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 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.
Here must be mentioned two families closely allied to the true Nematodes.
(i.) Chaetosomatidae.—This family includes three genera: Chaetosoma, Rhabdogaster, and Tristicochaeta. According to Metschnikoff,[[198]] although they are not true Nematodes, they have a great likeness to the group. He distinguishes them from the swimming members of the group as "creeping Nematoda." Chaetosoma, of which two species are known, C. ophicephalum and C. claparedii, has a head distinct from the body (Fig. 79). The mouth is at the anterior end, surrounded by a double semicircle of movable spicules; the whole body is covered by fine hairs, and on the ventral surface, just in front of the anus, is a double row of about fifteen cylindrical projections, by whose agency the animal creeps. The female C. claparedii is 1.5 mm. long, the male 1.14 mm. They were found creeping about on sea-weeds in the neighbourhood of Salerno.
