid. Contraction of muscles on one side causes the body to bend, while its straightening is driven by the elasticity of the stretched cuticle. Characteristically, the internal cuticle lining the foregut and hindgut lacks such a multi-layered structure precisely because it does not participate in the biomechanics of locomotion.

The Emergence of a complex and multifunctional cuticle played a crucial role in shaping the Organization of nematodes and marked a major step in the progressive evolution of this animal group. Among all other cuticle-covered worm-like organisms (such as gastrotrichs, rotifers, horsehair worms, and priapulids), only nematodes possess a complex cuticle that performs both protective and locomotor functions.

Beneath the cuticle lies the hypodermis, a modified immersed-type skin epithelium. The hypodermis is sharply differentiated into a thin subculticle layer (0.1–8 mm) spanning the entire body surface between the cuticle and the musculature, and longitudinal hypodermic cords that project deeply into the body cavity.

The nematode hypodermis consists of a limited number of Cells arranged in regular longitudinal rows—ranging from 5 to 12 depending on the species. Each hypodermic Cell is formed by a thin (0.1–0.8 mm) flattened portion contributing to the subculticle, and a large cytoplasmic process containing The Nucleus and most of the Organelles. This process forms an integral part of one of the hypodermic cords.

This structural arrangement of the hypodermis is dictated by its skeletal and mechanical functions. The biomechanics of nematode locomotion require a tight mechanical bond between the cuticle and muscles, which is provided by the hypodermic subculticle. Most organelles are concentrated within the hypodermic cords. Only in parasitic forms does the subculticle layer reach a significant thickness (up to 30 µm) and contain a network of supportive fibers.

As an important barrier tissue, the hypodermis regulates the selective permeation of substances and additionally serves as one of the primary storage tissues for accumulating fats and Glycogen.

The nematode musculature consists of a single layer of longitudinal Muscle cells forming two dorsal and two ventral bands. In most small, free-living, and parasitic nematodes, the number of muscle cells is fixed and small—most commonly eight in cross-section. Large parasitic nematodes possess significantly more muscle cells. Each muscle cell is divided into a contractile zone, a cytoplasmic part, and several branched innervation processes extending from The Cell body to the nerve cords (Fig. 191). The cytoplasmic zone contains the nucleus, Mitochondria, supportive fibers, glycogen granules, and other organelles and inclusions. For a long time, scientists believed that nematodes, like other lower worms, possessed smooth muscle cells. However, it has recently been established that nematode musculature consists of obliquely striated muscle cells, which structurally and functionally approach the striated muscles of Arthropods and Chordates.

Fig. 191. Muscle cells of Pontonema vulgare:

1 — cytoplasmic part; 2 — nucleus; 3 — contractile zone; 4 — innervation processes; 5 — ventral nerve cord

In nematodes, nerves do not extend to the muscles; instead, muscle cells reach toward the nerves with their innervation processes. Thus, the nematode musculature partially performs the function of conducting excitation via a system of innervation processes. Consequently, despite its anatomically simple structure, it exhibits a high level of specialization.

Nematodes are characterized by a primary body cavity, or schizocoel. However, in most microscopically small nematodes, the Organs are closely packed together, and the gaps between them are filled with a non-cellular substance consisting of a dense network of randomly interwoven fine fibrils. In large parasitic nematodes, the schizocoel appears as a large fluid-filled cavity. This fluid generates high internal pressure, which is essential during the undulating Movements of the animals. In small nematodes, high internal pressure is maintained by the cells of all tissues.

The Development of a large schizocoel offers distinct advantages: the fluid-filled cavity facilitates the uniform transmission of internal pressure to the cuticle, acts as an internal environment that ensures The transport of nutrients and metabolic wastes, and occasionally serves as a storage site for protein reserve granules.

Associated with the body cavity in nematodes are phagocytic cells located on the hypodermic cords (Fig. 192). They are numerous in free-living forms (order Enoplida) and limited to just a few pairs in parasitic ones. In Ascaris, these are four stellate cells located in the anterior part of the body on the lateral hypodermic cords. The function of these cells is to engulf insoluble Metabolic waste products and foreign particles.

The mouth of nematodes is surrounded by three to six lips, though some species lack lips entirely. It leads into the foregut (pharynx), lined with a thin cuticle. The pharynx is subdivided into an anterior section (stoma) and a posterior section (esophagus). The stoma frequently forms immovable cuticular thickenings called onchia, as well as movable ones known as Teeth. Some nematodes even possess jaws, while others feature a sharp stylet or spear. The pharynx acts as a pump for sucking in liquid food. The pharynx lumen is always triangular in cross-section—a structure considered optimal for animals feeding on liquid diets. In the posterior region of the pharynx in certain nematodes (e.g., Enterobius vermicularis), There is a bulbous expansion (bulbus) that functions like a pipette, increasing suction power.

192. Cytology/practical/72.html">Cross section of a female Ascaris:

1 — dorsal hypodermic cord; 2 — innervation processes of muscle cells; 3 — muscle cells; 4 — ovary; 5 — intestinal wall; 6 — cuticle; 7 — lateral hypodermic cord; 8 — longitudinal canal of the excretory system; 9 — uterus; 10 — oviduct; 11 — ventral hypodermic cord

The walls of the pharynx are formed by muscular, glandular, and supportive cells. Most commonly, three to five unicellular or bicellular Salivary Glands are embedded within its walls, though occasionally they may be numerous.

Digestion in nematodes begins in the oral cavity under the action of Enzymes produced by the pharyngeal glands. In some species, predominantly plant-parasitic nematodes, extracellular digestion takes place: saliva is secreted outward, liquefying plant tissues, after which the liquid food is ingested and fully digested in the midgut.

The Primary processes of digestion and nutrient absorption occur in the endodermal midgut.

The midgut is a straight tube lined with a single layer of epithelial cells. It is relatively short and lacks outgrowths or accessory glands. Its cells bear tiny projections on their free surface—microvilli—forming a continuous brush border. All cells share the same structure, but throughout its lifespan, each cell initially performs an absorptive function, followed by a secretory one, after which it dies and is shed into the intestinal lumen. Digestion begins in the intestinal lumen and is completed On the surface of the microvilli, where enzymatic activity is concentrated.

In addition to DIGESTION AND ABSORPTION, the midgut functions as a nutrient storage organ, accumulating glycogen, fats, and protein granules within its cells. The midgut also acts as an excretory organ, accumulating various Inorganic Components in its cells that are eliminated from the body along with sloughed-off cells (Fig. 193).

Fig. 193. Internal anatomy of Ascaris — female (a), male (b):

1 — lips; 2 — nerve ring; 3 — pharynx; 4 — phagocytic cells; 5 — ventral hypodermic cord; 6 — lateral hypodermic cord; 7 — Vagina; 8 — ovary; 9 — uterus; 10 — oviduct; 11 — midgut; 12 — hindgut; 13 — ejaculatory duct; 14 — testis; 15 — vas deferens

The hindgut is a short tube lined internally with a cuticle. A muscular valve is situated at the junction between the hindgut and the midgut.

Nematodes lack protonephridia. Their primary excretory organ is the cervical (or renette) gland, a massive cell located near the anterior end of the body with a duct opening externally via an excretory pore on the ventral side just posterior to the mouth. In soil-dwelling, freshwater, and parasitic nematodes (subclass Rhabditia), this gland features two long processes containing internal canals that extend along the lateral hypodermal cords from the posterior end of the body. In most nematodes, the entire cervical gland and its processes are composed of a single cell, and more rarely of two or three cells (Fig. 194). Phagocytic cells, mentioned previously, as well as the intestine, also participate in The excretion of metabolic waste products.

Fig. 194. Excretory system of nematodes (cervical gland): unicellular sac-like (a), tricellular (b), and lobed with long canals (c):

1 — excretory pore; 2 — processes of the excretory cell; 3 — excretory cell

The Nervous system of nematodes consists of a circumoesophageal nerve ring and eight meridional nerve cords, among which the ventral trunk is the most developed. A distinctive structural feature of the nematode nervous system, shared with gastrotrichs, is the absence of true ganglia. The circumoesophageal nerve ring is formed by nerve fibers—cellular processes whose cell bodies are situated around the ring. The highly developed ventral trunk is paired in origin; it bifurcates in front of the genital and anal openings and fuses again posterior to them. The ventral trunk comprises A large number of Nerve Cells and fibers. The other meridional nerves contain no Neurons and are formed solely by processes of cells whose bodies reside within the ventral trunk. These processes initially form half-ring commissures before merging into the meridional nerves. The thickest of these is the dorsal nerve, while the two lateral ones are the thinnest.

The nerve ring serves as the central organ where diverse information received from body-wide receptors is processed. The ventral and dorsal nerve trunks function to innervate the muscles, with the muscular innervation processes extending precisely to these trunks. Sensory functions are performed primarily by the lateral nerve trunks. In most free-living nematodes, the hypodermis contains a well-developed lateral nerve plexus that innervates sensory bristles, whereas in parasitic nematodes this plexus is reduced.

The nervous system of nematodes is directly connected to the hypodermis: in free-living forms, it lies within its superficial layers, whereas in parasitic forms, it is embedded in the hypodermal cords.

The low cell count in nematodes is also reflected in the composition of their nervous system. In some soil-dwelling free-living species, the entire neural apparatus consists of merely 200 neurons.

The Sense Organs of nematodes, particularly in free-living forms, exhibit a diverse structure. Because the nematode cuticle is insensitive to all types of stimuli, they lack free sensory nerve endings in the integument. All sensory endings are associated with specialized receptor organs, such as bristles and papillae, and it is exclusively through these structures that nematodes perceive stimuli. Any nematode sense organ comprises three components: a cuticular structure, nerve cell processes, and accessory cells (Fig. 195). The cuticular structures (bristle, pore, pocket) facilitate the penetration of specific stimuli to the sensory processes of the nerve cells, which are structurally modified cilia or flagella. The cell bodies themselves lie at a distance from the sense organs.

Fig. 195. Schematic diagram of the nervous system (a) and a sensory organ (b) of nematodes:

1 — nerves to sense organs; 2 — nerve ring; 3 — neuron clusters; 4 — ventral nerve trunk; 5 — lateral trunks; 6 — ring nerves; 7 — sensory papillae with nerves; 8 — dorsal nerve trunk; 9 — cuticle; 10 — sensory process of the nerve cell; 11 — axon; 12 — sensory neuron; 13 — enveloping cell; 14 — socket cell

The highest concentration of sense organs is found at the anterior (HEAD) end of nematodes, arranged in two to three concentric circles. These include labial and cephalic papillae—conical cuticular outgrowths surrounded at the base by a cuticular ridge—as well as bristles that are longer than the papillae (Fig. 196). Frequently, a pair of pocket-like or spiral cuticular depressions, known as amphids, is located at the anterior end. Presumably, all these organs function as chemoreceptors, although some simultaneously serve as tactile organs (mechanoreceptors). Certain free-living nematodes also possess simple photoreceptors (ocelli). Sense organs are most highly developed in free-living forms, whereas parasitic forms possess a significantly reduced number of receptors.

In free-living nematodes, sense organs are not restricted to the anterior end but densely cover the entire body. These are predominantly cuticular pores acting as chemoreceptors.

Fig. 196. STRUCTURE OF THE sense organs of Sphaerolaimus balticus — arrangement of sense organs at the anterior body end (a), labial papillae (b), cephalic and cervical bristles (c), amphids (d): 1 — mouth; 2 — amphids; 3 — cuticular bristles; 4 — papillae; 5 — sensory process of the nerve cell; 6 — cuticle; 7 — socket cell; 8 — enveloping cell

Nematodes are dioecious animals with internal Fertilization. Their reproductive system has a relatively simple tubular structure. In females, the thin free ends of both tubes are the Ovaries, where gametogenesis and egg Cell Formation take place. The ovaries transition directly into slightly wider tubular oviducts. The eggs entering the oviducts gradually move toward the uteri, which are formed by tubes of even greater diameter. The walls of the uterus contain muscular and glandular cells; here, fertilization of the eggs and the Formation of protective egg membranes occur. The two uteri merge into a single unpaired duct, the vagina, which opens on the ventral side of the body via the female genital pore. In some species, one of the reproductive tubes is reduced, making the reproductive system unpaired. Near the genital pore in certain species (e.g., Ascaris), there is a circular depression called the genital girdle (or vulvar region), which the male clasps with his tail during copulation (see Fig. 193).

The Male Reproductive System is similarly built from paired reproductive tubes, though often one of them is reduced. It consists of a long, thin testis where spermatozoa are produced. The testis continues into a vas deferens, followed by a slightly swollen Seminal Vesicle and a long, cylindrical, muscular-walled ejaculatory duct that opens together with the hindgut into the cloaca. The cloaca houses a specialized copulatory apparatus consisting of a pair of cuticular spicules operated by dedicated muscles. The spicules help anchor the male to the female's body, widen the female's vagina, and facilitate The transfer of spermatozoa during copulation (see Fig. 193).

Nematode spermatozoa lack flagella and move by means of pseudopodia.

Embryonic development is characterized by bilateral Cleavage of the zygote, resulting in an embryo shaped like a rhombus, tetrahedron, or T-figure. This cleavage is also determinate, meaning that as early as the two- or four-blastomere stage, the specific blastomeres destined to form particular tissues are already specified. Another distinctive feature of nematode development is that Gastrulation produces a slit-like blastopore that closes along its entire length except for the anterior and posterior edges, which transform into the mouth and anus, respectively. This gives rise to a complete, through-gut (Fig. 197). Recall that in all previously discussed groups of Eumetazoa (Cnidaria, Ctenophora, Platyhelminthes, Rotifera, Acanthocephala), the blastopore develops entirely into the mouth.

Fig. 197. Formation of the slit-like blastopore (a, b), its closure (c), and the bending of the embryo (d) in Pontonema vulgare

Postembryonic development in nematodes, as previously noted, is accompanied by molts, the number of which is fixed at four. Adults do not molt. In free-living nematodes, a first-stage larva hatches from the egg, and all four molts take place in the external environment. Parasitic nematodes of the subclass Rhabditia are characterized by partial embryonization of development, wherein one or two molts occur within the eggshell. In some animal parasites (order Strongylida), a third-stage larva hatches from the egg. Larvae of free-living nematodes share the same lifestyle as adults. In many parasitic forms, infective larvae inhabit intermediate hosts, soil, or water, and differ significantly in structure from the adult stage.

The class Nematoda is divided into three subclasses: Enoplia, Chromadoria, and Rhabditia.