| ||||||||||
Phylum of Metazoa
The nematodes are elongate worms ranging in length from 0.3 mm up to the 8.5 m of Placentonema gigantissimum in the placenta of whales; they may inhabit soil, fresh- and saltwater habitats, and are frequently encountered as parasites of plants, humans or animals. In general they are dioecius and in many species clear sexual dimorphism exists (Fig. 12, Hookworms/Fig. 1). Males are usually smaller than females (Table 1); both may have copulatory organs. The bilaterally symmetrical body of the unsegmented nematodes is covered by a typical cuticle which is formed by a hypodermis and must be shed during molt (Fig. 8, see ecdysis). The pseudocoelomatic body cavity of adults mostly contains a complete digestive tract, the anus of which is subterminally situated (Fig. 11, Fig. 12).
Food of various kinds (blood, body fluid, intestinal contents, mucus etc.) is taken up by means of the species-specific mouth (Fig. 10, Fig. 11). The excretory system, if present, empties through an anterior, ventromedial porus. Respiratory and circulatory systems are lacking; movements are brought about by contractions of the typically longitudinally oriented muscle cells (Fig. 8 C,D), with the fluid of the pseudocoel and the pressure of the cuticle working together as a hydrostatic skeleton. Apart from a few species (e.g., Strongyloides spp.) the ontogenesis of nematodes runs as metamorphosis involving four larval stages (L1-L4; Reproduction). Cell multiplication after hatching is very restricted (eutely) except within the reproductive system, midgut, epidermis and somatic musculature.
The classification of the nematodes is still a matter of controversy. In general two classes are accepted which are distinguished according to the suggestions of Maggenti (1981).
Class: Adenophorea (Asphasmidea)
Phasmids (i.e., minute, usually paired chemoreceptors) generally absent; amphids are postlabial and variable in shape, cephalic organs setiform to papilloid; setae and hypodermal glands usually present, hypodermal cells uninucleate; excretory organ, if present, single-celled; caudal glands mostly present; usually two testes in males; cuticle four layered; some selected parasitic species belong to the following orders:
Order: Trichocephalida
Family: Trichuridae (e.g. Trichuris/Fig. 1)
Family: Trichinellidae (e.g. Trichinella spiralis/Fig. 1)
Family: Cystoopsidae
Order: Mermithida
Family: Mermithidae
Class: Secernentea (Phasmidea)
Phasmids present posterior to the anus; hypodermis uni- to multinucleate; cuticle with two to four layers; males have only a single testis and are commonly provided with caudal alae (known as copulatory bursa); somatic setae or papillae absent on females; amphids usually open to exterior through pores located dorsolaterally on lateral lips or anterior extremity; cephalic sensory organs are pore-like, found on lips (16 in two circles with 6 inner and 10 outer), some selected orders with parasitic species are:
Order: Rhabditia
Family: Rhabditidae (e.g. Rhabdias bufonis)
Family: Strongyloididae (e.g. Strongyloides/Fig. 1)
Order: Strongylida
Superfamily: Ancylostomatoidea
Family: Ancylostomatidae (e.g. Hookworms/Fig. 1)
Family: Uncinariidae
Superfamily: Trichostrongyloidea
Family: Trichostrongylidae (e.g. Trichostrongylidae/Fig. 1)
Family: Dictyocaulidae (e.g. Lung Worms/Fig. 1)
Family: Heligmosomatidae
Superfamily: Metastrongyloidea
Family: Metastrongylidae (e.g. Lung Worms/Fig. 1
Family: Angiostrongylidae (e.g. Angiostrongylus cantonensis/Fig. 1)
Family: Protostrongylidae
Superfamily: Strongyloidea
Family: Strongylidae
Order: Ascaridida
Superfamily: Ascaridoidea
Family: Ascarididae (e.g. Ascaris/Fig. 1)
Family: Toxocaridae (e.g. Toxocara/Fig. 1)
Family: Anisakidae (e.g. Anisakis/Fig. 1)
Family: Cosmocercidae
Superfamily: Oxyuroidea
Family: Oxyuridae (e.g. Enterobius vermicularis/Fig. 1)
Superfamily: Heterakoidea
Family: Heterakidae
Family: Ascaridiidae
Superfamily: Dioctophymatoidea
Family: Dioctophymatidae
Order: Spirurida
Superfamily: Spiruroidea
Family: Spiruridae
Superfamily: Physalopteroidea
Family: Gnathostomatidae (e.g. Gnathostoma spinigerum/Fig. 1)
Family: Physalopteridae
Superfamily: Filarioidea
Family: Filariidae (e.g. Filariidae/Fig. 1)
Order: Camallanida
Superfamily: Camallanoidea
Family: Camallanidae
Superfamily: Dracunculoidea
Family: Dracunculidae (e.g. Dracunculus medinensis/Fig. 1)
Family: Philometridae
Family: Micropleuridae
Order: Diplogasterida
Order: Aphelenchida
Order: Tylenchida
Superfamily: Sphaerularioidea
Family: Sphaerulariidae
| ||||||||||
Nematodes are in general dioecious animals; relatively few species are hermaphrodites (e.g. cf. Rhabdias bufonis/Fig. 1) and, in those that are, the female and male gonads are formed consecutively.
The gonads of nematodes are tubes that are blind ending on one end. The other end of the female gonad is attached to the body wall and opens immediately outwards, whereas the male gonad opens into the rectum, which thus becomes a typical cloaca. The tubes of the male and female genital system float freely in the fluid of the pseudocoel (Fig. 11).
In general, nematodes have two female genital tubes; however, some trichurids and strongylids have only a single gonad. The tubes are subdivided into the ovaries, the oviducts, the receptacula seminis, and the uteri, which join to form the single vagina opening through the vulva to the exterior. The vagina lays on the ventral side, often in the midregion; however, in filariae it is found close to the very anterior end, whereas in strongylids it opens immediately beside the anal pore in the most posterior region.
In most nematodes the ovaries are of the telogonic type, where the oogonia are formed in the very tip of the ovary and descend the tube undergoing the various stages of oogenesis. Transovarially transmitted bacteria, which occur in some filariae, have been found in the epithelium of the cap region and in the oogonia. The rachis may be single or branched and in the former case the germinal cells surround the central rachis in a characteristic rosette-like pattern (Fig. 2). The oocytes detach from the rachis in the growth zone of the ovary or in the maturation zone just before the end of the ovary (Fig. 8).
The wall of the ovary consists of a basal lamina and a layer of epithelial cells, which in the proximal region of the ovary may contain basally arranged myofilaments. In hologonic ovaries, which are found in trichurid nematodes, the oogonia cover the entire wall of the ovarial tube. The germ cells develop via the various stages of oogenesis while moving from the wall towards the lumen of the ovary.
The oviduct is a short tube lined by epithelial cells which have many myofilaments at their base. The surrounding basal lamina interdigitates deeply with the epithelum, and the myofilaments are attached by hemidesmosomes to these protuberances of the lamina. The luminal surface of the epithelial cells protrudes microvilli. The oviduct forms a constrained tube through which the oocytes pass in a single file.
The seminal receptacle is a widening at the beginning of the uterus storing numerous spermatozoa until fertilization. Maturation of the eggs takes place in the uteri, which are lined by an epithelium covered by a basal lamina and some ring muscle cells. The muscle layer is thin in the wall of the seminal receptacle, but becomes prominent towards the end of the uterus. In the vagina uterina, the distal bifurcated portion of the vagina, the ring muscles are multilayered. In some groups of nematodes this region is modified to a strongly musculated ovijector, which serves as a valve. The surface of the proximal portion of the vagina, the vagina vera, is lined by cuticle.
The male genital tube consists of the blind-ending testis, the seminal vesicle, the vas deferens, and the ejaculatory duct which opens into the rectum (Fig. 11).
The testis of parasitic nematodes belonging to the Secernentae is of the telogonic type, where the terminal portion of the testis contains the spermatogonia (Fig. 3). During their further development in the testis, the germ cells are linked together by a rachis, which is a branched cytoplasmic core (Fig. 5). The wall of the testis consists of a rather thick basal lamina and the epithelial cells which in some species contain smooth muscle fibers in their basal portion. The hologonic type of testis, where the spermatogonia line the wall of the testis, is found in trichurid nematodes; further spermatogenesis occurs during migration towards the lumen of the testis.
The seminal vesicle is a storage organ for spermatozoa and in some species the final development to these stages takes place in this organ. The vas deferens and the ejaculatory duct lead to the cloaca. The wall of these ducts secretes substances that stimulate the transformation of the sperms into the amoeboid form (Fig. 4). The spicules are characteristic copulatory structures of male nematodes (Fig. 12, Fig. 13). In most nematodes there are two spicules, which often differ in length and shape, but in some species only a single spicule is found (Fig. 13). The spicules are needle-shaped and consist of thick cuticular material which surrounds a cytoplasmatic core with nerve processes. The nerve endings are covered by cuticular material. In many species the spicule wall is bent to form a hollow needle with an opening at its base and tip. The spicules are formed in a dorsal sac of the cloaca called the spicular pouch. The spicules can be moved back and forth by accessory muscles and during copulation they are inserted into the female vulva. A thickening of the dorsal wall of the spicular pouch, the gubernaculum, stabilizes the protruded spicule. Additional copulatory structures are found in some groups. The bursa is a lobular modification of the male posterior end, which is highly elaborated in stronglylid nematodes. The bursa surrounds the vulvular region of the female worm. In filarial nematodes the male worm twists its posterior region around the female worm several times. The ventral cuticle of this region carries longitudinal ridges (area rugosa) which interdigitate with the annulations of the female cuticle for better attachment.
The spermatogonia in the terminal region of the testis multiply by mitotic divisions (Fig. 3, Fig. 6A). It remains doubtful whether a cap cell gives rise to the spermatogonia or a population of stem cells lies in the terminal lumen and proliferates the spermatogonia, as has been observed in Dirofilaria immitis. When the cells enter the next region of the testis, they begin the prophase of the first meiotic division. In the diplotene the spermatocytes grow and differentiate the organelles characteristic of the nematode sperms. The meiotic divisions, resulting in four spermatids from each spermatocyte, occur either at the posterior portion of the testis or in the seminal vesicle. The chromatin is condensed into one or more bodies which are no longer surrounded by the nuclear envelope (Fig. 6C). Maturation to the fertile spermatozoa occurs in the vas deferens or in the female uterus (Fig. 6D).
Characteristic organelles of the spermatozoa of many nematode species are the membranous organelles which are already formed in the spermatocytes (Fig. 6B,C). In the mature spermatozoon they contain plicated membranes and are situated close to the plasma membrane. In the female uterus they become attached to the outer membrane and release substances (Fig. 6D). Furthermore, the spermatozoa may contain a system of microtubules, reserve materials such as lipid droplets or refringent bodies, and mitochondria. These organelles are not found in the spermatozoa of all species. The spermatozoa of nematodes have no acrosome or axonemal structures. Primitive nematode spermatozoa have a spherical shape, but most parasitic species possess elongated spermatozoa, which are often divided into two distinct parts (Fig. 2C). Inside the uterus the spermatozoa become amoeboid. Their organelles and the chromatin are concentrated at one pole, and the organelle-free portion develops the pseudopodium (Fig. 2C, Fig. 3D, Fig. 5D).
At the tip of the female gonad a syncytial mass or a cap cell may proliferate oogonia. The cap cell of Aspiculuris tetraptera, however, is morphologically different from the proliferating germ cells in the lumen and it has been suggested that the cap cell does not proliferate oogonia. The cap zone is part of the germinal zone in which the oogonia multiply by mitotic divison. The cytoplasm of all germinal cells is connected to a cytoplasmatic strand (rachis) running in the middle of the ovarial lumen. The oogonia have a spherical shape with little cytoplasm surrounding the nucleus (Fig. 2A, Fig. 3A, Fig. 25). Reaching the growth zone of the ovary the germinal cells cease mitotic activity and begin the prophase of the first meiotic division. The oocytes then remain in the diplotene stage and begin intense synthetic activity. In a first step they increase in size by adding more cytoplasm. The cells often elongate and become arranged radially around the rachis. During further development in the maturation zone, oocytes produce nutrient stores (e.g., lipid droplets, hyaline granules, dense granules and glycogen) and materials for eggshell formation (refringent granules, shell granules and glycogen). The oocytes continue the first meiotic division as they leave the ovary and pass through the oviduct.
When the oocyte enters the seminal receptacle, fertilization takes place. In Ascaris spp. the process has been described in detail; the pseudopodium of the spermatozoon contacts the oolemma and the gamete membranes interdigitate and fuse. The whole sperm then enters the cell. Penetration of the sperm is followed by formation of the eggshell and by completion of the two meiotic divisions resulting in expulsion of two polar bodies (Heterakis).
The eggshell is formed immediately after fertilization and usually contains four layers. The outermost (uterine) layer consists of material that is secreted by the uterine epithelial cells. The next, vitelline, layer originates from the vitelline membrane which is formed after fertilization of the oocyte. The underlying chitinous layer contains chitin and is often the thickest layer of the eggshell. The most internal layer is the lipid layer which is responsible for the extreme impermeability of the nematode eggshell. In Ascaris spp. it contains the ascarosides as characteristic lipids.
Embryogenesis is highly determined, and the cell lineage from blastomere to the particular organ can be followed. Chromatin diminution occurs during early embryogenesis of Ascaris spp. and related species where the nuclei of the somatic cell lines lose a large amount of their DNA.
Some nematodes lay eggs which are not embryonated and which need oxygen for further development. In other species embryogenesis occurs inside the uterus and embryonated eggs are laid from which the larvae soon hatch. Viviparous nematodes complete embryogenesis inside the uterus and larvae hatch before leaving the uterus. The ova of these species (e.g., Trichinella spp., filariae) form a very thin eggshell and embryogenesis starts immediately after the oocytes have been fertilized (Fig. 3B,C, Fig. 4). The microfilariae of some filarial species (e.g., Wuchereria bancrofti) are born enveloped by the thin sheath (= eggshell; Fig. 3D), whereas others (e.g., Onchocerca volvulus) are already hatched when they are born (Fig. 3E), i.e. they are described as unsheathed.
In nematodes the body cover consists of two layers. The hypodermis is a cellular or syncytial derivative of the ectoderm. It secretes a thick, mainly proteinous layer, the cuticle, which covers the outer surface of the worm entirely. The term hypodermis is used for the nematode epidermis because of its position below the cuticle.
The hypodermis secretes the cuticle, a complex layer which covers the entire surface and lines the buccal cavity, the esophagus, the rectum, the terminal portion of the vagina, and the excretory duct. Depending on the species the cuticle reaches 5–10% of the body's diameter and 10–20% of its volume, thus being an important system that is closely connected to the hypodermis by hemidesmosomes. The main functions of the cuticle are to protect the organism from environmental influences and, together with the high turgor pressure of the pseudocoel, to maintain the shape and serve as an antagonistic system for the somatic muscles. Furthermore it is active in the uptake of nutrients, a fact which is important in drug application. Considerable diversity is observed in the chemical and morphological composition of the cuticle. Usually it is composed of various layers and often its outer surface is covered with an additional envelope.
Such an external surface coat may consist of a thin polysaccharide-rich layer (e.g., microfilariae of Dirofilaria immitis), be overlaid by a coat which consists of three distinct lamellae (e.g., Trichinella spp., Fig. 8E), or be covered by a coat of irregular thickness (Onchocerca spp. females). Finally, degranulation of host eosinophils on the nematode cuticle may also result in a thick coat. Except for the last example it is not clear which parts of the coat are contributed by the host and which originate from the parasitic nematode.
The outermost layer of the cuticle is a thin epicuticle which is between 6 and 60 nm thick and consists of mostly 2 dark lamellae separated by a lighter interspace (Fig. 7B, Fig. 8E). In spite of the similarities to a cell membrane the epicuticle is probably not derived from the outer hypodermal membrane. The epicuticle of Trichinella spiralis differs significantly from membranes of this parasite (Fig. 8E). The epicuticle does not fracture between the lamellae and it lacks the particles which are characteristic of any type of cell membrane. However, intramembranous particles were found in the epicuticle of Nippostrongylus brasiliensis and second-stage larvae (L2) of Meloidogyne javanica. Nevertheless, the epicuticle is an extracellular structure which is not at all continuous with plasma membranes. Therefore, Locke proposed the name envelope for structures like this which have similar dimensions to plasma membranes and are formed at membrane surfaces.
In most nematodes the epicuticle is smooth, following the pseudometameric annulations of the underlaying cuticular layer. However, in members of the genus Onchocerca the epicuticle is folded independently of the underlaying cuticle (Fig. 8). Additionally the epicuticle of male 0. volvulus exhibits a honeycomb-like pattern. Female worms of this and other species have long protuberances, regular plications and piles; it might be expected that a continuous turnover of the epicuticle occurs in these worms.
The cuticle underlying the epicuticle shows great morphological diversity among the various groups of nematodes. Nevertheless, based on fine structural similarities, these layers can be subdivided into three major zones, named cortical, median and basal (Fig. 7, Fig. 8). The cortical zone forms the pseudometameric annulations and internally is often amorphous and electron-dense. The median zone may contain fluids, struts or globular bodies. The basal zone is in general extremely complex. It may consist of several laminae and contains fibers or striations. The differentiation into these three zones can be observed in developmental and adult stages of some nematode species, but is invisible in many others.
During ontogeny nematodes undergo four complete molts during which the old cuticle is shed and replaced by a new one (Fig. 9). The new cuticle is formed before molting at the hypodermal surface below the old cuticle. In several nematodes it has been observed that the newly formed epicuticle becomes extensively folded, and these folds smoothen during later intermolt growth. Thus, further elongation of the epicuticle is not necessary before the next molt, although the other cuticular layers and the worm may grow considerably.
After formation of the epicuticle, other cuticular material is released successively for the various layers. The hypodermis serves as a template for the formation of the various layers. During this early phase of cuticular secretion the layers are hardly distinguishable, but already they comprise all the basic structures needed for the layers when self-assembly of the cuticular structures occurs later. The diversity of the mechanical and chemical properties of the various zones results from formation of fibers or lamellae and from the concentration of certain molecules in particular areas. Further growth in length and width of the cuticle occurs by deposition of certain molecules in all layers. Cuticular growth is not only found during and shortly after the molt. Considerable growth of the cuticle occurs between molts and particularly after the last one, during maturation of the nematode, and is often combined with a thickening of particular zones of the cuticle.
The main components forming the cuticle are collagen-like proteins. In the cortical layer of large ascarids there is a tanned structural protein (cuticulin) which does not exhibit the striations characteristic of collagen and is not attacked by collagenase. The fibrillar lamellae of the basal zone include proteins which are more similar to mammalian collagen and which are lysed by collagenase, but it is characteristic of these proteins that the molecules are linked by disulfide bonds.
Other materials may be enclosed in the cuticle, e.g., large amounts of nonglycogen polysaccharides in Trichuris myocastoris or hemoglobin in Nippostrongylus brasiliensis. Numerous modifications of the cuticle may be found (Fig. 10). Alae are keel-like thickenings which follow the lateral lines and support undulating locomotion (Fig. 10C). Stiff longitudinal ridges support attachment by burrowing into the structures around which the worm is twisting (Fig. 7). Many nematodes have bosses which are scattered over the cuticle and might create a space between the cuticle and the host tissue. Peculiar cuticular formations (e.g., teeth) in or near the buccal opening are used during the uptake of food (Fig. 10). Various copulatory structures are differentiated by cuticular modifications at the posterior end of male worms (Fig. 12, Fig. 13).
The hypodermis underlies the cuticle and covers the somatic muscle cells as a thin cytoplasmatic layer. The hypodermis forms thick chords which are in contact with the pseudocoel between the four sectors filled by muscle cells. Depending on their position they are called dorsal, ventral and lateral hypodermal chords (Fig. 7, Fig. 14C). The hypodermis consists of multinucleate cells (=syncytia) with nuclei in the chords. Frequently, there is a row of large dorsal and another row of large ventral syncytia which are in contact in the lateral chords. In the middle of each chord a row of small cells may be situated between the large syncytia.
During secretion of the new cuticle the hypodermis is modified, showing the morphological features characteristic of intense protein synthesis. The nuclei are enlarged and contain a prominent nucleolus and extended chromatin. The number of the mitochondria is increased. The cytoplasm is filled with rough endoplasmic reticulum and many Golgi apparatus. The formation of vesicles, their transport to the outer membrane, and their release can be observed.
The hypodermis in the sectors where the muscle cells are (= interchordal hypodermis) is a thin layer crossed by numerous tonofibrils which are attached to the muscle cells by desmosome like junctions and to the cuticle by hemidesmosomes (Fig. 14B). These fibrils are a stable link between the systems acting antagonistically in nematode locomotion. Only a few mitochondria, ribosomes, Golgi complexes, and some multivesicular bodies are found in the interchordal hypodermis after molts (Fig. 14A). These organelles are often concentrated in corners where two muscle cells abut and the hypodermis is slightly thicker. In middle cells of the lateral hypodermis chords the channels of the excretory system are found.
A basal lamina entirely covers the basal zone of the hypodermis separating the hypodermis from the muscle cells and the chords from the pseudocoel. The adjacent membrane of the hypodermis often forms a basal labyrinth which can be very elaborate, particularly in the lateral chords, increasing the exchange of substances between body cavity and hypodermis (Fig. 15A).
Particular modifications of the hypodermis may be found in female filariae which feed via their body wall. These nematodes have extremely large lateral chords (Fig. 16). The outer hypodermal membrane is plicated into numerous lamellae underlaid by a zone of mitochondria and lysosome-like bodies (Fig. 15B). At the base of the chord the basal labyrinth is extremely elaborate. In female filariae which have changed to sessile life (and thus reduced their somatic muscles), even the interchordal hypodermis becomes thickened and resembles the hypodermal chords.
In trichurid nematodes the lateral chords contain a particular type of cells which are called hypodermal gland cells or bacillary cells (Fig. 17, Fig. 18, Fig. 19). The basal portion of these cells contains a large nucleus and many organelles, indicating intense physiological activity. The function of these cells is unknown, but secretory or osmoregulatory functions are assumed.
The somatic musculature of nematodes consists of a single layer of muscle cells which run in a longitudinal direction along the inner margin of the hypodermis. The hypodermal chords group the muscle cells into four sectors (Fig. 7, Fig. 8, Fig. 11, Fig. 25).
Each muscle cell consists of a contractile (= fibrillar) portion and of a noncontractile (= afibrillar) portion containing the nucleus (Fig. 7). The basic type of muscle cell is named platymyarian, the contractile portion of which is restricted to a small zone along the outer border of the muscle cell, i.e. running parallel to the hypodermis. In the muscle cells of the coelomyarian type the contractile portion also covers the lateral borders of the cells, whereas in the circomyarian type the whole inner side is covered by fibrillar material, thus surrounding the central afibrillar cytoplasm. Every muscle cell has at least one process leading to the dorsal or ventral hypodermal chord, where it forms synaptic connections to one of the motoneurons (Fig. 7, Fig. 20C) and thus receives its different stimulations.
In the contractile portion of the obliquely striated, supercontractile muscle cells of nematodes the fibrils have a particular arrangement (Fig. 20A,B). Similarly to cross-striated muscles, the A-band (actin and myosin filaments), H-bands (myosin filaments alone) and I-bands (actin filaments alone) can be distinguished. However, in contrast to cross-striated muscles, the Z-planes do not run transversely to the fibrils, but at an oblique angle. This means that the other bands are also arranged at an oblique angle to the fibrils. In muscle contraction, the interdigitation of the myofilaments also leads to shearing forces, resulting in an increased obliquity of the whole system. Therefore, obliquely striated muscle is able to vary in length to a greater extent than cross-striated muscle.
Nematodes are mostly relatively small organisms; their size is limited by the fact that they do not possess circulatory systems to accelerate the transport of nutrients to the various organs. The alimentary canal and the gonads are surrounded by the fluid-filled pseudocoel, which is not septate (Fig. 11). Movement of the fluid is maintained by movements of the somatic and organ muscles. In many nematodes the genital tubes are twisted around the intestine; this large-scale contact probably accelerates nutritional exchange.
The hydrostatic pressure of the body fluid is necessary (as a padding) to maintain the independence of the pumping movements of the esophagus and the undulating locomotion of the worm. In extremely small nematodes (below 0.3 mm) these functions would interfere with one another and the typical nematode morphology would become inefficient. There may also exist an upper size limit for effective functioning of the nematode's alimentary canal. The morphology of both extremes can be demonstrated with filarial nematodes. The size of the microfilariae (i.e., the first-stage larvae) is limited by the fact that they have to pass through the food canal formed by the mouth parts of sucking insects (Fig. 25). These microfilariae do not possess a functional esophagus or intestine, but form them after growing inside their vector. Adult female filariae are extremely long and thin and are able to take up nutrients via their body wall. In some of the longest species reaching up to 70 cm in length (e.g., Onchocherca females) the alimentary canal seems to have lost the ability to ingest food (Fig. 23).
The alimentary canal may be subdivided into mouth, buccal cavity, esophagus, intestine, rectum and anus (Fig. 11, Fig. 21, Fig. 22, Fig. 23).
The mouth was originally surrounded by six lips on which sensory papillae occur, but this basic pattern is considerably modified in most parasitic species. In strongylids the mouth is surrounded by one or two leaf crowns, in ascarids there are only three lips, and in filariae prominent lips are completely lacking (Fig. 11, Fig. 13).
The buccal cavity has a wide lumen in strongylid nematodes. The duct of the dorsal esophageal gland opens into this cavity and enzymes for extracorporal digestion are released together with other substances such as acetylcholinesterase, which is thought to have a role in attachment of the worm to the host's intestinal wall. The cutting plates of Necator americanus and the teeth of Ancylostoma spp. are modifications of the cuticular lining of the buccal cavity, and may even be lacking in many other nematode groups (Fig. 13).
The esophagus is a tube with a characteristic trifurcated cuticle-lined lumen, the outer wall of which is formed by the basal lamina. The tips of the luminal rays are connected to the basal lamina by tonofibrils. Muscle fibrils radiate from the cuticular lining of the lumen to the basal lamina (Fig. 21). The contraction of these fibrils opens the lumen of the esophagus, thus sucking in nutrients. The hydrostatic pressure of the body fluid closes the esophageal lumen. This pumping mechanism presses the food through the intestine to the anus. At the end of the esophagus there is an additional valve which prevents the reflux of the ingested material. A gland cell is situated in the dorsal and both subventral sectors of the posterior portion of the esophagus. The cells produce digestive secretions which are released through ducts into the lumen. The opening of the dorsal gland cell is situated far anteriad, often even in the buccal cavity, while the other openings are further posteriad. Some trichurid nematodes are endowed with a stichosome (Fig. 17, Fig. 18, Fig. 22), a multicellular organ that is very prominent in some stages and consists of unicellular stichocytes. It opens into the esophageal lumen and apparently functions as a secretory gland and storage organ.
The intestine is a cylindrical tube and its wall consists of a basal lamina and a single layer of epithelial cells which carry microvilli on their luminal surfaces (Fig. 23A,C). The microvilli stand close together and contain an axial core which is extended into the underlying cytoplasm and is connected to the terminal web. The terminal web is a porous layer of structural proteins lying below the bases of the microvilli and connected to the core of the microvilli. On its other side numerous microfilaments are continuous with the underlying cytoplasm (Fig. 23B).
The cytoplasm of epithelial cells of the anterior intestine contains mainly mitochondria, rough endoplasmic reticulum, and Golgi complexes producing digestive enzymes which are released into the intestinal lumen. The cells of the middle and posterior region contain more structures which are associated with absorption, intracellular digestion, and storage of reserves and/or waste products. Intestinal cells usually contain energy reserves such as glycogen and lipid droplets. The outer membrane of the intestinal cells is often folded into a basal labyrinth which is covered by the basal lamina as the outer lining of the intestinal cylinder. The intestinal cells of bloodsucking nematodes usually contain large amounts of concentric granules. These granules contain iron originating from the hemoglobin of host erythrocytes. In several species further disintegration leads to the complete disappearance of these granules. Glycogen is rather rare in the intestinal cells of blood-feeding nematodes, whereas in others it forms large aggregations. The intestinal cells of adult females of Onchocerca volvulus and O. gibsoni are extremely thick, thus reducing the intestinal lumen to a system of intercellular clefts (Fig. 23D).
The rectum is lined by cuticle. In male nematodes the germinal tube opens into the rectum, thus giving rise to a cloaca, the wall of which forms retractible copulatory organs (Fig. 12, Fig. 13).
The excretory system of nematodes was named as such from morphological descriptions, but its function is rather osmoregulatory, ion regulatory, and even secretory rather than excretory. The tubular type of excretory system is the most common type among parasitic nematodes. It consists of a system of tubes and one or two gland cells which have a joint excretory duct. The lateral tubes run inside the lateral chords of the hypodermis (Fig. 11, Fig. 14C, Fig. 24D). The hypodermal cytoplasm and the excretory tube are separated by membranes. The cytoplasm of the tube contains numerous canaliculi which open into the main canal (Fig. 24). In the anterior region of the worm, the lateral tubes are connected by a transverse canal. An excretory duct which is lined with cuticle runs from this transverse canal to the excretory pore. The subventral gland cells are connected to the transverse canal (Fig. 24). These gland cells have a secretory function, and they have been shown to release acetylcholinesterase and protease in some species.
In the first-stage larvae of filariae (microfilariae) the excretory system consists of a single cell, and adult worms of this group apparently lack excretory systems. The hypodermal gland cells or bacillary cells are thought to have an osmoregulatory or secretory function in trichurid nematodes (Fig. 17, Fig. 18).
Single and sometimes branched cells are frequently found in the pseudocoel adjacent to the gonads or other internal organs, and in the anterior or posterior end portions of nematodes. These cells are called coelomocytes and are assumed to be phagocytic and to purify the body fluid (Fig. 24).
Most research on nematode behavior concerns free-living bacteriophagous or plant-parasitic species. These commonly respond to a variety of different stimuli, e.g. water-soluble and volatile chemicals, temperature, touch, light, and electric potentials. They mainly seem to use chemotactic mechanisms to find their food or hosts in the soil. In the bacteriophagous nematode Caenorhabditis elegans, responses to hundreds of chemicals have been tested and 14 types of sensory neurons described. Alone in the amphids, which are sensory structures in the head, the functions of 11 types of neurons were analyzed using laser microbeam ablation and various genetic methods.
The behavior of infective larvae of animal-parasitic nematodes is not as varied as that of the free-living species. This may be based on the fact that the infective stages do not feed, defecate, oviposit, mate or undergo morphogenesis. The typical infective larva is enclosed within two cuticles. The outer cuticle or sheath originates from an incomplete molt and protects the infective larva from environmental stress. The sheath does not prevent the detection of environmental stimuli by the larvae as the amphids, as complex sensory organs, communicate with the environment via openings in the cuticle.
Whereas much information is available on the behavior and the physiological processes of infective nematodes which must be eaten by their hosts, little is known about the way larvae which penetrate the host's skin find and identify their host. Their behavior has functions in the following phases of host-finding:
|
