Fig. 66.—Ascaris lumbricoides Cloq. ♂, natural size, cut open along the dorsal middle line. a, Oesophagus; b, intestine; c, testis; d, vas deferens; h, lateral excretory canals.
With rare exceptions, e.g. Filaria attenuata, where it is double, the male reproductive organ consists of a single tube divisible into a testis proper, a vas deferens, a vesicula seminalis, where the spermatozoa are stored up, and a ductus ejaculatorius. The tube stretches through the body in a straight line in the small free-living forms, but is thrown into loops and coils in the larger parasitic Nematodes. Within the testis the mother-cells of the spermatozoa are attached to a rhachis or axial cord; the mother-cells divide, and their products ultimately form spermatozoa. The latter have a very peculiar shape; in accordance with the universal absence of cilia in the Nematoda the spermatozoon has no flagellum, and at first consists of a spherical nucleated cell, on one side of which a cap or covering of some refractive substance appears. The cap elongates and becomes conical, whilst the protoplasmic portion of the spermatozoon throws out pseudopodia and becomes amoeboid, but ultimately rounds itself off again. The spermatozoa do not attain maturity until they reach the uterus of the female.
The internal female reproductive organs are, with few exceptions (Trichina, etc.), double, but the vagina, which is lined with cuticle continuous with that covering the body, is always single. They are usually much coiled, and may be divided into ovary, oviduct, and uterus. The ova arise from a polynucleated mass of protoplasm or syncytium (Fig. 65, o) at the upper end, and acquire distinctness as they approach the oviduct. Fertilisation takes place in the uterus, but the segmentation may not begin until some time after the eggs are laid; in Dochmius, however, it is well advanced at this period, and in many genera, e.g. Pseudalius, Trichina, Dracunculus, etc., the whole development of the larva takes place in the body of the mother.
Fig. 67.—Ascaris lumbricoides Cloq. ♀, natural size, cut open along the median dorsal line to show the internal organs. a, The muscular oesophagus; b, the intestine; c, the ovary; d, the uterus; e, the vagina; f, the external opening; h, the excretory canals; i, their opening.
Embryology.—The eggs of many of the parasitic forms require a considerable degree of warmth to develop. Those of Ascaris lumbricoides require a temperature of 20° C., those of Trichocephalus 22.5° C., and those of Oxyuris vermicularis, 40° C. The latter develop in a few hours, the eggs of Dochmius in a few days, whilst those of A. lumbricoides take weeks or even months, and the young of Trichocephalus seldom develop within a year.[[168]] The ova only develop in a damp atmosphere, and they can be arrested at almost any stage, and for considerable periods, by desiccation.
Our knowledge of the processes by which the fertilised egg-cell develops into the larva is very imperfect. As a rule the segmentation is complete and equal; it results in the formation of a blastula, which may take the form of a hollow sphere of cells—A. megalocephala—or the cavity may be reduced, and the blastula may consist of a double-layered plate, as in Cucullanus.[[169]] The distinction into cells which will form the three embryonic layers, the ectoderm, mesoderm, and endoderm, is very early evident,—in the eight-cell stage. By the growth of one side of the blastula and the tucking in of the other the blastula becomes converted into a gastrula, which is a two-layered stage with a cavity opening to the exterior by a pore termed the blastopore. In Nematodes the blastopore is elongated and slit-like; it either forms the mouth (Cucullanus) or closes from behind forwards, the mouth ultimately arising at the point where the blastopore finally closed (Rhabdonema nigrovenosum). The mesodermal cells lie between the ectoderm and the endoderm; they ultimately develop into the muscles of the body-wall, the lateral excretory canals, and the reproductive organs; the last-named two systems arise each[[170]] from a single cell. The nervous system arises from the ectoderm, which also forms the sub-cuticle, and is turned in slightly at the mouth and anus; the remainder of the alimentary canal develops from the endoderm.
The post-embryonic development, which is very variable, and in many cases very extraordinary, will be dealt with under the several families.
Classification.—The classification of the Nematodes is a matter of very considerable difficulty; their structure is unusually monotonous, and, owing perhaps to their largely parasitic mode of life, they show practically none of those external features which are so useful to the systematist in other groups. Schneider in his Monograph divides the group into three subdivisions—(i.) the Polymyarii, in which numerous muscle cells are seen in a transverse section; (ii.) the Meromyarii, in which only eight are seen, two in each quadrant; and (iii.) the Holomyarii, in which the muscles are either not divided, or only divided by longitudinal lines. This grouping has, however, to some extent broken down, since Bütschli[[171]] and others have shown that the third subdivision is founded on insufficient observation, whilst the first two include, in different subdivisions, Nematodes which are closely allied in all respects except as regards their muscle cells.