In this article we will discuss about Ascaris Lumbricoides:- 1. Habit, Habitat and External Features of Ascaris Lumbricoides 2. Body Wall of Ascaris Lumbricoides 3. Body Cavity or Pseudocoel 4. Digestive System 5. Respiratory and Excretory System 6. Nervous System 7. Sense Organs 8. Reproductive System 9. Life History 10. Parasitic Adaptations.
Contents:
- Habit, Habitat and External Features of Ascaris Lumbricoides
- Body Wall of Ascaris Lumbricoides
- Body Cavity or Pseudocoel of Ascaris Lumbricoides
- Digestive System of Ascaris Lumbricoides
- Respiratory and Excretory System
- Nervous System of Ascaris Lumbricoides
- Sense Organs of Ascaris Lumbricoides
- Reproductive System of Ascaris Lumbricoides
- Life History of Ascaris Lumbricoides
- Parasitic Adaptations of Ascaris Lumbricoides
Contents
- 1. Habit, Habitat and External Features of Ascaris Lumbricoides:
- 2. Body Wall of Ascaris Lumbricoides:
- 3. Body Cavity or Pseudocoel of Ascaris Lumbricoides:
- 4. Digestive System of Ascaris Lumbricoides:
- 5. Respiratory and Excretory System of Ascaris Lumbricoides:
- 6. Nervous System of Ascaris Lumbricoides:
- 7. Sense Organs of Ascaris Lumbricoides:
- 8. Reproductive System of Ascaris Lumbricoides:
- 9. Life History of Ascaris Lumbricoides:
- 10. Parasitic Adaptations of Ascaris Lumbricoides:
1. Habit, Habitat and External Features of Ascaris Lumbricoides:
Ascaris lumbricoides is an endoparasite in the small intestine of man lying freely in the lumen. It has been living in man from time immemorial. It is cosmopolitan in distribution. It is found more commonly in children than in adults. Sometimes it migrates from intestine to stomach and comes out through the mouth or nostrils of the host.
As many as 1000 to 5000 adult worms may inhabit a single host. Mode of nutrition is holozoic, as it feeds on host’s partly digested food by sucking action of its pharynx. It produces anti-enzymes to protect itself from the action of the host enzyme. Sexual dimorphism is well distinct; only sexual reproduction takes place, asexual reproduction does not occur. Life cycle is simple and monogenetic; no secondary host.
External Features of Ascaris Lumbricoides:
(i) Shape and Size:
Ascaris lumbricoides is elongated, cylindrical, and tapering at both ends. It is a large sized nematode showing sexual dimorphism, i.e., sexes are separate. The female is 20-41 cm .(8-16 inches) long and 4—6 mm in diameter, but the male is smaller, being 15-31 cm (6-12 inches) and 2-4 mm in diameter; its posterior end (tail) is curved ventrally.
(ii) Colouration:
Generally nematodes have no colour, the external cuticle is whitish or yellowish but some, like Ascaris have a definite reddish tint caused by the presence of haemoglobin.
(iii) Morphology:
The anterior end of both the sexes exhibits the same structure. The body is covered with a smooth, tough and elastic cuticle which is striated transversely and gives the pseudo-segmented appearance to the worm. The cylindrical body has four longitudinal epidermal chords visible externally; the narrow one mid-dorsal, one mid-ventral and two thick ones are lateral.
The dorsal and ventral chords appear white, while the laterals appear brown. In nematodes the anterior mouth is bounded by six lips or labia but they are reduced by fusion to three in Ascaris, one elliptical mid-dorsal and two oval latero-ventral in position. Therefore, the mouth of Ascaris is a triradiate aperture.
The dorsal lip has 2 double sensory papillae, and each latero-ventral lip has 1 double sensory papilla; these four papillae form an outer labial circle though most nematodes have 6 papillae in the outer labial circle. Nematodes also have an inner labial circle of 6 papillae, but in Ascaris, as in most parasitic nematodes, the papillae of the inner labial circle are absent.
The latero- ventral lips have a lateral papilla each and cuticular excavation called amphid which is reduced in parasitic nematodes. Amphids are olfactory chemoreceptors. The lips have fine teeth. Behind the lips there is a pair of cervical papillae, one on each side close to the nerve ring. All papillae are sensory.
Near the posterior end is a transverse anus with thick lips, but the male has a cloaca from which two equal chitinous spicules or penial setae project. In the male near the cloaca there are cuticular elevations ventrally, about 50 pairs of pre-anal papillae and 5 pairs of post-anal papillae, these are concerned with copulation.
There is a short post-anal tail which is straight in the female but sharply curved in the male. The female genital aperture called vulva, or gonopore is on the ventral side, about one-third of the length from the anterior end. Behind the lips, there is an excretory pore mid-ventrally.
2. Body Wall of Ascaris Lumbricoides:
The body wall of Ascaris consists of outer cuticle, middle sub-cuticle or epidermis or hypodermis and inner muscle layer.
(i) Cuticle:
It is a thick, tough, wrinkled and transparent outermost layer of the body wall, and it is continuous with the cuticular lining of the pharynx and rectum. It is non-cellular and made of albuminous protein which is resistant to the digestive juices of the host but it is permeable to salts and water. The cuticle is not chitin and it is soluble in KOH, but true chitin is present in egg shells.
However, under light microscope, cuticle can be identified into four distinct layers being made of different chemical composition and different structural arrangements; these are:
(i) Cortical or cortex layer, being made of keratin and resistant to the action of host’s digestive enzymes.
(ii) Matrix layer, it is of a spongy consistency having a protein matricin rich in sulphur; it is an elastic layer and contains several fine fibres.
(iii) Fibre layer, it is in fact the inner layer but not the last layer of cuticle; it has dense connective tissue with interlacing collagen fibres, and
(iv) Basement membrane, it constitutes the innermost layer of cuticle.
But the recent electron microscopic studies of the cuticle of Ascaris revealed the presence of one more layer in addition to the above four layers.
However, cuticle under electron microscope appears to be made of following layers:
(i) Lipoid layer, it is about 1000 A° thick thin layer of an osmophilic membrane;
(ii) Cortical or cortex layer, it is composed of (a) an outer cortical layer and an inner cortical layer;
(iii) Matrix layer, it is composed of an outer fibrillar layer, and a boundary layer;
(iv) Fibre layer, it is composed of collagen fibres arranged in three strata; and
(v) Basement membrane, it is a thin layer surrounding the epidermis or hypodermis.
The cuticle moults four times in the lifetime and the moultings occur only during the period of growth.
(ii) Epidermis:
Below the cuticle is a syncytial epidermis with many nuclei but no cell walls, the nuclei lie in the longitudinal epidermal chords only, the number of epidermal cells is small. The epidermis secretes the cuticle and forms four longitudinal thickenings, the epidermal chords. Two of these are thicker lateral lines or chords and two thinner are dorsal and ventral lines.
The lateral lines contain excretory canals, and the dorsal and ventral lines contain nerves. Epidermis of free-living nematodes contains unicellular epidermal glands. Fat and glycogen reserves are abundantly found in the epidermis.
(iii) Muscle Layer:
Internal to epidermis and lying between the epidermal chords is the body wall musculature consisting exclusively of a single layer of longitudinal fibres running along the length of the body.
Each muscle cell or fibre has two zones, an outer fibrillar zone of longitudinally striated, spindle-shaped muscular part which is contractile and lies towards the epidermis; and a second protoplasmic zone which is club-shaped, bladder-like mass of protoplasm with a nucleus and a network of supporting fibrils form a fibrous process or muscle tail.
Muscle tails of the upper half are inserted into the dorsal line and joined to the dorsal nerve, and in the lower half, the muscle tails are inserted into the ventral line and joined to ventral nerve. Dobell (1965) has emphasised that muscle tails are cellular extensions which form synapses on the motor nerves of the dorsal and ventral nerve cords.
The muscles lie in four quadrants being separated by the longitudinal chords. Each quadrant has about 150 muscle cells. Contractions of these elongated muscles cause a twisting and bending of the body; it results into undulating movement of Ascaris to counteract the peristaltic activity of host’s intestine.
When the muscle cells are many in each quadrant and extend well into the body cavity, then this condition is called polymyarian, as in Ascaris.
When muscles are flattened and only 2 or 3 occur in each quadrant, the condition is termed meromyarian, as in Oxyuris. But when the muscles are small and closely packed together forming a complete layer then this type is holomyarian as in Trichuris.
In a transverse section the longitudinal muscles show a peripheral U-shaped fibrillar zone enclosing a club- shaped protoplasmic zone from which muscle tails project. Special muscles occur in the pharynx, vagina of female, and in connection with the spicules of the male.
3. Body Cavity or Pseudocoel of Ascaris Lumbricoides:
The space between the body wall and alimentary canal is not a coelom but a pseudocoel because it is bounded by muscles outside and the cuticle of the intestine inside. It is formed by the breakdown of connective tissue cells. In the young animal the organs are packed in parenchyma, but almost all of it disappears in the adult so that the organs hang loosely.
Pseudocoel has fibrous tissue and fixed cells called coelomocytes or pseudocoelocytes or giant cells, there are four such cells occupying fixed positions along the lateral chords, they are branched and giant-sized, so that they fill the body cavity, the confluent vacuoles of these giant cells represent the pseudocoel, hence, the pseudocoel is an intracellular space.
Pseudocoel is filled with a clear, protein-rich fluid, the pseudocoelomic fluid or perienteric fluid which distributes digested food and transports various metabolites.
It is composed of 93 per cent water and remaining solids like protein, glucose, non-protein, nitrogenous substances, sodium chloride, phosphate, etc. Reproductive organs lie free in the pseudocoel. In free-living forms, pseudocoelocytes are small in size and more numerous.
Nematodes show certain histological peculiarities, they have a constancy in the number of cells in the body which have been formed by the time hatching takes place, because cell division stops after hatching (except in the reproductive organs); hence, with growth the cells elongate and become giant-sized, especially the muscle cells, coelomocytes in the pseudocoel, and excretory canal cells; a cell may be more than 1 cm long.
There is tendency towards formation of syncytia, probably nuclei increase in number by fragmentation or amitosis and the cell walls break down.
4. Digestive System of Ascaris Lumbricoides:
(i) Alimentary Canal:
It consists of the mouth, a short pharynx or oesophagus forming the foregut; a long tubular intestine or the midgut and a short rectum or hindgut.
(ii) Mouth:
As already referred to, the mouth is a triradiate aperture situated at the anterior tip surrounded by three lips or labia.
(iii) Pharynx:
The terminal mouth leads into a cylindrical thick-walled pharynx or oesophagus which has a posterior swelling called end bulb which is provided with valves, the pharynx has muscular walls having radial muscle fibres which dilate the lumen.
Internally it is lined by cuticle which, at the margin of mouth, is continued with the cuticle of the body wall. The pharynx has 3 large branching gland cells which open by cuticular ducts into the lumen; these are in fact, pharyngeal or oesophageal glands.
The cavity of the pharynx has three deep longitudinal grooves lined by cuticle, and in a transverse section the lumen appears triradiate, connective tissue fibres arise from each of the three internal grooves and go to the cuticle covering the pharynx, they maintain the triradiate shape of the lumen. This much constitutes the stomodaeum or foregut.
(iv) Intestine:
The pharynx opens posteriorly into a thin-walled dorsoventrally flattened intestine or midgut which extends almost the entire length of the body. It is formed of a single layer of columnar epithelial cells lined externally by a thin layer of cuticle. The free inner margin of each cell is produced into several finger-like projections, the microvilli (Kessel et al., 1961).
They form a sort of tightly packed brush border which increases the surface area. The intestine has no muscle layer.
(v) Rectum:
The intestine is followed by the, hindgut or rectum which is also flattened dorsoventrally. Its wall consists of tall columnar cells and lined internally by cuticle and externally by few muscle fibres. In male the rectum opens out by cloaca because it receives the ejaculatory duct but in female the rectum opens out by a transverse slit-like aperture, the anus.
The anal aperture is guarded by anterior and posterior lips and is provided with a few special dilator muscles running from the rectum to the body wall, called depressor ani. Their contraction from time to time causes the faecal matters to be discharged out. The rectum also has large unicellular rectal glands; 3 in the female and 6 in the male.
(vi) Food, Feeding and Digestion:
Food of Ascaris Lumbricoides comprises blood, tissue exudes and partly or fully digested food of the host. Food is sucked in by the suctorial action of the pharynx.
Digestion is extracellular which occurs in the intestinal lumen; the process of digestion is facilitated by the enzymes like proteases, amylase and lipase secreted by the gland cells of the pharynx. The digested nutrients are absorbed in the intestinal wall and finally distributed by the pseudocoelomic fluid.
Excess food is generally stored as reserve glycogen and fat in the intestina wall, muscles and syncytial epidermis. Intracellular digestion has also been reported to occur in the cells of intestinal wall as they engulf solid particles to digest intracellulary. The undigested wastes, if any, are defecated out by the contraction of special muscles of rectum through anus or cloaca.
5. Respiratory and Excretory System of Ascaris Lumbricoides:
There are no respiratory organs, but parasites carry on anaerobic or anoxybiotic respiration as the oxygen content in the intestine of host is very poor. It obtains energy by the breaking down of glycogen into CO2 and fatty acids which are excreted though the cuticle like those of flatworm parasites, e.g., Fasciola and Taenia.
The chief fatty acids produced as excretory wastes are butyric, valerianic and caproic acids. It also consumes oxygen when available. The small amount of hemoglobin iii the body wall and pseudocoel fluid takes up the oxygen even when it is present in low tension, so that aerobic respiration may also take place.
Excretory System of Ascaris:
Excretory Organs:
In marine nematodes, the excretory system consists of one or two large renette gland cells lying ventrally in the pseudocoel near the junction of the pharynx and intestine, a duct arises from each renette cell, these ducts join and open by an excretory pore mid-ventrally.
There is much evidence to support the view that from this glandular system arose a tubular excretory system which is like an H with two longitudinal excretory canals connected by a bridge, the transverse canal from which arises a common excretory canal leading to an excretory pore.
In Ascaris Lumbricoides (Fig. 46.7) there is a longitudinal excretory canal in each lateral line, the anterior limbs of the H are reduced, and the transverse canal is branched to form a network from which arises a short common excretory canal to open by a minute ventral excretory pore just behind the lips. The canals are more developed on the left side than on the right.
The canals are lined by a firm membrane and covered with a layer of cytoplasm; they are intracellular excavations in the single giant cell whose nucleus lies on the transverse canal. The excretory system has no internal openings, cilia, flame cells, or nephridia.
Physiology:
Excretory physiology of Ascaris Lumbricoides is very poorly understood. However, the nitrogenous waste chiefly comprises urea which diffuses into the pseudocoelomic fluid.
The excretory canals are said to secrete this urea which is eliminated through the excretory pore; some ammonia and urea are also passed out along with the faecal matters. Observations are also there to suggest that Ascaris Lumbricoides excretes more urea when water is scarce.
6. Nervous System of Ascaris Lumbricoides:
The nervous system of Ascaris Lumbricoides (Fig.46.8 and 46.9) is well developed and complicated and like the excretory system it is also situated in the body wall, i.e., it is hypodermic. However, it consists of circumpharyngeal ring or nerve ring and nerves originating from it.
There is a circumpharyngeal ring around the pharynx, it is made of nerve fibres and some diffusely arranged nerve cells. Associated with this ring are many ganglia; there is an unpaired dorsal ganglion, and close to it is a pair of sub-dorsal ganglia. On each side of the ring is a lateral ganglion which is divided into six ganglia. On the lower side of the ring is a pair of large-sized ventral ganglia.
Each ganglion has a fixed number of nerve cells. From the circumpharyngeal ring arise six small nerves anteriorly, each having a ganglipn, they are arranged radially and go to the sense organs of the anterior end (papillae and amphids).
Posteriorly six long nerves arise from the ring and run to the posterior end; of these six nerves one is a mid-dorsal nerve and one is a mid-ventral nerve, the former lies in the dorsal line and the latter in the ventral line. The mid-ventral nerve is the main nerve and it is ganglionated along the anterior length, it may be called the nerve cord. Near the anus it has an anal ganglion which sends nerves to the tail.
The other four posterior nerves are thinner, they are one pair of dorsolateral nerves and a pair of ventrolateral nerves, they lie on the sides close to the excretory canals. The dorsal and ventral nerves are connected by a number of transverse commissures, and the ventral nerve and lateral nerves are joined together by many ventrolateral commissures. Posteriorly the innervation is more complicated in males than in females.
7. Sense Organs of Ascaris Lumbricoides:
Due to parasitic mode of life, Ascaris Lumbricoides has developed sense organs (Fig. 46.10) which are very simple. They are either as minute elevations or pits in the cuticle of the body.
However, they are as follows:
(i) Labial Papillae:
The labial papillae are four, two on the dorsal lip and one each on the ventrolateral lips, each is a double sense organ. Each labial papilla consists of a fine fibre of sensory nerve surrounded by many supporting cells (Fig. 46.10 B). These are gustatory or taste organs.
(ii) Amphids:
The amphids are two, situated one each, on the ventrolateral lips. These are small pits containing glandular and nerve cells supplied by amphidial nerve from the lateral or amphidial ganglia (Fig. 46.10 A). These are olfactory chemoreceptors.
(iii) Phasmids:
The phasmids are unicellular glands situated one on each side of the tail behind the anus. These are pit-like and chemoreceptors (Fig. 46.10 E).
(iv) Cervical Papillae:
The cervical papillae are a pair of small pits situated just a little behind the oral lips in the lateral sides of the body. These are bulb-like nerve endings with supporting cells (Fig. 46.10 F). These are probably tactile in function.
(v) Cephalic Papillae:
The cephalic papillae are also pit-like being formed of nerve fibres surrounded by supporting cells but the nerve fibre has a lens-like expansion just beneath the cuticle and then narrows to form canal which widens before opening at the surface (Fig. 46.10 C).
(vi) Genital Papillae:
The genital papillae are found in males. These consist of nearly 50 pairs of preanal and 5 pairs of postanal papillae . These are also formed of 1-3 nerve fibres embedded in supporting cell (Fig. 46.10 D). These are also tactile in function and help during copulation.
The various receptor organs of nematodes are of taxonomic importance.
8. Reproductive System of Ascaris Lumbricoides:
In Ascaris Lumbricoides, like other nematodes, sexes are separate, i.e., dioecious and readily distinguishable externally, i.e., sexual dimorphism is well distinct.
The male is smaller in size than the female, its tail is curved, while female’s tail is straight, it bears cloaca and a pair of spicules or penial setae but female possesses anus and spicules absent, male possesses pre- and post-anal papillae which are absent in female. The male system is reduced to a single tube but the female system is double.
Male Reproductive Organs:
These are confined to the posterior part of the body and consist of testis, vas deferens, seminal vesicle, ejaculatory duct and penial setae.
(i) Testis:
Testis is monorchic, i.e., single in Ascaris Lumbricoides but it may be diorchic, i.e., two testes in some nematodes. However, the testis of Ascaris Lumbricoides is a long, thread-like, coiled tube.
Its wall is composed of a single layer of cuboidal cells being surrounded by the basement membrane. Its central axis is solid cytoplasmic rachis; the rachis is surrounded by clusters of amoeboid cells in various stages of their development. In fact, these are developing sperms.
(ii) Vas Deferens:
The testis continues distally into a short and thick coiled tube of the same diameter, the vas deferens. However, it is distinguished from the testis in possessing a central lumen in place of the solid rachis.
(iii) Seminal Vesicle:
The vas deferens joins posteriorly with a much thicker, wider somewhat muscular and straight tube called seminal vesicle. It lies below the intestine in the posterior one- third of the pseudocoel.
(iv) Ejaculatory Duct:
The seminal vesicle narrows at its posterior end to form a short, but muscular ejaculatory duct which opens into the rectum to form the cloaca. This duct bears a number of prostatic glands whose secretions help in copulation. The cloaca opens out by cloacal aperture.
(v) Penial Setae:
Dorsal to the cloaca is- a pair of muscular sacs, the penial sacs or spicule pouches, the two spicule pouches unite and join the cloaca. The pouches contain a pair of spicules or penial setae which are cuticular with a cytoplasmic core. The spicules can be protruded and retracted through the cloacal aperture by the action of special sets of protractor and retractor muscles.
They serve in copulation to open the female genital pore, and, thus, help to transfer sperms, their function is aided by a chitinous plate, the gubernaculum lying in the wall of the cloaca.
Female Reproductive Organs:
These are confined in the posterior two-third of the body and consist of ovaries, oviducts, uteri and vagina.
In fact in Ascaris Lumbricoides, like most of the nematodes, there is a set of two parallel tracts of female reproductive organs, i.e., an ovary, oviduct and a uterus in one tract; this condition is called didelphic, although monodelphic (one tract) and polydelphic (many tracts) conditions are also found.
(i) Ovaries:
The paired ovaries of Ascaris Lumbricoides are long, thread-like and highly twisted tube-like and terminate blindly in the pseudocoel. Internally it has a single layer of cuboidal epithelial cells, a cytoplasmic central rachis and externally a basement membrane surrounding the epithelial cells. The rachis is encircled round by group of developing ova.
(ii) Oviducts:
The ovaries are continued posteriorly into somewhat broader oviducts having similar structure to ovaries but in place of solid rachis there is a lumen in the oviducts.
(iii) Uteri:
Each oviduct is further continued into still broader, thicker and muscular uterus; it has a thick inner layer of circular muscles and a thin outer layer of oblique muscles. The first part of uterus serves as seminal receptacle where sperms, after copulation, are stored and where fertilisation occurs, the remaining uterus stores fertilised eggs, and its cells produce yolk and material for egg shells.
(iv) Vagina:
The two uteri unite and open into a short of gonad. Maturation of median, muscular vagina lined with cuticle. The vagina the gonad not shown. opens by a transverse gonopore or vulva which lies mid-ventrally about 1/3 of the length from the anterior end.
In some nematodes the end part of the uterus or the end part of the vagina, when present, forms muscular ovejectors which by peristaltic movement force the eggs one by one through gonopore.
Formation of Gametes in Ascaris Lumbricoides:
Gonads may be hologonic or telogonic. In hologonic gonads germ cells arise along their entire length. In Ascaris Lumbricoides the gonads are telogonic in which germ cells arise at the proximal end only which is called germinal zone or the zone of proliferation. Next part of the gonad is a growth zone where gametogonia enlarge. In the ovary, the elongated developing eggs are arranged radially around a central cytoplasmic rachis.
In the testis the developing amoeboid sperms are packed around the central rachis.
In the last of part the gonads gametocytes are formed and become free from the rachis, here they undergo maturation division to form eggs or sperms. The last part of gonads where developing gametes undergo maturation are ‘referred to as the maturation zone. As referred to, the sperms are amoeboid in shape, while eggs are elliptical in shape.
9. Life History of Ascaris Lumbricoides:
The life history of Ascaris Lumbricoides is monogenetic as it involves only one host, i.e., man.
However, the life history of Ascaris Lumbricoides can be studied as under:
(i) Copulation and Fertilization:
Copulation takes place in host’s intestine. During copulation male Ascaris Lumbricoides moves in such a way that its cloacal aperture faces the vulva of the female and then male thrusts its penial setae to open the vulva of female.
Then, soon the cloacal wall of male contracts causing transfer of sperms into the vagina of the female and they come to lie in the seminal receptacle part of the uteri, wait for eggs to come through the oviduct for fertilisation.
During fertilisation the entire sperm enters the egg. Soon, after fertilisation the glycogen globules of the egg migrate to the surface to form the fertilisation membrane which soon hardens into a thick, clear inner chitinous shell. Soon, thereafter, the fat globules of the egg form a lipoid layer below the chitinous shell.
Now, as the fertilised egg passes down, the uterine wall secretes an outer thick, yellow or brown albuminous (proteinous) coat or outer shell having a characteristic wavy surface or rippling’s. These eggs are now known as mammiliated eggs; such eggs are elliptical in shape measuring 60-70 µ by 40-50 µ.
(ii) Zygote:
So, to say the zygote has a thick, clear inner shell, a lipoidal layer and an outer shell which is warty and yellow or brown in colour.
The fertilised eggs (zygotes) are laid by female Ascaris in the small intestine of the host and they pass out with the faeces; they are un-segmented when they leave the host. One female may lay from 15,000 to 200,000 eggs in a day; the egg production of Ascaris is astounding, one mature female may have up to 27 million eggs.
The eggs become stained yellowish or brown in the intestine. Eggs fall on the ground and can remain alive for months in the moist soil though complete drying kills them. In order to develop they require oxygen, moisture and a temperature lower than that of the human body, the most favourable temperature is 85°F. They require a period of incubation outside the human body.
(iii) Early Development (Outside the Host):
The stages of early embryonic development, say the cleavage or segmentation, etc., start in the soil. The pattern of cleavage is spiral and determinate.
The fertilised egg undergoes two cleavages to form four cells or blastomeres; in fact the first cleavage results in a dorsal cell AB and a ventral cell P1, the second cleavage causes AB to divide into an anterior cell A and a posterior cell. B, while the ventral cell P1 divides into a dorsal cell EMST and a ventral cell P2.
These four cells are at first arranged in the shape of a T in Ascaris, but later they become arranged in a rhomboid shape, as P2 comes to lie posterior to EMST, which is characteristic of nematodes. However, these four cells are now called A, B, P2 and S2 or EMST. These cells undergo further cleavage to form smaller blastomeres.
However, in the next cleavage A and B divide into A1, A2 and B1 B2 cells respectively, P2 divides into P3 and C, while EMST into MST and E. Thereafter, P3 and E divide into P4 and D and E1 and E2 respectively. The P4 further divides into G1 and G2.
The fate of the various cells resulted so far is fixed, i.e., the descendants of A and B will give rise to the entire ectoderm, except that of the posterior end, MST form the mesoderm of the body wall, pseudocoel cells, and the lining of the stomodaeum, the descendants of E (E1 and E2) give rise to the entire endoderm of the intestine, the descendants of P4 (G1 and G2) will give rise the germ cells and C and D will together take part in the formation of ectoderm and mesoderm.
Thus, the cleavage of embryonic cells continues giving rise to a blastula at the 16-celled stage which is characterised by having a cavity, the blastocoel. Then gastrula is formed by epiboly or overgrowth of ectodermal cells over the endodermal cells, and by invagination of stomodaeum and endodermal cells.
Finally a juvenile is resulted in about 10-14 days from the beginning of segmentation. Structurally, a juvenile possesses an alimentary canal, a nerve ring and a lateral excretory system.
This juvenile resembles very much with Rhabditis (a soil nematode), hence, it is also referred to as rhabditiform larva or rhabditoid. This larva moults within the egg shell in about seven days and becomes the second stage juvenile or second stage rhabditoid; this stage of the life history of Ascaris is infective to the host.
Data suggest that under favourable conditions of oxygen, moisture and temperature, the eggs of Ascaris lumbricoides with infective juveniles may remain viable for about six years in the soil.
(iv) Infection to Host:
We know that there is no secondary host in the life history of Ascaris Lumbricoides, hence, infection to host (man) occurs when he swallows the infective eggs of Ascaris with contaminated food or water.
Thus, when the infective eggs reach in the small intestine of the host, the egg shells are dissolved by the action of host’s digestive juices and the infective second stage juveniles are set free. These juveniles are about 0.2-0.3 mm long and 13-15 µ in diameter and have all the structures of the adults except the reproductive organs.
(v) Later Development in New Host:
They do not develop in the intestine but go on a typical wandering tour of 10 days.
They bore through the intestine wall and enter the mesenteric circulation and pass through the hepatic portal vein to enter the liver, from where they enter the hepatic vein and through the postcaval vein come to the right side in the heart, from where they are carried several times through the body along with the blood stream, then they go through pulmonary arteries into the lungs.
In the lungs, juveniles rupture the capillaries and enter the alveoli where they live for some days, here they grow and moult to become 3rd stage larvae which moult again to become 4th stage larvae.
From the alveoli of the lungs, the 4th stage juveniles make their way through the bronchioles and bronchus into the trachea and then to the throat from where they are swallowed into the oesophagus and reach the small intestine for the second time.
During this 10-day tour, the juveniles have grown about ten times and are 2 to 3 mm long. In the intestine the fourth and final moulting takes place, and in 60 to 75 days, they grow into adult males and females and attain sexual maturity. The length of life of the parasite in the host averages only 9 months to a year.
However, the life history of Ascaris lumbricoides can be represented as:
Adults → fertilised eggs pass out → larvae develop in egg shell and moult twice → swallowed by man → intestine where juveniles hatch → bore through intestine → mesenteric veins → hepatic portal vein → liver → hepatic vein → postcaval vein → right side of heart → pulmonary artery → lungs → alveoli where third moulting occurs → bronchioles → bronchus → trachea → glottis → oesophagus → intestine where fourth moulting occurs → grow into adults.
10. Parasitic Adaptations of Ascaris Lumbricoides:
Ascaris Lumbricoides, like those of other helminth intestinal parasites, exhibits a number of adaptive features to live comfortably and lead a successful life.
However, some of them are as under:
1. Body is long and cylindrical in shape with both the ends pointed.
2. Body is covered externally with resistant cuticle and it also secretes antienzymes; both these protect the worm from the digestive enzymes of the host.
3. Adhesive organs are not found in Ascaris; lack of such organs is compensated by a very poor power of locomotion as by slight movement it maintains its position in the intestine of the host and also counteracts the intestinal peristalsis.
4. Cilia are completely wanting.
5. As the parasite feeds on digested and predigested food of the host, its alimentary canal is simple and poorly developed but pharynx being muscular facilitates ingestion by suctorial action.
6. Since circulatory system is absent, hence, its function of distributing the digested food to the body tissues and transfer of waste substances from the tissues to the excretory canals are performed by pseudocoelomic fluid.
7. Mode of respiration is anaerobic because it lives in an environment which is nearly fully deficient of oxygen.
8. Sense organs are very poorly developed because it leads a sheltered life.
9. Production of huge number of eggs compensates the wastage that takes place during their transfer from one host to new host.
10. The hard shelled covering of the eggs protects the developing juveniles from environmental hazards.
11. The eggs, being very small in size, are easily dispersed.
12. Mode of direct infection due to absence of secondary host from the life history has increased the chances for juveniles to reach a fresh human host.
