Vuillemin and Legrain[194] point out that while _Heterodera_ is injurious to cultivated plants growing in damp soil, its presence is advantageous to those that grow in deserts. It is very common in the Sahara, and attacks many plants which are {157}immune from it elsewhere. It causes the rootlets to swell out, and the bladder-like extensions thus formed act as reservoirs for water. Many other species attack plants; _Tylenchus millefolii_ Löw forms galls on _Achillea_, _T. dipsaci_ Kühn. on the teazle. They all seem to have great powers of resisting desiccation. The former species, when dried and placed in a herbarium in May, gave rise to active worms when moistened the following October; and the corn eel-worm is said to survive twenty-seven years in a state of suspended animation. On the other hand, although these Nematodes like moisture, they cannot withstand submersion in water for any time. They can resist a considerable degree of cold, and a species, _Aphelenchus nivalis_ Auriv.,[195] has been described from Spitzbergen, where it lives in the snow amongst a small red alga, _Sphaerella nivalis_. VII. FAMILY ENOPLIDAE. Small, as a rule free-living, usually marine Nematodes, without a second oesophageal bulb. Eyes and mouth-armature often present. Fine hairs and bristles sometimes surround the mouth. Genera: _Enoplus_, _Dorylaimus_, _Enchelidium_, and others. The genus _Enoplus_ is exclusively marine, living amongst Algae and Hydroids in shallow water and moving actively about, but never coiling into spirals. De Man[196] describes _Enoplus brevis_ Bast. as being attacked by a plant parasite, probably a Bacterium, of a greenish colour, which infested the muscles and gave them a peculiar colour. Numerous other species have been described by De Man from the coast of Holland. It is probable that some of them are the free stages of parasitic forms; a brackish water species found in the East Indies (_Dorylaimus palustris_) is regarded by Carter as the larva of _Filaria medinensis_. _Oncholaimus echini_ Leyd. is parasitic in the intestine of the sea-urchin _Echinus esculentus_. _Tricoma cincta_[197] has a strongly striated cuticle, which gives it almost the appearance of segmentation. _Fimbria tenuis_ has numerous hairs on the tail, and the mouth is surrounded by bristle-bearing papillae. {158}Here must be mentioned two families closely allied to the true Nematodes.