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General Morphology of Nematodes
General
Morphology of Nematodes
Plant parasitic
nematodes as well as the free living soil and fresh water nematodes are
microscopic, metamerically unsegmented (segmentation apparent in some
cases is confined to cuticle only), thread like slender animals which
are generally bilaterally symmetrical (condition whereby any two parts
of the body are exact counterparts of one another) and vary in length
between 0.2 to 10.0mm. The majority of them, however, are Jess than 0.2
mm in length. Typically, the nematode body is cylindrical (elongate),
sometimes fusiform; gradually tapering at the both extremities. The
adult females of some groups exhibit a further modification in shape in
as much as some may be pear, lemon, reniform or saccate shaped. For
example, the females of the genera Meloidogyne, Heterodera, Nacobbus,
Tylenchulus and Rotyler.chulus assume one of these different
shapes.
The
nematode body is circular in cross section, with the principal axis as
the longitudinal axis. The width of the nematode body varies with the
total length, the long thin forms being generally filiform, with almost
the same body width throughout the length, e.g., members of the
genus Ecphydaphora, In the shorter nematodes, body width is
narrow at the extremities and wide in the middle and may be as much as
15-16 per cent of the total length, e.g., members of the family
Crkonematidae.
The nematode
body mainly consists of three tubes, the outer tube or the
body wall and musculature, the inner tube or the digestive tract and the
reproductive parts which are embedded in the false coelom of the
nematode body. There is no definite division of
parts of the
nematode body, but for convenience sake, the body can be sub-divided
into recognizable parts. The anterior end, bearing the mouth, pharynx
and cephalic papillae, is termed as the head. This is followed by
generally a well defined oesophagus, intestine and finally the tail
which characterizes the portion between the anus and tip of the nematode
body. Longitudinally, the body is-divided into four regions: dorsal,
ventral and two lateral sides. When at rest, nematodes generally have a
tendency to curve ventrally. However, the ventral side can be easily
ascertained by locating the anal opening or the excretory opening or the
vulval opening in adults. The position of the vulval opening is
variable, usually located in the middle of the body but in certain cases
it may be slightly above the anal opening.
Body Wall:
The body wall is composed of cuticle, hypodermis. And somatic
musculature.
Cuticle: It is
a protective covering and extends over the nematode body and into the
various openings. It is non-cellular colorless and tough and may be
smooth or deeply striated into annules as in the genus Criconemoides
or marked by punctations or spines or various other markings. There
may be longitudinal thickenings or expansion of the cuticle which are
termed as alae. These may extend laterally throughout the length of the
nematode body (termed as lateral field) or may be confined to the two
extremes of the nematode body.
Normally,
there are four moults during the life cycle of a nematode and with each
moult, the old cuticle is shed off from the body as also the cuticle of
the pharynx, vagina and rectum. In some instances, however, the old
cuticle may be retained at the final moult as in the case of members of
the genus Hemicycliophora.
External structures ; Lateral fields:
These help in the free movement of annules and are variable in number
(two to six) in different genera and species. In a cross section, these
appear as in folds of the cuticle. These are also referred to as
incisures, alae, lateral striae, lateral alae, etc. generally, these
incisures start near about the position of the nerve ring and terminate
near the anal
region. The
transverse striae are broken by these incisures but in some species, the
transverse striae, at places, run across the incisures giving rise to
aerolation. In some genera of plant parasitic nematodes, the striae tend
to disappear or are difficult to observe because of the
huge gonads or
obesity of the female body as in the genera Angulna, Ditylenchus,
Heterodera, etc. However, in the larval forms it can be observed
easily. The incisures, which are often diagnostic in value, can be seen
best in freshly killed nematodes. The most reliable' method of
ascertaining the number of lateral measures is to cut a cross section of
the nematode body near about the mid region and mounting it for an
end-on view.
Cuticle Layer:
Historical studies have demonstrated that cuticle is a complex
structure, which may be different with different group, of nematode, or
even different in different growth stages of a single nematode species.
Although, in the cuticle of Ascaris lumbricoides, and many as nine
layers have been identified but in general the external cuticle layer,
could be broadly grouped in three main layers: (1) cortical; (2) median
(matrix,; and (3) basal (oblique fiber) layer,. Recently, Deontostoma, (Bnoplia)
has been proposed to possess the model cuticle since the
genus is considered to posses, nearly all the complements of strata'.
According to this model the nematode cuticle is divided into four
fundamental layers-epicuticle, exocuticle, mesocuticle and endocuticle.
Beside, these four basic layers, anteriorly and posteriorly, some
nematodes may have additional deposits called 'infracuticle.
Chemically,
the cuticle is-apparently composed of, at least in the case of Ascaris
lumbricoides, albumins, a glycoprotein, a fibriod a collagen and a
keratin. Chemically, the external cortical layer show, characteristic,
of keratin because of its sulfur content and ratio of histidine, lysine
and arginine. But x-ray defractor patterns show, its similarity with
collagen. Quinones, polyphenols and polyphenol oxidase activity ha, been
detected in some nematodes of Heterodera species. In a study on
Ditylenchus dipsaci, it has been demonstrated that the
imperviousness, of the cuticle i, due to a membrane which is
thermolabile-probably a wax or sterol which disappears when the
nematode, are killed by heat. Under this layer lie, the cortex followed
by matrix which is rich in matricin (rich in sulphur). The fibre layer
consists of collagan.
Fifteen
amino acids in the cuticle component of Meloidogyne
javanica
and M. hapla along with presence of phenolic compounds and enzyme
polyphenol oxidase have been recorded. Qualitatively and/or
quantitatively, the cuticle components may vary with different .genera.
This is reflected by their differential permeability to fixatives and
stains. An electron microscopic study of the cuticle of M. javanica
has shown that the cuticle consists of an osmophilic external
cortical layer and an internal cortical layer which is morphologically
different from the thick fiber layer. The outermost layer is about 7 xnp
thick and corresponds with a triple layered plasma membrane. The
internal cortical layer is biochemically active and has enzymes and RNA.
Generally, marine nematodes have greater permeability of the cuticle
than the animal or plant parasitic nematodes. The cuticle of the living
nematode may act as a semipermeable membrane for selective entry of
metabolites.
As has been
stated already, the cuticle may be smooth or marked by transversely
striated lines to resemble segmentation. In Pratylenchus and
Tylenchorhynchus species, the striae are fine in comparison to
deep striatjons in Hemicydiophora and Hoplolaimus species.
In Criconematinae, the striations become even more prominent and
resemble almost segmentations. Asa icsult of swelling of the females in
the Heteroderids, the transverse striae are apparently modified into
perinea) patterns and zig-zag pattern.
Other cuticular
markings are punctattons suggested by some of the openings of pore
canals6 in the cortical layer and may occur either in
transverse or longitudinal rows. The cuticle of fieierodera
species is abnormally thick and contain pore canals6 and
polyphenol oxidase6. Spines, bristles, warts or setae are
also cuticular markings and these may be localised or occur all over the
body. Cuticle may be also inflated in some cases appearing as swellings
in a blister like manner.
Besides
transverse striations, longitudinal markings may be present on cuticle
and these may be simply interrupting the transverse striations or may
be ridges or alae. The term alae are used here to designate thickenings
or projections extending throughout the nematode body or may be confined
to the anterior or posterior regions. The longitudinal alae, usually
four in number are sub-lateral
in position.
The lateral fields are two such longitudinal cuticular thickenings.
Besides the
longitudinal alae, modifications may be present at the two extreme ends,
the anterior region being known as cervical alae and mainly observed in
animal parasitic nematodes. The caudal alae, which is located in the
posterior region of the males, serves as the clasping organ at the time
of copulation. However, it is not an essential feature of the males. It
may be present in some genera and absent in others. It may also have
genital papillae, in some cases, which serve as tactile organs. There
may be six or more pairs as in Rhabditida or three or fewer as in
Tylenchida.
Besides
forming the top layer of the nematode body, the cuticle is involved also
in the formation of lips, setae and sensory organs like deirids, amphids
and phasmids.
The
internal cuticle, which invaginates mouth, rectum, cloaca, vagina and
excretory pore, is characterized by having epi and exocuticle.
More On General Morphology
of Nematodes
● Hypodermis
● Muscles
● EXCRETORY
SYSTEM
● DIGESTIVE
SYSTEM
● NERVOUS
SYSTEM
●
REPRODUCTIVE SYSTEM
●
Male Reproductive System
●
Female Reproductive System
● Sex
Determination
●
Embryonic Development
●
Post-embryonic Development
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