PARASITES AND PARASITIC DISEASES OF DOMESTIC ANIMALS


The Nematodes

Reproductive System

The sexes are separate in most species of nematodes and males are almost always smaller than females since females need to accommodate the production of large quantities of eggs. This disparity in size between male and female nematodes in shown in the accompanying image of Syngamus trachea in copulation: the smaller, pale blue, male (0.5 cm long) is dwarfed by the larger female (2.0 cm long) whose body is packed with coils of its yellowish-white reproductive tract.

The female reproductive system is tubular and in most nematodes of veterinary importance consists of two ovaries, each of which connects to an oviduct, and a uterus. The two uteri end in a common vagina which opens to the outside by a vulva which is often covered by a protective flap of cuticle, the vulva flap. In female members of the order Strongylida a muscular structure called an ovejector controls the exit of eggs from the uterus.

The male reproductive system in nematodes of veterinary importance is a single tube differentiated into testis, seminal vesicle and vas deferens and terminating in a muscular ejaculatory duct which empties into the cloaca.

In most nematode species there are two accessory male organs, spicules and a gubernaculum as shown in the accompanying image of Haemonchus contortus. Spicules (A) are made of cuticle, are often paired and used in copulation to dilate the female vulva. The gubernaculum (B) is a cuticular modification of the dorsal wall of the cloaca and is used to guide the spicules down through the cloaca so as to penetrate the female vulva. In this particular species, the spicules normally protrude from the cloaca even at rest. In members of the order Strongylida  a copulatory bursa is used to grasp the female at the level of the vulva, the rigid spicules are then extruded  and inserted into the vagina to keep it open so that amoeboid sperm can be transferred for fertilization to occur.

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Parasites and Parasitic Diseases of Domestic Animals
Dr. Colin Johnstone (principal author)
Copyright 1998 University of Pennsylvania
This page was last modified on February 11, 2000