Among recent advances having medical import in our knowledge of the Nematodes, the chief are those dealing with the parasites of the blood.
These enenlies are as a rule so conspicuous that we do not look on their depredations as diseases, though the gradual deterioration of hay under the exhausting effects of root-parasites like Rhinanthus, and the onslaught of Cuscuta when unduly abundant, should teach us how unimportant to the definition the question of size may be.
Gunther, " On the Structure and Affinities of Mnestra parasites Krohn; with a revision of the Classification of the Cladonemidae," Mitt.
The term "biting-lice" is sometimes given to these parasites, in allusion to the mandibulate character of their mouth-parts, which serves to distinguish them at once from the true lice of the order Rhynchota in which the jaws are haustellate.
Of all parasites the one which by its mere presence is the most dangerous is the larva of Taenia echinococcus.