F. sanguinis hominis, found in Australia, China, India, Egypt, and Brazil, the sexual female of which inhabits the lymphatic glands, is the cause of elephantiasis, lymph scrotum, etc. and produces living embryoes circulating with the blood, giving rise to chyluria and haematuria, and passing into a larval state within a mosquito; Dracnnculus (Filaria) medinensis, the female only of which is known, attains a length of l-6 ft, lives encysted under the skin of the lower leg or shoulder, produces living young which become larvae in a Cyclops, and is distributed over certain districts of Asia (Arabia Petraea, Persian Gulf, Caspian Sea, Ganges), of Africa (Upper Egypt, Abyssinia, Guinea), in several W. Indian Is., and formerly in Brazil; Trichina spiralis, rare in England, found in the Pig as well as Man, the sexual worm of which inhabits the intestines, is viviparous, and its progeny migrate through the tissues and encyst in the muscles, causing what is known as Trichiniasis; Trichocephalus dispar, not uncommon in England and Ireland and the Continent, infrequent in Scotland, which infests the caecum and upper part of the colon, and the young of which appear to enter the body with water or food; Rhabdonema strongyloides (=Leptodera s.

Anguillula stercoralis and intestinalis), which occurs in Cochin China and Europe, and has been stated, but probably erroneously, to be the cause of a peculiar dysentery or diarrhoea, at least in Europe, the disease being really due to associated Dochmius (Anchylostomd) duodenalis 1.

1 The Oxyuris vermicularis lays ova containing at the time of their escape ready formed embryoes. Hence the great ease with which infection not only spreads, but is maintained in the case of this parasite. The embryoes of Ascaris megalocephala are developed more quickly in damp air than in water. The embryoes are set free from their shells if the latter are strewn on moist earth, and they continue to live if supplied with slices of pear, etc. The natural mode of development probably follows a similar course. See Hallez, C. R. 101, 1885. Von Linstow believes that Julus guttulatus is the intermediate host of A. lumbricoides; Z. A. ix. 1886.

2 The embryo of Ollulanus, a viviparous genus, may wander from the intestines of the Cat to its lungs, sometimes with results fatal to the host. After encystation it undergoes fatty degeneration. This is an instance of a parasite going astray. See Stirling, Q. J. M. xvii. 1877.

It is probable that the life-histories of a very large number of parasitic Nematoda fall under this heading (10): among others, perhaps those of Dracunculus medinensis and Filaria Bancrofti s. F. sanguinis hominis. The larva of the former inhabits a Cyclops; of the latter a certain sp. (?) of Mosquito; but the mode in which both are introduced into the human subject is still unknown.

The classification of the Nematoda is not easy. The class was divided by Schneider into three sections: (i) Polymyarii, with many muscle-cells in each quadrant of the body, e. g. Ascaris; (2) Meromyarii, with the muscle-cells disposed in eight rows, two rows in each quadrant, e. g. Oxyuris; and (3) Holomyarii, with the musculature of the body either undivided or divided only by the longitudinal thickenings of the subcuticula, e. g. Trichina, Mermis. But this last section appears to be founded on error.

Orley has proposed to establish three sub-divisions: (1) Nematentozoa, (2) Rhabditiformae, and (3) Anguillulidae, for the characters of which see his paper in A. N. H. (5), ix. 1882, p. 301 et seqq.

Claus distinguishes eight families: (1) Ascaridae (Ascaris, Heterakis, Oxyuris); (2) Strongylidae (Eustrongylus, Strongylus, Dochmius, Anchylostoma, Sclerostoma, Syngamus, Pseudalius, Olullanus); (3) Trichotrachelidae (Trichocephalus, Trichosoma, Trichina)) (4) Filariidae (Filaria, Dracunculus, Ichthyonemd); (5) Mermithidae (Mermis, Sphaerularid); (6) Gordiidae, see infra; (7) Anguillulidae, free-living in fresh-water or earth or in plants (Tylenchus, Anguillula, Angiostomum, Leptodera, Felodera); (8) Enoplidae, free-living and mostly marine.

1 Cf. A. N. 51, 1, 1885, pp. 11-12.

The Gordiidae are regarded by recent writers as a group distinct from the Nematoda, and Vejdovsky has proposed the name Nematomorpka for them. The principal points of anatomy are the following: There is an ornamented cuticle, pierced by pores containing processes of the subcuticula or hypodermis, which consists of distinct cells in the anterior and posterior regions of the body, of a granular nucleated matrix in the middle region. The muscle-cells are numerous, longitudinal, and disposed in a single layer. The body-cavity is filled up to a certain period by cells with distinct walls, which, during the evolution of the genital products, are reduced to an epithelium lining the body-walls, covering the 'egg-' and 'sperm-sacs,' and forming epithelioid mesenteries, which apparently constitute the walls of the efferent genital ducts, etc. The nervous system is represented by a peripharyngeal ring, a ventral cord composed of ganglion cells and nerve-fibres, and a caudal ganglion. Papillae similar to those of Nematoda are the sole organs of special sense, and are especially well developed on the ventral aspect of the tail of the male. There is a digestive tract, with cellular walls; the mouth appears to be occluded in some species when they become free; the anus is ventral and posterior.

A dorsal canal is regarded by Vejdovsky as excretory. The sexes are separate, the tail of the male deeply cleft. The female genitalia consist of paired metamerically-arranged ovaria; of two laterally-placed 'egg-sacs,' in which the ova ripen; of two median 'egg-receptacula,' which narrow posteriorly into the oviducts; of a large receptaculum seminis dorsal to the digestive tract, with two ducts leading one into each oviduct; of an atrium, which has glandular walls, opens posteriorly, and receives the two oviducts. As to the male organs, the true testes are not known (Vejdovsky); there are two vesiculae seminales, the homologues of the 'egg-sacs,' supra; and two vasa deferentia, continuous with the vesiculae and opening into the dilated termination of the digestive tract, which is capable of partial evagination. Villot's account differs from that of Vejdovsky, which has been followed here.