Gordioidea. The ovum of Gordius undergoes a regular segmentation. According to Villot (No. [391]) it forms at the close of segmentation a morula, which becomes two-layered by delamination. The embryo is at first spherical, but soon becomes elongated.
By an invagination at the anterior extremity the head is formed. It consists of a basal portion, armed with three rings of stylets, and a conical proboscis, armed with three large stylets. When the larva becomes free the head becomes everted, though it remains retractile. By the time the embryo is hatched a complete alimentary tract is formed with an oral opening at the end of the proboscis, and a subterminal ventral anal opening. It is divided into an œsophagus and stomach, and a large gland opens into it at the base of the proboscis.
The body has a number of transverse folds, which give it a ringed appearance.
Metamorphosis and life history.
Nematoidea. Although a large number of Nematodes have a free existence and simple life history, yet the greater number of known genera are parasitic, and undergo a more or less complicated metamorphosis[155]. According to this metamorphosis they may be divided into two groups (which by no means closely correspond with the natural divisions), viz. those which have a single host, and those with two hosts. Each of these main divisions may be subdivided again into two.
In the first group with one host the simplest cases are those in which the adult sexual form of parasite lays its eggs in the alimentary tract of its host, and the eggs are thence transported to the exterior. The embryo still in the egg, if favoured by sufficient warmth and moisture, completes its development up to a certain point, and, if then swallowed by an individual of the species in which it is parasitic in the adult condition, it is denuded of its shell by the action of the gastric juice, and develops directly into the sexual form.
Leuckart has experimentally established this metamorphosis in the case of Trichocephalus affinis, Oxyurus ambigua, and Heterakis vermicularis. The Oxyuridæ of Blatta and Hydrophilus have a similar life history (Galeb, No. [386]), and it is almost certain that the metamorphosis of the human parasites, Ascaris lumbricoides and Oxyurus vermicularis, is of this nature.
A slightly more complicated metamorphosis is common in the genera Ascaris and Strongylus. In these cases the egg-shell is thin, and the embryo becomes free externally, and enjoys for a shorter or longer period a free existence in water or moist earth. During this period it grows in size, and though not sexual usually closely resembles the adult form of the permanently free genus Rhabditis. In some cases the free larva becomes parasitic in a freshwater Mollusc, but without thereby undergoing any change. It eventually enters the alimentary tract of its proper host and there become sexual.
As examples of this form of development worked out by Leuckart may be mentioned Dochmius trigonocephalus, parasitic in the dog, and Ascaris acuminata, in the frog. The human parasite Dochmius duodenale undergoes the same metamorphosis as Dochmius trigonocephalus.
A remarkable modification of this type of metamorphosis is found in Ascaris (Rhabdonema) nigrovenosa, which in its most developed condition is parasitic in the lungs of the frog (Metschnikoff, Leuckart, No. [388]). The embryos pass through their first developmental phases in the body of the parent. They have the typical Rhabditis form, and make their way after birth into the frog’s rectum. From this they pass to the exterior, and then living either in moist earth, or the fæces of the frog, develop into a sexual form, but are very much smaller than in the adult condition. The sexes are distinct, and the males are distinguished from the females by their smaller size, shorter and rounded tails, and thinner bodies. The females have paired ovaries with a very small number of eggs, but the testis of the males is unpaired. Impregnation takes place in the usual way, and in summer time about four embryos are developed in each female, which soon burst their egg-capsules, and then move freely in the uterus. Their active movements soon burst the uterine walls, and they then come to lie freely in the body cavity. The remaining viscera of the mother are next reduced to a finely granular material, which serves for the nutrition of the young forms which continue to live in the maternal skin. The larvæ eventually become free, and though in many respects different from the parent form which gave rise to them, have nevertheless the Rhabditis form. They live in water or slime, and sometimes become parasitic in water-snails; in neither case however do they undergo important changes unless eventually swallowed by a frog. They then pass down the trachea into the lungs and there rapidly develop into the adult form. No separate males have been found in the lungs of the frog, but it has been shewn by Schneider (No. [390]) that the so-called females are really hermaphrodites; the same gland giving origin to both spermatozoa and ova, the former being developed before the latter[156]. The remarkable feature of the above life history is the fact that in the stage corresponding with the free larval stage of the previous forms the larvæ of this species become sexual, and give rise to a second free larval generation, which develops into the adult form on again becoming parasitic in the original host. It constitutes a somewhat exceptional case of heterogamy as defined in the introduction.