It is to be presumed that this dreadful parasite has been introduced into man by means of the flesh of the goat, and perhaps of the rabbit. Linguatulæ are found in their primary agamous form, in open cavities like the nasal fossæ. Leuckart was the first to show that the linguatulæ, which lived at first encysted in the peritoneum of the rabbit, completed their evolution and became perfect in the nasal fossæ of the dog. The _Linguatula serrata_ (Fig. 65), which lives primarily in the goat, the guinea-pig, the hare, the rabbit, &c., 232 is found accidentally in man, and perfect in certain mammals. Examples have been given of sick persons being completely cured by the evacuation of worms from the nostrils; these worms were, doubtless, linguatulæ. Fulvius Angelianus and Vincentius Alsarius speak of a young man who had suffered for a long time from head-ache, and who passed a worm from his nostrils. It was as long as the middle finger. There is little doubt that this was the _Linguatula tænioïdes_. These parasites may perhaps sometimes lose their way in their peregrinations. Some years ago a lioness died of peritonitis at Schönbrunn, and, after death, the liver, the spleen, and other organs were found to be filled with encysted linguatulæ. [Illustration: Fig. 64.--Isolated hook of Linguatula.] [Illustration: Fig. 65.--Linguatula magnified six times. Four hooks are seen around the mouth in front. _c_, the anus.] The nematode worms are long and rounded, like the ordinary ascarides of infants, which take up their abode in all the organs of animals of the various classes of the animal kingdom. About a thousand varieties are known, varying in length from a few millimètres to forty or 233 fifty centimètres. They are not all parasites, as has been thought, since some are found in the sea, and others in damp earth, in putrid matter, and even on plants and their seeds. The migrations of nematodes are subjects of great interest. Their changes of form are usually not very considerable; but the modifications in their sexual apparatus, whether in the same individual, or in the succeeding generations, are very curious.