_Ascaris lumbricoides_, Linneus.--This common parasite was for a long while regarded as identical with the great lumbricoid of the horse, but the question has been finally settled by Schneider, who has shown that the human worm, although identical with Dujardin’s _Ascaris suilla_ of the hog, is nevertheless quite distinct from the _Ascaris megalocephala_ of solipeds. The large lumbricoid occasionally found in the ox belongs to the human worm. Our large human helminth resembles the common earth-worm in general appearance only. The males usually measure from four to six inches in length, and the females from ten to fourteen inches. Some have been reported up to seventeen or eighteen inches in length. The body is smooth, fusiform, and elastic, and marked by numerous fine transverse rings. It is attenuated towards either extremity, the anterior end terminating in a prominently three-lobed mouth The tail is bluntly pointed. The female is much shorter than the male, having a diameter of nearly a quarter of an inch. The male is supplied with a double spiculum, its tail being always more or less curved towards the central surface. The female reproductive orifice is situated above the centre of the body. According to Schneider, the tail supports from 138 to 150 caudal papillæ, that is, from 69 to 75 on either side of the median line. Below the anus the papillæ are regularly arranged in pairs, seven in number, the two uppermost pairs being double. Notwithstanding the advantage which the size of this entozoon affords us in the matter of observation and experiment, we are yet ignorant as to the precise mode in which the young gain access to the human body. From what has been said respecting the quick growth of _Ascaris mystax_ in the dog, and from what has been observed respecting the rapid growth of the so-called _A. suilla_ in the hog, we know that the worm requires but a short time to pass from the larval to the sexual state. The view of Hering, Mosler, Davaine, and others, who suppose that these worms are reared in a direct manner by swallowing the ova, is, as Leuckart observed, not yet proved. We are not in full possession of the facts of larval development. It is true that Professor Heller’s interesting “find” has shown that when these worms first gain access to the human body their size is quite insignificant. At the post mortem of an imbecile, Heller discovered eighteen young worms, varying in size from about 1/9″ to 1/2″ in length (2·75 to 13 mm.). The sexes were indistinguishable. As a set-off against this, Leuckart’s repeated attempts to rear _Ascaris lumbricoides_ and _A. mystax_ by means of direct feeding-experiments with the eggs all failed. Thus, we are yet left in doubt as to the destiny of the larvæ during the period which elapses between the time of their escape from the egg and the time of their entry into the human body. So important is the question as to the mode of origination, growth, and subsequent development of the