For the most part free living and of small size. The oesophagus has usually a double swelling or two oesophageal bulbs. The male has two equal spicula. Genera: _Diplogaster_, _Mononchus_, _Rhabditis_, _Tylenchus_, _Anguillula_, and many others. 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. {155}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.