INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

KINGDOM METAZOA

SUBKINGDOM EUMETAZOA

SECTION TRIPLOBLASTICA, or BILATERIA ANIMALS

SUBSECTION ECDYSOZOA

PHYLUM NEMATODA

Nematodes (from Greek nema - thread, eidos - form) are predominantly free-living or, less frequently, parasitic animals adapted to life in diverse environments (marine and freshwaters, soils, and the bodies of humans, animals, and plants). This is one of the most abundant animal groups: over 20,000 species are known (approximately 1.6 thousand in the fauna of Ukraine), but according to scientists' estimates, the actual number of extant species may reach a million.

Free-living species are typically microscopic in size (0.3–1 mm), plant parasites are somewhat larger (8–10 mm), and the longest species are found among vertebrate parasites. The maximum length (up to 8 m) is attained by Placentonema gigantissima, a parasite of the sperm whale.

The body of nematodes is covered by a multilayered cuticle. The musculature is represented exclusively by longitudinal Muscles. A primary body cavity, the schizocoel, is present. The gut is complete, and the Pharynx has a triangular lumen. The excretory system consists of modified cutaneous glands. The Nervous System comprises a circumesophageal ring and longitudinal cords; true ganglia are absent. Nematodes are dioecious, with reproduction exclusively sexual. Egg Cleavage is bilateral and determinate. Development proceeds without metamorphosis; larval growth is accompanied by molting, whereas adult nematodes do not molt. Nematodes are characterized by Cell constancy (eutely) and a complete lack of regenerative capacity.

The body plan of nematodes lacks diversity, despite their colonization of various habitats. The body is most often fusiform, tapered at the ends, and less commonly sac-like or spherical (Fig. 265). In cross-section, it is circular (hence the alternative name of the phylum - Roundworms).

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Fig. 265. Exterior view of nematodes (after Paramonov and Sonin):

a - free-living marine nematode Steineria mirabilis; b - Meloidogyne sp.; c - Aphelenchoides composticola - plant parasites

The Skin-muscular sac consists of a dense multilayered cuticle, the hypodermis, and a single layer of longitudinal muscles. The cuticle is a structurally complex, elastic, and resilient multilayered formation. For example, in ascarids, up to 10 structurally distinct layers are recognized (Fig. 266). The outer layers are permeated by a system of microscopic canaliculi. The thickness of the cuticle depends on environmental conditions: it is thicker in parasitic species than in free-living ones. The cuticle performs multiple Functions: protective (against damage by hard soil particles or within a biologically active environment in parasitic species), transport (allowing the active penetration of certain chemical elements and low-molecular-weight compounds), and gas exchange. The cuticle also plays a key role in nematode locomotion: it acts as an antagonist to the longitudinal musculature of the skin-muscular sac. Contraction of the muscles on one side of the body causes it to bend, while extension occurs upon their relaxation due to the elasticity of the cuticle, which is maintained by the high internal pressure of the coelomic fluid. The hypodermis is a modified sunken-type integumentary epithelium differentiated into a thin subkuticle underlying the cuticle, and hypodermal chords that project deeply into the body cavity (Fig. 267). The hypodermis performs a supportive and mechanical function, ensuring a tight mechanical link between the cuticle and muscles, and serves as an important barrier tissue that regulates the passage of substances through the integument, as well as a storage tissue accumulating fats and Glycogen. Its most critical function is The formation of the cuticle.

Fig. 266. Diagram of nematode cuticle Structure (after Malakhov):

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

Fig. 267. Cross-section of a female ascarid (schematic) (after Strelkov et al.):

1 - dorsal hypodermal chord; 2 - nerve processes of Muscle Cells; 3 - muscle cells; 4 - Ovary; 5 - gut wall;

6 - cuticle; 7 - lateral hypodermal chord; 8 - longitudinal excretory canal;

9 - Uterus; 10 - oviduct; 11 - ventral hypodermal chord

Specialized Skin glands are associated with the nematode integument, with their ducts opening onto the body surface. Some of these glands—esophageal—belong to the Digestive System, while others belong to the excretory system (cervical glands) or Sensory Organs (phasmids).

The musculature consists of obliquely striated muscle cells that, in their Structure and function, approach the striated muscles

of Arthropods and Chordates. A muscle cell is subdivided into a contractile zone, a cytoplasmic region, and several branched innervating processes that extend toward the nerve cords and also contact the processes of adjacent muscle cells (Fig. 268). Unlike most animals, in nematodes it is not the nerve endings that extend toward the muscle cells; rather, the reverse occurs: processes of the muscle cells extend toward the nerves.

The body cavity, or schizocoel, is well-developed and performs transport and skeletal functions. However, in microscopic nematodes, it is practically absent; the Internal Organs fit closely together, and the spaces between them are filled with a non-cellular ground substance. In large parasitic nematodes, the schizocoel is a spacious fluid-filled cavity that generates the high internal pressure necessary for the undulating Movements of the animals.

Fig. 268. Muscle cells of Pontonema vulgare (after Malakhov): 1 - cytoplasmic part; 2 - Nucleus; 3 - contractile zone;

4 - innervation processes; 5 - ventral nerve trunk

Digestive system. The intestine in nematodes is complete, consisting of three sections: the foregut (pharynx), midgut, and hindgut, which opens externally via the anus (Fig. 269). The Mouth opening is located at the anterior end of the body and is surrounded by Lips: most species have six, but in specialized forms, their number is reduced to three, and sometimes lips are absent. The mouth leads into the pharynx, lined with a thin cuticle; single- or bicellular Salivary Glands are embedded in the pharyngeal walls. The pharynx is divided into two parts: the anterior (stoma) and the posterior (Esophagus). Thickening of the cuticle is often observed in the stoma: immobile thickenings (onchi) and mobile Teeth. Some forms possess jaws, while plant-feeding phytonematodes have a sharp spear or stylet. In some species, the posterior part of the pharynx forms an expansion—the bulb—which increases the suction force, as the pharynx functions as a pump.

Fig. 269. Internal Structure of Ascaris (after Strelkov et al.): a - female; b - male:

1 - lips; 2 - nerve ring; 3 - pharynx; 4 - phagocytic cells; 5 - ventral hypodermal cord; 6 - lateral hypodermal cord;

7 - Vagina; 8 - ovary; 9 - uterus; 10 - oviduct; 11 - midgut; 12 - hindgut; 13 - ejaculatory duct;

14 - Testis; 15 - vas deferens

Among free-living species, there are saprophages as well as predators. Phytonematodes feed on plant juices and Tissues, while parasites of humans and animals feed on the tissues and Blood of their hosts, as well as the contents of their intestines. Nematodes also include commensals and polyphages. In some parasitic forms, the digestive system is reduced or entirely absent. For instance, in Representatives of the genus Filaria, the posterior end of the intestine is blindly ending, whereas in species of the order Mermithida, the rudimentary midgut functions as a peculiar fat body, accumulating reserve nutrients.

The excretory system is represented by modified cutaneous glands (Fig. 270). It consists of two long intracellular canals that originate from glandular cells and extend from back to front inside the longitudinal lateral cords of the hypodermis. Near the anterior end of the body, the canals open to the outside through a common opening—the excretory pore. These are the so-called cervical glands, which may number two or one. In most nematodes, the entire cervical gland with its processes is a single cell, and more rarely, two or three. Cervical glands can also perform an osmoregulatory function. Special phagocytic cells (ranging from two to six in parasitic species to numerous ones in free-living species) located in the schizocoel on the hypodermal cords also participate in excretion. These cells are capable of accumulating solid Metabolic waste products, removing them from METABOLISM, as well as phagocytosing foreign particles and microorganisms, thereby performing a protective function.

Fig. 270. Excretory system of nematodes (cervical gland) (after Malakhov):

a - saccate unicellular; b - tricellular; c - lobate with long canals:

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

Respiratory organs are absent. Many parasitic species, especially intestinal parasites, are characterized by anoxybiosis. Free-living forms perform gas exchange through the integument.

The nervous system consists of a circumoesophageal nerve ring and several (8–12) longitudinal trunks, of which the ventral one is the most developed (Fig. 271). True ganglia are absent. In free-living nematodes, the nerve trunks lie in the superficial layers of the hypodermis, whereas in parasitic ones, they lie within the hypodermal cords. Nerve branches extend from the circumoesophageal ring and longitudinal trunks toward the body surface, forming the peripheral part of the nervous system.

Fig. 271. Diagram of The structure of: a - the nervous system of nematodes (after Dogiel); b - a sense organ (after Malakhov):

1 - nerves to Sense Organs; 2 - nerve ring; 3 - cluster of Neurons; 4 - ventral nerve trunk; 5 - lateral trunks;

6 - circular nerves; 7 - sensory papillae with nerves; 8 - dorsal nerve trunk; 9 - cuticle; 10 - sensory process of the nerve cell; 11 - axon of the sensory cell; 12 - sensory cell; 13 - enveloping cell; 14 - socket cell

Sense organs are most fully developed in free-living nematodes, particularly at the anterior end of the body, where they are arranged in two to three circles. The cuticle of nematodes is insensitive to stimuli, and their integument lacks free sensory nerve endings. Sensitivity is localized only in specific areas of the integument that feature special cuticular formations: labial and cephalic papillae, which are short conical cuticular outgrowths surrounded at the base by a cuticular collar; longer setae; and amphids, which are pouch-like or spiral depressions of the cuticle (Fig. 272). Their common feature is that each consists of a cuticular structure (seta, pore, pouch), accessory cells, and a sensory process of a nerve cell whose body lies far from the sense organ (Fig. 271, b). The described organs function as mechano- and chemoreceptors. In free-living forms, the entire body is covered with sensory setae and cuticular pores, the latter acting as chemoreceptors. Clusters of these structures are also concentrated at the posterior end of the body—around the anus and the copulatory apparatus of males. In species of the subclass Rhabditia, the caudal region of the body features phasmids, which are unicellular glands acting as chemoreceptors. Some species also possess simple ocelli.

Fig. 272. Diagram of the structure of sense organs in Sphaerolaimus balticus (after Malakhov):

a – arrangement of sense organs at the anterior end of the body; b – labial papillae; c – cephalic and cervical setae; d – amphid (enlarged):

1 – mouth; 2 – amphid; 3 – cuticular setae; 4 – papillae; 5 – sensory process of the nerve cell; 6 – cuticle;

7 – socket cell; 8 – envelope cell

Reproductive System. Nematodes are typically dioecious with internal Fertilization; physiological or true hermaphrodites occur occasionally. The former, externally resembling females, acquire The ability to produce spermatozoa as well. Dioecious species exhibit Sexual Dimorphism, with males consistently being smaller. In both sexes, the reproductive system appears as long tubes. In females, the ultra-fine terminals of the two tubes—the Ovaries—lack a distinct internal lumen.

They transition into wider tubular oviducts, which in turn lead into even broader uteri. Merging together, the latter form the vagina, which opens via the female genital pore on the ventral surface of the body (Fig. 267, 269, a). The Male Reproductive System is similarly built from paired reproductive tubules, though often one of them is reduced. In males, the thread-like testis continues into a tubular vas deferens, followed by an ejaculatory duct that opens, along with the hindgut, into the cloaca, which houses a specialized copulatory apparatus consisting of a pair of cuticular needles (spicules) and accessory structures (Fig. 269, b). Nematode spermatozoa lack flagella and move by means of pseudopodia. Occasionally, parthenogenesis occurs in nematodes.

Reproduction. Nematodes are oviparous, though some species (e.g., the trichina worm Trichinella spiralis) areovoviviparous. The eggs hatch into larvae that resemble adults in body shape, albeit with an underdeveloped reproductive system. Since the tough cuticle impedes animal growth, the larvae molt periodically; the old cuticle detaches from the hypodermis and is shed, while the hypodermis forms a new one. The number of molts in nematodes is fixed at four. Adult nematodes neither grow nor molt.

Life cycles. Free-living nematodes have short-term life cycles lasting two to three days. The life cycles of parasitic nematodes are highly diverse: in some cases, they are completed without a change of host (various species of ascarids, whipworm, hookworm, etc.), while in others, they involve alternating hosts. Invertebrates (Mollusks, insects, crustaceans, etc.) most commonly serve as intermediate hosts, and vertebrate animals do so less frequently.

The phylum Nematoda comprises four classes.