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which remains over from the egg and by the remains of the mother which they have taken into their alimentary canal, make their way through the earth, and ultimately coming across the root of a beet, begin to bore into it. This they do by means of a spine which can be protruded from the mouth. Once within the root, they absorb the cell sap of the parenchyma and begin to swell until their body projects from the surface of the root in the form of a tubercle (fig. 6). The reproductive organs do not begin to appear until the larva has twice cast its skin. After this a marked sexual dimorphism sets in. The female, hitherto indistinguishable from the male, continues to swell until she attains the outlines of a lemon. Doing this she bursts the epidermis of the rootlet, and her body projects into the surrounding earth. The male has a different life-history (fig. 7). After the second larval moult, he passes through a passive stage comparable to the pupa-stadium of an insect, and during this stage, which occurs inside the root, the reproductive organs are perfected. The male next casts his cuticle, and by means of his spine bores through the tissues of the root and escapes into the earth. Here he seeks a female, pairs, and soon afterwards dies. The eggs of the female give rise to embryos within the body of the mother; her other organs undergo a retrogressive change and serve as food for the young, until the body-wall only of the mother remains as a brown capsule. From this the young escape and make their way through the earth to new roots. The whole life-history extends over a period of some 4-5 weeks (fig. 7), so that some 6-7 generations are born during the warmer months. If we assume that each female produces 300 embryos, and that half of these are females, the number of descendants would be, after six generations, some 22,781 milliards (A. Strubell, Bibl. Zool., 1888-1889). Other species which have been recorded in the United Kingdom are Tylenchus devastatrix (Kuhn), on oats, rye and clover roots; T. tritici, causing the ear-cockle of wheat; Cephalobus rigidus (Schn.), on oats; Heterodera radicicola (Greef), on the roots of tomatoes, cucumbers, potatoes, turnips, peach-trees, vines and lettuce, and many other plants.

Fig. 5.

A, View of the heart of a dog infested with Filaria immitis Leidy; the right ventricle and base of the pulmonary artery have been opened: 𝑎, aorta; 𝑏, pulmonary artery; 𝑐, vena cava; 𝑑, right ventricle; 𝑒, appendix of left auricle; 𝑓, appendix of right auricle.

B, Female F. immitis, removed from the heart to show its length.

Fig. 6.

A, 𝑎, Female Heterodera schachtii Schmidt, breaking through the epidermis of a root; the head is still embedded in the parenchyma of the root.

B, 𝑎, larvae boring their way into a root; 𝑏, larva of the immobile kind surrounded by the old skin, living as an ectoparasite on the outside of the root. (From Strubell.)

Fig. 7.

A,	Male Heterodera schachtii, greatly magnified.
	𝑎,	Head lappets.
	𝑏,	Mouth cavity.
	𝑐,	Spine,
	𝑑,	Muscle of spine.
	𝑒,	Gland.
	𝑓,	Oesophagus.
	𝑔,	Bulb.
	ℎ,	Nerve-ring.
	𝑖,	Excretory pore
	𝑗,	Oesophagus.
	𝑘,	Testis.
	𝑙,	Intestine.
	𝑚,	Muscles moving spicule.
	𝑛,	Spicule.

B,	First motile larva.
C,	Second immovable parasitic larva casting its skin.
D,	A female with one half of the body-wall taken away to show the coiling generative organs.
	𝑎,	Boring apparatus.
	𝑏,	Oesopiageal bulb.
	𝑐,	Excretory pore.
	𝑑,	Alimentary canal.
	𝑒,	Anus.
	𝑓,	Ovary.
E,	A male shortly before casting its larval skin.

See N. Nassonov, Arch. Mikr. Anat. (1900); Arch. parasit. (1898); Rabot, Lab. Warsaw (1898); Zool. Anz. (1898); L. Jägerskiold, Centrbl. Bakter. (1898); J. Spengel, Zool. Anz. (1897); H. Ehlers, Arch. Naturg. (1899); O. Hamann, Die Nemathelminthen (1895).  (F. E. B.; A. E. S.) 

NEMATOMORPHA. This zoological group includes Gordian worms which are found swimming in an undulatory manner or coiling round water-weeds in ponds and puddles, or knotted together in an apparently inextricable coil. They may be several inches in length and are no thicker than a piece of whip-cord. The male is distinguishable from the female by the presence of a fork at the posterior end of the body. The body is covered by a cuticle which is sculptured and the various markings are of systematic importance: it is secreted by a hypodermis which also includes nerve-cells and some gland-cells. In the adult aquatic stage the alimentary canal shows signs of degeneration, and it seems probable that in this stage Gordian worms take no food. The mouth is terminal or subterminal; there is a weak. sucking pharynx situated behind the brain, and a long intestine lying along the medio-ventral body-cavity; it ends in a cloaca which receives the vasa deferentia in the male. There is a single unsegmented nerve-cord which runs along the ventral middle line and enlarges posteriorly into a caudal ganglion and anteriorly in a ganglion, the brain, which is not supra-oesophageal. The peripheral nervous system is minutely described by T. H. Montgomery. There is a median eye on the head.