Nematodes

NEMATODES, Nematoda Rudolphi, 1808 (from Greek nema - thread), or roundworms, are, from the point of view of those authors who recognize the existence of the phylum Vermes, an independent class of the subphylum Nemathelminthes; many modern researchers abolish Vermes as a phylum, considering Nematodes to be a class of the independent phylum Nemathelminthes. Nematodes are organisms not divided into metameres (in contrast to annelids; see Annelids); the body is in most cases cylindrical or spindle-shaped; they are dioecious. Some authors also include Acanthocephala (see Acanthocephalans) as a separate class in Nemathelminthes. Nemathelminthes are divided into two classes: Nematoda and Gordiacea. Nematodes are either free-living organisms or parasites of animals or plants. Free-living nematodes inhabit fresh waters and seas, damp soil, decomposing organic matter, etc. Many species are capable of inhabiting diverse substrates; thus, for example, freshwater forms can settle in soil and silt, and soil nematodes can transition to rotting matter and grow and multiply there, and even transition to temporary parasitism on plants or, more rarely, on animals. Examples of such nematodes are species of Rhabditis and Diplogaster; representatives of these genera can multiply on rotting plants or animals (fungi, earthworms, mollusks, etc.); some nematodes are recorded as facultative and rare parasites of humans. Some free-living nematodes in the larval stage can settle in invertebrate animals, in which, however, for the most part they do not develop further and consequently are not true parasites. In these cases, free-living nematodes use their "hosts" mainly for their further dispersal; at the same time, the "hosts" into which the nematodes penetrate provide the latter with abundant nutritional material after their death. Thus, for example,

^J21

Figure 1. Figure 2. Figure 1. Various types of structure of the muscular system in nematodes: a - Polymyarii; b - Mero-myarii; c - Holomyarii. Figure 2. Diagram of the structure of a nematode: 1 - labial papillae; 2 - nerve ring; 3 - cervical papillae; 4 - esophagus; 5 - intestinal tube; 6 - telogonia; 7 - spicules; 8 - pre-anal pedunculated papillae; 9 - gubernaculum; 10 - opening of the cloaca; 11 - post-anal pedunculated papillae; 12 - caudal alae; 13 - post-anal sessile papillae; 14 - pre-anal sessile papillae; 15 - opening of the vulva; 16 - intestinal tube; 17 - lateral ala; 18 - uterus; 19 - ovaries; 20 - seminal receptacles; 21 - anus. Rhabditis pellio settle in earthworms, Leptodera appendiculata in snails, Rhabditis obtusa in the hindgut of a beetle. Such settling presents an example of the transition from a free-living mode of life to parasitism. On the other hand, among parasitic nematodes there are also certain groups of forms that spend part of their life as free-living organisms. Thus, for example, in Mermithidae the mature form is free-living, while the larva undergoes a parasitic stage of development in insects; Strongylata, conversely, spend part of their larval development in the external environment; finally, a form such as Strongyloides is characterized by the alternation of parasitic and free-living generations. In their structure, free-living and parasitic nematodes do not present essential differences from each other. Parasitic nematodes can be parasites of vertebrate and invertebrate animals or plants. They are organisms with an elongated body of thread-like or spindle-shaped form, from 1 mm to 1 m in length; mostly dioecious. Externally, the body is covered with a cuticle, internal to which is a single-layer epithelium (the hypodermal layer of the cuticle); still further inward lies the musculature. According to the structure of the musculature, Schneider (1860) divided nematodes into three groups: 1. Mero-myarii, with 8 muscle cells in the circumference of the body (on each cross-section) of the nematode, 2 in each sector formed by the dorsal, ventral, and lateral fields of the body. 2. Polymyarii, with more numerous muscle cells in each sector of the cross-section of the body. 3. Holomyarii, where the muscle cells lie closely adjacent to each other, forming an uninterrupted layer (Fig. 1). In the vast majority of cases, the muscle cells lie in a single annular layer and are elongated in the longitudinal direction; upon general contraction, they shorten the length of the body of the nematode, while upon unilateral action they bend the body in the corresponding direction. Most nematodes have a through, straight digestive tube opening at the head end with a mouth often surrounded by cuticular outgrowths - "lips"; the mouth is sometimes followed by a buccal capsule or in other cases a "pharynx" leading to the esophagus, which may consist of two sections - muscular and glandular; in some forms of nematodes (mainly Oxyurata) at the end of the esophagus there is an expansion, the so-called "bulbus", which may include a special masticatory apparatus in the form of chitinous plates. The intestine opens ventrally before the posterior end of the body via the anal opening (Figure 2). The nervous system consists of a nerve ring surrounding the anterior end of the esophagus; nerve trunks depart from it anteriorly and posteriorly, sending their branches to the peripheral nervous apparatus - "papillae", situated mainly at the head end ("head papillae"), in the region of the esophagus ("cervical papillae"), and on the tail, predominantly in males ("genital papillae"). The excretory system consists mostly of two symmetrical canals fusing in the anterior part of the body and opening here with an excretory pore. The sex organs have the appearance of convoluted tubes, and all parts of the genital apparatus both in the male (testis, vas deferens) and in females (ovary, oviduct, uterus, vagina) are merely specific sections of one continuous tube. The female genital organs are often double and open via the vulvar opening. In males, near the excretory duct of the sex organs located ventrally on the tail end, there are also auxiliary elements: spicules, a gubernaculum, representing chitinous formations playing a role in the act of copulation (Fig. 3). Reproduction of nematodes occurs mostly by the females laying eggs at various developmental stages in different species - from the pre-segmentation stage to a fully formed larva; viviparity is also encountered in nematodes belonging to the most diverse suborders. Among human parasites, viviparity occurs in the guinea worm. Postembryonic

Figure 3. Sagittal section of the tail end of Ascaris: 1 - m. retractor spiculae; 2 - spicule; 3 - m. protractor spiculae; 4 - vas deferens; 5 - intestine.

development of nematodes occurs in the majority of cases by a direct route (without intermediate hosts); in this case, infection of the animal is carried out either by the entry of eggs or larvae into its digestive tract or by the active penetration of larvae through the integument; sometimes development is complicated by so-called heterogony (see), e.g., in Strongyloides stercoralis. In other cases, the development of nematodes occurs with the help of intermediate hosts, which are mainly invertebrates (principally insects, see Mosquitoes). The development of nematodes in the host organism sometimes proceeds via complex pathways, with the help of so-called migration (see Infestation, Ascarids, Human Helminthiases). The localization of nematodes in the organism of the definitive host serves principally as the digestive tract, as well as subcutaneous tissue (various Filariata), muscles (Trichinella, etc.), blood, abdominal cavity (in humans Dipetalonema perstans, syn. Acanthocheilonema perstans), eye conjunctiva and tear ducts (Thelazia), etc. The systematics of nematodes is at present still comparatively weakly studied. It is based mainly on morphology (structure of lips, cuticular elements, external elements of the male genital apparatus) and partly on anatomy (esophagus, genital apparatus, etc.). Nematodes are divided into 9 suborders: Filariata, Spirurata, Dioctophymata, Ascaridata, Oxyurata, Strongylata, Rhabdiasata, Tricho-cephalata, Mermithata, of which all nine have representatives among human parasites. For larval and immature forms that cannot be precisely identified, collective names have been proposed, such as: Agamonema, or Agamonematodum for nematodes in general; Agamofilaria (for adult immature Filariata); Microfilaria (for larvae of Filariata localized predominantly in the blood); Agamospirura (for immature Spirurata); Agamascaris (for immature Ascaridata); Agamomermis (for immature Mermithata). Nematodes constitute numerically the richest group of parasitic worms of humans, in whom a total of about 70 species have been recorded. Some human nematodes are cosmopolitans, encountered in all parts of the world (Ascaris, pinworm, whipworm). The geographical distribution of others is limited by specific climatic conditions; these are mostly forms whose larvae spend a certain stage of their development in the external environment (Ankylostomidae, Strongyloides); a third group of nematodes has an even narrower distribution area, depending also on the presence of corresponding intermediate hosts (filariae, guinea worm).