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General Morphology of Nematodes
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REPRODUCTIVE SYSTEM
REPRODUCTIVE
SYSTEM
Nematodes are
generally amphigonus (dioecious), existing as separate males and females
and are oviparus. The sexes can be easily identified by the primary and
secondary sexual characters. The males possess the copulatory apparatus
with prominent spicules at the posterior end of the nematode body while
the females possess the vaginal aperture, on the ventral side, in the
middle or the
posterior half
of the body. Rarely, it may be located in the anterior region of the
body. Other distinguishing features of males, which may or may not be
present in all the cases, are comparatively smaller size, curvature of
the posterior end and presence of a caudal alae or bursa and genital
papillae and other accessory copulatory structures. In the marine and
most of the plant parasitic nematodes, both the sexes occur in almost
equal proportion. In the terrestial and fresh water forms, particularly
the latter, the females predominate in the population. The scarcity or
absence of males indicates a tendency towards hermaphroditism or
parthenogenesis. The hermaphroditism is usually of the protoandric
type—the gonad releases the sperms first which are stored in a special
structure and subsequently when an ovum is produced by the same gonad,
the stored sperms fertilize the ovum to form an egg.
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Apart
from the sexual differences, the males and females, in some genera
of plant parasitic nematodes, exhibit extreme sexual dimorphism. For
example, in the genera Heterodera and Meloidogyne, the
females are swollen to lemon or pear shape and become a reproductive
sac and the males remain slender and worm like. In other forms, both
the sexes may remain vermiform but the males may lack a distinct
feeding apparatus as in the Criconematids or the genus Radopkolus.
Intersexes are; also encountered in some populations like
Ditylenchus triformis and Meloidogyne species. In such
cases, the females do not develop fully and possess rudiments of
male characters. Probably, this results from partial sex reversal
under environmental influence or nutritional stress,
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Gonads.
It is essentially similar in both
sexes of all nematodes. It is composed of one or two tubular gonads,
varying greatly in length. These may be also straight or sinous,
reflexed or coiled back and forth. Females with two complete genital
tubes are referred to as didelphic (diovarial) and those with only one
tube are called monodelphic (monovarial). In the didelphic forms, if the
uteri are opposed at the point of origin, then they are called
amphidelphic. If they are parallel and anteriorly directed at the
origin, then they are called prodelphic. If the uteri are parallel and
directed posteriorly
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