The Nematode parasites of the Invertebrata are usually immature forms which attain their full development in the body of some vertebrate; but there are a number of species which in the sexually adult condition are peculiar to the Invertebrata.2 The Nematoda contain about as many parasitic species as all the other groups of internal parasites taken together; they are found in almost all the organs of the body, and by their presence, especially when encysted in the tissues and during their migration from one part of the body to another, give rise to various pathological conditions.
Gunther, " On the Structure and Affinities of Mnestra parasites Krohn; with a revision of the Classification of the Cladonemidae," Mitt.
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.
C. Claus, " Ober die Struktur der Muskelzellen and fiber den Korperbau von Mnestra parasites Krohn," Verhandl.
On the 6th of November in that year he plainly saw the living parasites under the microscope in the blood of a malarial patient, and he shortly afterwards communicated his observations to the Paris Academie de Medecine.