Transcript Phylum : Aschelminthes - GCG-42

Dr. Sanjay Kumar Jha
Department of Zoology
P.G.G.C.G. - 42, Chandigarh
Systematic Position
Phylum
Class
Order
Family
Genus
Species
-
Aschelminthes
Nematoda
Ascaroidea
Ascaridae
Ascaris
lumbricoides
NATURAL HISTORY
 Ascaris is one of the best known parasitic roundworms, partly because of its large
size and partly because it is a common human parasite. Human and pig
roundworms are morphological identical and considered to belong to the same
species but cannot survive in one another’s host and hence called var. humanis and
var. suum for human and pig respectively. Other ascarid species, Toxicara canis and T.
cati infects dogs and cats respectively.
 HABITAT: Ascaris lumbricoides var. humanis lives in the small intestine of man, lying
freely in the cavity; sometimes moves to bile duct into the liver, which they greatly
damages; may also move backward into appendix or go forward into the stomach,
oesophagus and comes out through mouth or nostrils. Childrens of slum-areas are
more prominent host of this worm due to their unsanitary and unhealthy habits. It
is cosmopolitan in distribution with very high incidence of infection in Philippines
(70%) and Korea (50%).
 HABITS: Ascaris moves about in the intestinal cavity by dorsoventral bending of
its body; holozoic in nutrition and takes host’s digested food by sucking action;
respiration generally anaerobic; produces enzyme inhibitors to protect itself from
host’ digestive enzymes; reproduction sexual; sexes are separate (male smaller than
female and curved posteriorly); no intermediate host; juveniles resemble the adults
and moult their cuticle four time during their growth.
External Characters
 Form, Colour and Size: Female Ascaris is larger
(20 to 40 cm) while male is smaller (15 to 30 cm)
long; smooth, cylindrical body which tapers either
end and is pink or yellow in colour; body not
divisible into regions; have four whitish or
brownish longitudinal lines (1 dorsal, 1 ventral
and 2 lateral) due to internal thickenings of the
epidermis in these regions.
 Body Parts: The anterior end has triangular
mouth bounded by three lips (1 dorsal and 2
ventrolateral) having fine denticulate margin and
bear sensory papillae. Dorsal lip has 2 double
papillae while each ventrolateral lip has 1 double
papilla, 1 simple papilla and a reduced sense organ
called Amphid. Close behind the lips is a pair of
Cervical papillae, one on each side of the body
little behind excretory pore. Anus is on the
ventral side, a little in front of the hind end in the
form of transverse slit.
(Contd.)
EXTERNAL CHARACTERS
In males, anus also serves as genital aperture
hence called as cloacal aperture. Two
minute chitinous rods often projects through
it called as penial spicules or copulatory
setae which was used by males during
copulation. The females has a separate genital
aperture called the vulva situated midventraly
about 1/3 of the body from the anterior end.
The part of body behind the anus forms the
tail in both the sexes. The curved hind end
of the male bears on the ventral side a
cushion-like prominence just in front of the
anus, about 50 pairs if simple preanal
papillae and 5 pairs of postanal papillae.
Out of these 5 pairs of postanal papillae, the
anterior 2 pairs have double postanal
papillae while the rest 3 pair have single
postanal papilla.
EXTERNAL CHARACTERS (Contd.)
 BODY WALL: The body wall of Ascaris has
three regions: cuticle, epidermis and muscle
layer.
(a) Cuticle: is a thick, firm but elastic,
nonchitinous layer having three main layers cortex (outer & inner), median layer and basal
layer containing collagen and other chaemical
compounds. A thin epicuticle covers the cortex
externally. Cuticle is secreted by the underlying
epidermis and is moulted only four time during
growth (in adults only growth takes place).
(b) Epidermis: It is composed of remarkably
few cells. It is however, syncytial (multinucleate)
and appears as a finely granular and fibrous layer.
It is expanded inward to form 4 longitudinal
ridges, the epidermal chords (1 middorsal, 2
midlateral and 1 midventral in position) visible
through the cuticle as four longitudinal lines.
Lateral cords are much thicker than dorsal and
ventral cords. Basal lamina lies under the
epidermis.
EXTERNAL CHARACTERS (Contd.)
(c) Muscle layer: Only longitudinal muscle
fibres are present. The circular fibres are
altogether lacking in nematodes. The muscle layer
is not continuous but divided by epidermal
chords into four longitudinal tracts: 2
dorsoventral and 2 ventrolateral. The number of
muscle cells is fixed with 150 cells in each tract of
muscle layer. The muscle cell have two distinct
portions: the outer contractile fibrillar portions
and the inner granular cytoplasmic portion or
bladder which projects into the pseudocoel and
is drawn out into a long, slender process, the
muscle tail. The muscle tail of upper and lower
half of the body extends to the dorsal and
ventral nerve cord present in dorsal and ventral
epidermal chords respectively.
 BODY CAVITY: There is a space between body
wall and viscera not lined by mesodermal
epithelium, hence, pseudocoel filled with
pseudocoelomic fluid containing few large
cells, pseudocoelomocytes occupying fixed
positions.
PHYSIOLOGY
 LOCOMOTION: Power of locomotion is poor in Ascaris due to the
presence of stiff cuticle and lack of circular muscles. Slow locomotion
shown by the worm is due to dorsoventral bending of the body brought
about by alternate contraction of dorsal and ventral muscles in the
anterior part of the body which pass over rest of the body. Extention of
the body after contraction is accomplished by elasticity of the cuticle and
partly by the hydrostatic pressure of the pseudocoelomic fluid.
 RESPIRATION: Respiration is generally anaerobic in Ascaris because
the intestinal contents are deficient in free oxygen. In this respiration,
glucose breaks down into CO2 and variety of fatty acids, chiefly
valerianic acid. These fatty acids along with traces of other substances,
gives the living worm, a characteristic tinned-pineapple like smell. CO2
leaves the body by diffusion through the skin whereas fatty acids are
removed via excretory system. Ascaris resorts to aerobic respiration
when oxygen is available. This respiration is aided by the haemoglobin
present in small amounts in the body-wall and in the fluid filling the
pseudocoelom. Ascaris is thus a facultative (optional) anaerobe and not
obligate (compulsory) anaerobe.
Digestive System
The alimentary canal of Ascaris is complete, i.e.,
it has mouth as well as anus. The alimentary
canal is a straight tube and extends almost the
entire length of the body, having three
distinct regions - foregut or stomodaeum or
pharynx; midgut or intestine; and the
hindgut or proctodaeum or rectum. The
foregut and hindgut have ectodermal lining
with continuous cuticle covering whereas the
midgut have only endodermal lining. The
alimentary canal consists of tri-lipped
mouth, suctorial pharynx, intestine,
rectum and anus. The worm mostly absorbs
predigested food of the host and rest of
digestion occurs by various enzymes secreted
(proteases, amylases and lipase) by pharyngeal
gland cells and intestinal cells following by
intestinal cell food capture by food vacuoles.
Nervous System
The nervous system of Ascaris lumbricoides also includes
Central and Sympathetic Nervous System.
(1) Central Nervous System: The C.N.S. of Ascaris
includes circumpharyngeal nerve ring round the
pharynx chiefly composed of nerve fibres with very
few nerve cells. On anterior side it gives off 6
papillary nerves to the sensory papillae of the lips.
Out of these nerves, 2 are dorsolateral, 2 lateral and
2 ventrolateral in position and each one bears a small
papillary ganglion just beyond its origin. 7 ganglia
are also associated with the nerve ring including a
median dorsal ganglion, a pair of subdorsal
ganglion, a pair of lateral ganglion (each
subdivided into six smaller ganglia) and a pair of
large ventral ganglia. From these ganglias, eight
nerves proceed backward respectively. Amphidial
ganglion arising form subdivision of one of the
lateral ganglion sends its nerves to each amphids.
(2) Sympathetic Nervous System: Ascaris also
possesses two sympathetic systems namely,
pharyngeal and rectal.
Sensory System
The sensory organs or sensillae of Ascaris include
Papillae, Amphids and Phasmids.
(1) Papillae: occurs in lips and on the sides of the
anterior end in both the sexes and in front of the
anterior end in both the sexes and in front and
behind the cloacal aperture in the male. There are
four types of papillae in Ascaris, viz., Labial Papilla,
Cephalic Papilla, Cervical Papilla and Genital
Papilla. All papillae are tactile in function and
almost consists of same structural composition, i.e.,
bulb of nerve fibre endings under cuticle
surrounded by suppporting cell with or without
accompanying cell.
(2) Amphids: two amphids, one on each ventrolateral
lip a little above the labial papilla. They also consists
of supporting cell opening out by a pore having
bundle of nerve fibres. They act as chemoreceptors.
(3) Phasmids: they are pair of unicellular glands that
open out on the sides of the tail. These are
glandulosensory in nature.
Excretory and Osmoregulatory System
The excretory system of Ascaris comprises of a pair of
longitudinal excretory canals, one extending
through each epidermal chord. The left excretory
canal is wider than right, connected near the anterior
end of the body by a transverse network of fine
canaliculi lying beneath the pharynx making
excretory system ‘H-shaped’. The anterior end of H
are, however, much shorter than the posterior limbs.
The left canal opens out at the excretory pore that
lies a little behind the mouth. The nucleus of the
system lies on the left arm of the network and forms
the largest cell of the worm’s body. No flame cells or
cilia to move the excretory fluid but ultrafilteration is
done due to high hydrostatic pressure of the
pseudocoelomic fluid and pushed further in the canal
by muscular undulation of the body.
Functions: Water and ion regulation is the only
function, along with some non-nitrogenous waste
materials. Evidences suggested that ammonia and
urea are eliminated by way of alimentary canal.
Reproductive System
Sexes are separate in Ascaris. The gonads are thread like and
consists of flattened epithelium and central rachis,
around which gametes clustered. Gonads are telogonic
divided into germinative, growth and maturation
zone.
A. Male Reproductive System: lies in the posterior 2/3rd
of the body and includes monorchic Testis (occurred
in single set), a twisted Vas deferens followed by
straight wide tube, the seminal vesicle, ejaculatory
duct opening into the cloaca. A pair of sacs, the
spicule pouches, lie above the cloaca having the
penial setae or copulatory spicules that is used for
opening the vulva of the female.
B. Female Reproductive System: lies mainly in the
middle third of the pseudocoel and are didelphic
(occur in double set). It comprises of a pair of long,
slender, contorted Ovaries, two twisted tubes, the
Oviducts, wider straight tubes, the uteri of cuboidal
epithelium and circular and oblique muscles which
leads to vagina and genital pore. The proximal part
of uteri functions as seminal receptacles used for
storing sperms.
LIFE HISTORY
Early Development: The gravid proglottids break loose from strobila singly or in
chains of 4 to 5 and pass out with the faeces of the host. Development of zygote
begins within the worm and by the time the proglottis shed, it has already changed
into an embryo. Cleavage is complete but unequal forming blastomeres of 3 types of
cells: 2-3 macromeres, 3-5 mesomeres and numerous micromeres. The
micromeres form the embryo with 3 pairs of claw-like hooks at the posterior end.
This 6-hooked embryo is called as Hexacanth. The mesomeres and macromeres
form the inner and outer embryonic membranes around the hexacanth. The inner
membrane later forms a thick, chitinous, radially striated secondary shell or
embryosphore. The hexacanth with embryosphore, outer embryonic membrane and
shell or capsule wall is known as the Onchosphere of about 40 µm in diameter.
Onchosphere comes out when the proglottis disintegrates in human faeces and
remain viable for several months in the soil. They do not develop further until taken
up by a pig along with human faeces.
Infection of the Pig: Inside pig shell wall and embrosphore dissolves due to acidic
juices of stomach and a new larval form Hexacanth appears which bore through the
intestinal mucosa of pig and via blood reaches to different part of striated muscles
(tongue, neck, heart and limbs). Here they develops into another larval form called
Bladdorworm or Cysticercus which waits for its infection to man via undercooked
pork meat.
Effect on the Host (Pathogenicity)
Tapeworm infection produces little or no effect on a person with
a sound health.
1) It damages to intestinal lining by hooks and absorbs the tissue
fluid by its scolex.
2) Weak person and children may develop a disease named
Taeniasis characterised by abdominal pain, indigestion,
vomiting, constipation, loss of appetite and weight, insomnia,
lowered resistance to other diseases and nervous disorder.
3) More than one worm may block the intestine and may prove
fatal.
4) When onchospheres accidently reaches human stomach along
with contaminated food or water they reaches the brain or eyes
and encysts their causing a more fatal disease called
Contorl Measures
 Proper disposal of human faeces by underground sewerage
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system.
Preventing the pigs from visiting human ground faeces sites by
isolating them or not defecating in open.
The cysticerci can be killed by thoroughly cooking pork-meat
before eating it.
Vermifuges may be used to remove the adult worms from the
human body. This can only remove the strobila, not the scolex.
The scolex can be removed only by surgical operation.
Inspection of pork by health officials can help checking the
human infection.