The female aperture is usually on the ventral aspect and near the middle of the body, but it may be close to the mouth, or just in front of the anus, as in some Strongyli. It is generally a cross slit with more or less prominent margins, and in Trichocephalns is supported by a spinulose papilla. The female sexual tube is divisible into vagina, uterus, oviduct (or tuba), and ovary. The vagina is always short, and in small Nematoda scarcely visible. It is an invagination of the cuticle and subcuticula. The remainder of the tube has a wall composed of a delicate membrane lined by an epithelium which has a characteristic shape in its different sections. In the uterus and lower part of the oviduct it consists of polyhedric plates, usually numerous, bearing papilliform projections, and separated by furrows. The papillae probably secrete a substance which cements the ova together while the furrows give shelter to the sperm. The epithelium of the ovary consists of nucleated protoplasmic bands; that of the upper part of the oviduct is a nucleated protoplasmic layer in A. megalocephala. The lower part of the oviduct and the uterus have an external muscular layer of chiefly transverse fibres, whilst the vagina has fibres which radiate towards the lateral areae.
The genital tube is of greatest length in the larger Nematoda, in which it is disposed in many longitudinal coils; it is short and simple in the smaller species. It may be single and directly prolonged from the vagina as in Trichina, etc.; double, and then one branch may run towards the head, the other towards the tail, the vagina being at right angles, e. g. Oxyuris, Strongylus, etc.; or the two branches may run either parallel or at an acute angle to one another. A greater number of branches than two is rare. The spermatozoa accumulate near the junction of the uterus and oviduct; hence this part is sometimes termed spermatheca. They become mixed with the descending ova, and those which do not effect impregnation appear to reascend to the spot named by means of the intraepithelial furrows.
Remarkable peculiarities are the following: - In the impregnated female Sphaerularia which inhabits the coelome of the Humble Bee, the uterus undergoes prolapsus, and while the body of the Worm is only -gV in. long, the prolapsed uterus with the contained genitalia and a loop of intestine grows to a length of one inch. A similar prolapsus appears to occur in the female Simondsia from the stomach of the Pig. The male Trichodes crassicauda passes bodily into the uterus, and the male Syngamus trachealis has been said, but probably erroneously, to undergo fusion with the female1.
1See Schneider, Die Nematoden, p. 113, sub sp.
The ova originate in the upper part of the ovary, and it is said from a syncytium disposed like the corresponding part in the male. In the small Nematoda there is a single row of ripening ova connected by a slender rhachis; in the larger a number which are grouped round a central simple rhachis. The ova gradually increase in size, and their protoplasm becomes more or less granular as they approach the oviduct, where they are set free. A vitelline membrane is sometimes formed before impregnation, sometimes after. There is in the latter case a micropyle, which E. van Beneden states is closed by the fusion to its margins of the membrane covering the conoid process (ante, p. 681) of the entering spermatozoon. There is an eggshell which is thin where the embryo hatches in utero, and in some other instances. In other cases it is thick, and then often ornamented externally, and brownish in colour, and lined by one or two internal membranes2. The egg is usually oval in shape. The number formed varies, and the size of the worm does not necessarily regulate the size of the ovum.
Segmentation may not begin until some time after the laying of the egg, e. g. in A scar is lumbricoides; it takes place before laying, as in Dochmius - or the embryo may be already formed as in Oxyuris, or even escape from the shell in utero, e.g. Olhdanus, Pseudalius, Trichina, Dracunculus, Filaria Ban-crofti. Segmentation is total, and the odd numbers are found among the blastomeres. The latter possess the power of amoeboid motion. In some cases where the embryoes undergo development in utero, the rest of the maternal viscera are destroyed by their growth and movements, but the cuticle persists, and is turned into a moving sac of embryoes, e. g. in the female form of Angiostomum 3.
1 The body of the female Sphaerularia was supposed by Sir J. Lubbock to be an adherent male. As to Syngamus, it is difficult to see how the ova escape, as they undoubtedly do escape, if fusion occurs.
2 How the egg-shell and its contained membranes arise is a matter of dispute. According to E. van Beneden, the egg itself of A. megalocephala gives origin to a delicate vitelline membrane, and then successively to two peri-vitellar envelopes, its protoplasm retreating after each formation, and leaving a space filled by liquid. The shell proper is formed by the uterus. The same view is held by Leuckart. From Schneider's account, it seems that the thin egg-shell is a vitelline membrane; the thick, a vitelline membrane modified by subsequent growth, due to the fact that the protoplasm does not retreat from it; the two internal membranes are formed at a later period. If, he says, the impregnated egg of A. megalocephala is removed from the uterus and suffered to lie in water, the thick shell developes as usual, thus proving its non-uterine origin.
3 The history of the embryonic development presents difficulties. Nusbaum speaks of an invaginate Gastrula in Ascaris megalocephala (Z. A. vi. 1883). Hallez observed an invagination in the same animal, and the derivation of both oesophagus and rectum from endoderm (C. R. 101. 1885). In Cucullanus elegans, Butschli states that there is a plate formed of two layers of cells, epiThree stages accompanied by moults of the cuticle have been distinguished in the growth of a Nematode after hatching - the embryo, larva, and the adult worm. The first is very small; mouth, anus, and digestive tract may or may not be discernible; the oesophagus is proportionately longer, and the sexual gland is present as a uni- or multicellular rudiment. The tail is often singularly different to that of the adult, and the head may be provided with a boring spine, lost in the next stage, e. g. oral spine of Cucullanus, of Mermis, and ventral boring process of some Ascarid larvae. The larva has the various organs distinct; its tail resembles that of the adult female, and has acquired sensory papillae sometimes of great size; the genus and even species is discernible.
Under the larval skin the characters of the adult are assumed, and after a moult the worm grows in size and matures genital products.