Many species of this family live in humus or decaying matter; others live on, or are parasitic in, plants; some, such as Anguillula aceti, which is found in vinegar and in paste, live in organic fluids.
The part played by the presence of these Nematodes in the soil is not thoroughly understood; sometimes they occur in great numbers, and even when not directly parasitic in plants, probably do them much damage. Cobb[[190]] has recently described from Australia and Fiji over eighty species, one-half of them new, which occur mostly in the earth, and many of them among plant roots. They frequently crawl up on to plants, especially on to seedlings. An instance of this is given as follows: "The edible part of three bunches of nice-looking celery bought of a Chinaman in Sydney was cut off as far up as it was tender, nearly to the first leaflets. It was washed by hand in a tin dish in tank water, free from Nematodes. The washings gave about 200 to 300 Nematodes, belonging to five different genera."
It is very probable that many of the free-living forms which have received distinct specific names may ultimately turn out to be but stages in the life-history of some of the parasitic species. Von Linstow[[191]] has pointed out that the free form of A. diplogaster, if found alone, would be placed in the genus Diplogaster; similarly the bisexual form of Ascaris nigrovenosa is known as Rhabditis nigrovenosa.
Those Nematodes which live parasitically in plants, e.g. many of the genera Tylenchus and Aphelenchus and Heterodera, as well as those which only pierce the epidermis of the roots (the remaining species of the above-named genera), are provided with a spine which works to and fro through the mouth and assists the animal to bore into the tissues of the plant. Tylenchus devastatrix lives and reproduces in leaves and stems (never in the roots, except in the case of hops[[192]]) of many cultivated plants, such as rye, oats, onions, etc. "Clover sickness" is probably caused by this Nematode. The plants become infected by the thread-worms in the soil during the spring; their presence causes swellings and often kills the plant, in which case the worms return to the soil or remain in the straw.
Tylenchus tritici Need. is the cause of "ear-cockles" in corn. These take the form of brown or purple galls, which replace the grains of corn, and which contain hundreds of minute Nematodes. In these galls they are motionless, and are capable of surviving in dryness for at least twenty years; but when moistened,—for instance, by the gall falling on damp earth,—they resume their vitality and make their way to the young wheat plants, and then, wriggling up the leaves and stems, find their way to the ear. Here they pair, and producing a gall-like growth in the flower, lay numerous eggs, from which arise the Nematodes of the ear-cockle.
Fig. 77.—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.)
Heterodera schachtii[[193]] Schmidt, is the cause of the "beet sickness," and forms galls or swellings on the roots of many plants, in England especially on the roots of tomatoes and cucumbers. The free larvae live in the earth and make their way into the smaller rootlets; here the female larvae shed their skin, lose their characteristic Nematode form, and become citron-shaped (Fig. 78, D). The male larvae undergo a change, and after a period of rest cast their skin and, leaving the rootlet, seek out the females. The female does not undergo this second ecdysis, but its generative organs grow and mature in what is practically a larval stage. The embryos develop within the body of the mother, and, escaping through the uterus, ultimately cause her death. They then make their way into the earth. The cycle of the development takes but four or five weeks, so that, as in the case of Tylenchus devastatrix, there are several broods in a year; T. tritici, on the other hand, has but one.
Fig. 78.—A, Male Heterodera schachtii strongly 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, intestine; 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, oesophageal bulb; c, excretory pore; d, alimentary canal; e, anus; f, ovary: E, a male shortly before casting its larval skin.