| While there is much diversity represented in the reproductive
structures of the Phylum Nemata, there are many features that are typical
of the phylum. Male nematodes are usually smaller than their female counterparts.
Basic male reproductive structures include: one testis, a seminal vesicle
and a vas deferens opening into a cloaca. One testis is most common, but
two testis are found in some species, while in others one testis is reduced.
Spermatogonia are produced in the testis and stored in the seminal vesicle
until the nematode mates. This
is illustrated in drawings by H. Hirschmann, in Plant Parasitic Nematodes
Vol. I, 1971, Academic Press pg. 52-53. The presence of one or two copulatory
spicules help dialate the vulva and can also serve as a canal for the spermatozoa.
The spicules are made from hardened cuticle, terminating in sensory dendrites
near the tip. Often the body wall around the cloaca is modified into a
bursa, which helps orient the male nematode and then helps hold the two
nematodes together.
Spermatozoa are amaeboid, and can have many different modifications. Some spermatozoa are round to ovoid in shape while others bear a resemblance to flagellated sperm. Different types of spermatozoa characterize different taxonomic groups of nematodes. Basic female structures include: one or two ovaries, seminal receptacles, uteri, ovijector and a vuvla. The ovary produces oogonia, which later develop into oocytes. The seminal receptacles, sometimes developed into a spermathecea, stores the spermatozoa until they are needed to fertilize an ooctye. The fertilized oocyte then develops into an egg in the uterus. The uteri often ends in an ovijector. The ovijector is very muscular and uses body movement combined with the high internal body pressure of the nematode to expel the egg through the vagina. All nematodes lay eggs. Syngamy, or cross fertilization, is common in most nematodes. Hermaphroditism also occurs, with the nematode gonads producing spermatozoa first and storing them until the eggs are produced. Parthenogenesis is also a normal means of reproduction in some nematodes. |